Libguestfs - Apr 2020 - [PATCH nbdkit v2 1/3] file: Move file operators to a new common/fileops mini-library.

Needs some work still, see in particular the commit message
for patch 3.

Rich.

Writing "file-like" plugins is hard because you have to implement your
own .zero, .trim, .extents, etc, and that is very complicated.
However implementations of these functions already exist in the file
plugin.  By factoring out the file plugin into a separate "fileops"
mini-library we can reuse these implementations in other plugins.

This refactoring commit creates a new mini-library called fileops, and
uses it to implement the file plugin.

Note that the name or prefix "file" leaks into fileops in a few
places: the debug option is still called ‘-D file.zero=1’ and the
nbdkit --dump-plugin output will still contain ‘file_blksszget’ etc.
However I think that is fine as it should make usage more consistent
across future file-like plugins.
---
 configure.ac               |   1 +
 Makefile.am                |   1 +
 common/fileops/Makefile.am |  45 +++
 plugins/file/Makefile.am   |   2 +
 common/fileops/fileops.h   | 158 +++++++++++
 common/fileops/fileops.c   | 547 +++++++++++++++++++++++++++++++++++++
 plugins/file/file.c        | 541 ++----------------------------------
 7 files changed, 773 insertions(+), 522 deletions(-)

diff --git a/configure.ac b/configure.ac
index c5db962c..7a859bca 100644
--- a/configure.ac
+++ b/configure.ac
@@ -981,6 +981,7 @@ AC_CONFIG_FILES([common/protocol/generate-protostrings.sh],
 AC_CONFIG_FILES([Makefile
                  bash/Makefile
                  common/bitmap/Makefile
+                 common/fileops/Makefile
                  common/gpt/Makefile
                  common/include/Makefile
                  common/protocol/Makefile
diff --git a/Makefile.am b/Makefile.am
index ec8ae05d..6c6e5053 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -81,6 +81,7 @@ SUBDIRS = \
 if HAVE_PLUGINS
 SUBDIRS += \
 	common/bitmap \
+	common/fileops \
 	common/gpt \
 	common/regions \
 	common/sparse \
diff --git a/common/fileops/Makefile.am b/common/fileops/Makefile.am
new file mode 100644
index 00000000..1fce2d81
--- /dev/null
+++ b/common/fileops/Makefile.am
@@ -0,0 +1,45 @@
+# nbdkit
+# Copyright (C) 2020 Red Hat Inc.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met:
+#
+# * Redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer.
+#
+# * Redistributions in binary form must reproduce the above copyright
+# notice, this list of conditions and the following disclaimer in the
+# documentation and/or other materials provided with the distribution.
+#
+# * Neither the name of Red Hat nor the names of its contributors may be
+# used to endorse or promote products derived from this software without
+# specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS
IS'' AND
+# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+# THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+# PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
+# CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
+# USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+# ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+# OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+# OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+# SUCH DAMAGE.
+
+include $(top_srcdir)/common-rules.mk
+
+noinst_LTLIBRARIES = libfileops.la
+
+libfileops_la_SOURCES = \
+        fileops.c \
+        fileops.h \
+	$(NULL)
+libfileops_la_CPPFLAGS = \
+	-I$(top_srcdir)/include \
+	-I$(top_srcdir)/common/include \
+	-I$(top_srcdir)/common/utils \
+	$(NULL)
+libfileops_la_CFLAGS = $(WARNINGS_CFLAGS)
diff --git a/plugins/file/Makefile.am b/plugins/file/Makefile.am
index 7fc40cf4..f3bd86c0 100644
--- a/plugins/file/Makefile.am
+++ b/plugins/file/Makefile.am
@@ -42,6 +42,7 @@ nbdkit_file_plugin_la_SOURCES = \
 
 nbdkit_file_plugin_la_CPPFLAGS = \
 	-I$(top_srcdir)/include \
+	-I$(top_srcdir)/common/fileops \
 	-I$(top_srcdir)/common/include \
 	-I$(top_srcdir)/common/utils \
 	$(NULL)
@@ -51,6 +52,7 @@ nbdkit_file_plugin_la_LDFLAGS = \
 	-Wl,--version-script=$(top_srcdir)/plugins/plugins.syms \
 	$(NULL)
 nbdkit_file_plugin_la_LIBADD = \
+	$(top_builddir)/common/fileops/libfileops.la \
 	$(top_builddir)/common/utils/libutils.la \
 	$(NULL)
 
diff --git a/common/fileops/fileops.h b/common/fileops/fileops.h
new file mode 100644
index 00000000..f8eb4b1a
--- /dev/null
+++ b/common/fileops/fileops.h
@@ -0,0 +1,158 @@
+/* nbdkit
+ * Copyright (C) 2020 Red Hat Inc.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ * * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution.
+ *
+ * * Neither the name of Red Hat nor the names of its contributors may be
+ * used to endorse or promote products derived from this software without
+ * specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS
IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+ * PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
+ * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+/* This mini-library helps when writing plugins which are like
+ * nbdkit-file-plugin.  It is also used to implement
+ * nbdkit-file-plugin itself.  What this means is if your plugin
+ * (after perhaps some custom setup) serves a single, whole, local
+ * file or block device then you can implement most of the data
+ * serving part of the plugin using these generic fileops.  The
+ * advantage is that this mini-library deals with the complexity of
+ * implementing callbacks such as .zero and .trim efficiently.
+ *
+ * To use it:
+ *
+ * - your plugin must define and use NBDKIT_API_VERSION == 2
+ *
+ * - your plugin per-connection handle must either have type ‘struct
+ *   fileops *’, or include ‘struct fileops’ as the first member
+ *
+ * - add FILEOPS_READ_WRITE_CALLBACKS or
+ *   FILEOPS_READ_ONLY_CALLBACKS to your ‘struct nbdkit_plugin’
+ *
+ * - call init_fileops from your .open callback, and close_fileops
+ *   from your .close callback
+ *
+ * - optionally call fileops_dump_plugin from your .dump_plugin
+ *   callback if you have one
+ *
+ * - the fileops mini-library is linked statically into the plugin
+ *
+ * This mini-library is safe to use from any thread model, including
+ * NBDKIT_THREAD_MODEL_PARALLEL (in fact, parallel is the recommended
+ * thread model).
+ *
+ * See plugins/file/file.c for an example.
+ */
+
+#ifndef NBDKIT_FILEOPS_H
+#define NBDKIT_FILEOPS_H
+
+#include <config.h>
+
+#include <stdbool.h>
+
+/* For SEEK_HOLE. */
+#include <unistd.h>
+#include <sys/types.h>
+
+/* Either use this as the per-connection handle, or it must appear as
+ * the first member of the per-connection handle.  Don’t access these
+ * fields directly from your plugin.
+ */
+struct fileops {
+  int fd;
+  int sector_size;
+  bool is_block_device;
+  bool can_punch_hole;
+  bool can_zero_range;
+  bool can_fallocate;
+  bool can_zeroout;
+};
+
+/* Initialize the fileops struct.  ‘fd’ is a file descriptor opened on
+ * the local file or block device that you want to serve.  Call this
+ * from your .open callback after allocating the handle and setting up
+ * the file descriptor.
+ */
+extern int init_fileops (int fd, struct fileops *fops);
+
+/* Close the file descriptor and perform any other cleanup (but it
+ * doesn’t free the struct or handle).  Use this in your .close
+ * callback.
+ */
+extern void close_fileops (struct fileops *fops);
+
+/* You may optionally define a .dump_plugin callback which calls this. */
+extern void fileops_dump_plugin (void);
+
+/* Use the fileops callbacks by adding either
+ * FILEOPS_READ_WRITE_CALLBACKS or FILEOPS_READ_ONLY_CALLBACKS to your
+ * ‘struct nbdkit_plugin’.
+ */
+#define FILEOPS_READ_WRITE_CALLBACKS            \
+  .can_trim           = fileops_can_trim,       \
+  .can_fua            = fileops_can_fua,        \
+  .pwrite             = fileops_pwrite,         \
+  .flush              = fileops_flush,          \
+  .trim               = fileops_trim,           \
+  .zero               = fileops_zero,           \
+  FILEOPS_READ_ONLY_CALLBACKS
+
+#define FILEOPS_READ_ONLY_CALLBACKS             \
+  .get_size           = fileops_get_size,       \
+  .can_cache          = fileops_can_cache,      \
+  .pread              = fileops_pread,          \
+  .errno_is_preserved = 1,                      \
+  FILEOPS_MAYBE_EXTENTS                         \
+  FILEOPS_MAYBE_CACHE
+
+#ifdef SEEK_HOLE
+#define FILEOPS_MAYBE_EXTENTS                   \
+  .can_extents        = fileops_can_extents,    \
+  .extents            = fileops_extents,
+#else
+#define FILEOPS_MAYBE_EXTENTS /* nothing */
+#endif
+
+#ifdef HAVE_POSIX_FADVISE
+#define FILEOPS_MAYBE_CACHE                     \
+  .cache             = fileops_cache,
+#else
+#define FILEOPS_MAYBE_CACHE /* nothing */
+#endif
+
+/* Don’t call these directly. */
+extern int64_t fileops_get_size (void *handle);
+extern int fileops_can_trim (void *handle);
+extern int fileops_can_fua (void *handle);
+extern int fileops_can_cache (void *handle);
+extern int fileops_flush (void *handle, uint32_t flags);
+extern int fileops_pread (void *handle, void *buf, uint32_t count, uint64_t
offset, uint32_t flags);
+extern int fileops_pwrite (void *handle, const void *buf, uint32_t count,
uint64_t offset, uint32_t flags);
+extern int fileops_zero (void *handle, uint32_t count, uint64_t offset,
uint32_t flags);
+extern int fileops_trim (void *handle, uint32_t count, uint64_t offset,
uint32_t flags);
+extern int fileops_can_extents (void *handle);
+extern int fileops_extents (void *handle, uint32_t count, uint64_t offset,
uint32_t flags, struct nbdkit_extents *extents);
+extern int fileops_cache (void *handle, uint32_t count, uint64_t offset,
uint32_t flags);
+
+#endif /* NBDKIT_FILEOPS_H */
diff --git a/common/fileops/fileops.c b/common/fileops/fileops.c
new file mode 100644
index 00000000..6eb22aab
--- /dev/null
+++ b/common/fileops/fileops.c
@@ -0,0 +1,547 @@
+/* nbdkit
+ * Copyright (C) 2020 Red Hat Inc.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ * * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution.
+ *
+ * * Neither the name of Red Hat nor the names of its contributors may be
+ * used to endorse or promote products derived from this software without
+ * specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS
IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+ * PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
+ * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <config.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <inttypes.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/ioctl.h>
+
+#include <pthread.h>
+
+#if defined (__linux__) && !defined (FALLOC_FL_PUNCH_HOLE)
+#include <linux/falloc.h>   /* For FALLOC_FL_*, glibc < 2.18 */
+#endif
+
+#if defined (__linux__)
+#include <linux/fs.h>       /* For BLKZEROOUT */
+#endif
+
+#define NBDKIT_API_VERSION 2
+#include <nbdkit-plugin.h>
+
+#include "cleanup.h"
+#include "fileops.h"
+#include "isaligned.h"
+
+#ifndef HAVE_FDATASYNC
+#define fdatasync fsync
+#endif
+
+/* Any callbacks using lseek must be protected by this lock. */
+static pthread_mutex_t lseek_lock = PTHREAD_MUTEX_INITIALIZER;
+
+/* To enable: -D file.zero=1 */
+int file_debug_zero;
+
+static bool
+is_enotsup (int err)
+{
+  return err == ENOTSUP || err == EOPNOTSUPP;
+}
+
+/* Print some extra information about how the plugin was compiled. */
+void
+fileops_dump_plugin (void)
+{
+#ifdef BLKSSZGET
+  printf ("file_blksszget=yes\n");
+#endif
+#ifdef BLKZEROOUT
+  printf ("file_blkzeroout=yes\n");
+#endif
+#ifdef FALLOC_FL_PUNCH_HOLE
+  printf ("file_falloc_fl_punch_hole=yes\n");
+#endif
+#ifdef FALLOC_FL_ZERO_RANGE
+  printf ("file_falloc_fl_zero_range=yes\n");
+#endif
+}
+
+int
+init_fileops (int fd, struct fileops *fops)
+{
+  struct stat statbuf;
+
+  if (fstat (fd, &statbuf) == -1) {
+    nbdkit_error ("fstat: %m");
+    return -1;
+  }
+
+  fops->fd = fd;
+  fops->is_block_device = S_ISBLK (statbuf.st_mode);
+  fops->sector_size = 4096;  /* Start with safe guess */
+
+#ifdef BLKSSZGET
+  if (fops->is_block_device) {
+    if (ioctl (fops->fd, BLKSSZGET, &fops->sector_size))
+      nbdkit_debug ("cannot get sector size: %m");
+  }
+#endif
+
+#ifdef FALLOC_FL_PUNCH_HOLE
+  fops->can_punch_hole = true;
+#else
+  fops->can_punch_hole = false;
+#endif
+
+#ifdef FALLOC_FL_ZERO_RANGE
+  fops->can_zero_range = true;
+#else
+  fops->can_zero_range = false;
+#endif
+
+  fops->can_fallocate = true;
+  fops->can_zeroout = fops->is_block_device;
+
+  return 0;
+}
+
+void
+close_fileops (struct fileops *fops)
+{
+  close (fops->fd);
+}
+
+/* For block devices, stat->st_size is not the true size.  The caller
+ * grabs the lseek_lock.
+ */
+static int64_t
+block_device_size (int fd)
+{
+  off_t size;
+
+  size = lseek (fd, 0, SEEK_END);
+  if (size == -1) {
+    nbdkit_error ("lseek (to find device size): %m");
+    return -1;
+  }
+
+  return size;
+}
+
+/* Get the file size. */
+int64_t
+fileops_get_size (void *handle)
+{
+  struct fileops *fops = handle;
+
+  if (fops->is_block_device) {
+    ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&lseek_lock);
+    return block_device_size (fops->fd);
+  } else {
+    /* Regular file. */
+    struct stat statbuf;
+
+    if (fstat (fops->fd, &statbuf) == -1) {
+      nbdkit_error ("fstat: %m");
+      return -1;
+    }
+
+    return statbuf.st_size;
+  }
+}
+
+int
+fileops_can_trim (void *handle)
+{
+  /* Trim is advisory, but we prefer to advertise it only when we can
+   * actually (attempt to) punch holes.  Since not all filesystems
+   * support all fallocate modes, it would be nice if we had a way
+   * from fpathconf() to definitively learn what will work on a given
+   * fd for a more precise answer; oh well.  */
+#ifdef FALLOC_FL_PUNCH_HOLE
+  return 1;
+#else
+  return 0;
+#endif
+}
+
+int
+fileops_can_fua (void *handle)
+{
+  return NBDKIT_FUA_NATIVE;
+}
+
+int
+fileops_can_cache (void *handle)
+{
+  /* Prefer posix_fadvise(), but letting nbdkit call .pread on our
+   * behalf also tends to work well for the local file system
+   * cache.
+   */
+#if HAVE_POSIX_FADVISE
+  return NBDKIT_FUA_NATIVE;
+#else
+  return NBDKIT_FUA_EMULATE;
+#endif
+}
+
+/* Flush the file to disk. */
+int
+fileops_flush (void *handle, uint32_t flags)
+{
+  struct fileops *fops = handle;
+
+  if (fdatasync (fops->fd) == -1) {
+    nbdkit_error ("fdatasync: %m");
+    return -1;
+  }
+
+  return 0;
+}
+
+/* Read data from the file. */
+int
+fileops_pread (void *handle, void *buf, uint32_t count, uint64_t offset,
+               uint32_t flags)
+{
+  struct fileops *fops = handle;
+
+  while (count > 0) {
+    ssize_t r = pread (fops->fd, buf, count, offset);
+    if (r == -1) {
+      nbdkit_error ("pread: %m");
+      return -1;
+    }
+    if (r == 0) {
+      nbdkit_error ("pread: unexpected end of file");
+      return -1;
+    }
+    buf += r;
+    count -= r;
+    offset += r;
+  }
+
+  return 0;
+}
+
+/* Write data to the file. */
+int
+fileops_pwrite (void *handle, const void *buf, uint32_t count, uint64_t offset,
+                uint32_t flags)
+{
+  struct fileops *fops = handle;
+
+  while (count > 0) {
+    ssize_t r = pwrite (fops->fd, buf, count, offset);
+    if (r == -1) {
+      nbdkit_error ("pwrite: %m");
+      return -1;
+    }
+    buf += r;
+    count -= r;
+    offset += r;
+  }
+
+  if ((flags & NBDKIT_FLAG_FUA) && fileops_flush (handle, 0) == -1)
+    return -1;
+
+  return 0;
+}
+
+#if defined (FALLOC_FL_PUNCH_HOLE) || defined (FALLOC_FL_ZERO_RANGE)
+static int
+do_fallocate (int fd, int mode, off_t offset, off_t len)
+{
+  int r = fallocate (fd, mode, offset, len);
+  if (r == -1 && errno == ENODEV) {
+    /* kernel 3.10 fails with ENODEV for block device. Kernel >= 4.9 fails
+       with EOPNOTSUPP in this case. Normalize errno to simplify callers. */
+    errno = EOPNOTSUPP;
+  }
+  return r;
+}
+#endif
+
+/* Write zeroes to the file. */
+int
+fileops_zero (void *handle, uint32_t count, uint64_t offset, uint32_t flags)
+{
+  struct fileops *fops = handle;
+  int r;
+
+#ifdef FALLOC_FL_PUNCH_HOLE
+  if (fops->can_punch_hole && (flags & NBDKIT_FLAG_MAY_TRIM)) {
+    r = do_fallocate (fops->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
+                      offset, count);
+    if (r == 0) {
+      if (file_debug_zero)
+        nbdkit_debug ("fops->can_punch_hole && may_trim: "
+                      "zero succeeded using fallocate");
+      goto out;
+    }
+
+    if (!is_enotsup (errno)) {
+      nbdkit_error ("zero: %m");
+      return -1;
+    }
+
+    fops->can_punch_hole = false;
+  }
+#endif
+
+#ifdef FALLOC_FL_ZERO_RANGE
+  if (fops->can_zero_range) {
+    r = do_fallocate (fops->fd, FALLOC_FL_ZERO_RANGE, offset, count);
+    if (r == 0) {
+      if (file_debug_zero)
+        nbdkit_debug ("fops->can_zero-range: "
+                      "zero succeeded using fallocate");
+      goto out;
+    }
+
+    if (!is_enotsup (errno)) {
+      nbdkit_error ("zero: %m");
+      return -1;
+    }
+
+    fops->can_zero_range = false;
+  }
+#endif
+
+#ifdef FALLOC_FL_PUNCH_HOLE
+  /* If we can punch hole but may not trim, we can combine punching hole and
+   * fallocate to zero a range. This is expected to be more efficient than
+   * writing zeroes manually. */
+  if (fops->can_punch_hole && fops->can_fallocate) {
+    r = do_fallocate (fops->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
+                      offset, count);
+    if (r == 0) {
+      r = do_fallocate (fops->fd, 0, offset, count);
+      if (r == 0) {
+        if (file_debug_zero)
+          nbdkit_debug ("fops->can_punch_hole &&
fops->can_fallocate: "
+                        "zero succeeded using fallocate");
+        goto out;
+      }
+
+      if (!is_enotsup (errno)) {
+        nbdkit_error ("zero: %m");
+        return -1;
+      }
+
+      fops->can_fallocate = false;
+    } else {
+      if (!is_enotsup (errno)) {
+        nbdkit_error ("zero: %m");
+        return -1;
+      }
+
+      fops->can_punch_hole = false;
+    }
+  }
+#endif
+
+#ifdef BLKZEROOUT
+  /* For aligned range and block device, we can use BLKZEROOUT. */
+  if (fops->can_zeroout && IS_ALIGNED (offset | count,
fops->sector_size)) {
+    uint64_t range[2] = {offset, count};
+
+    r = ioctl (fops->fd, BLKZEROOUT, &range);
+    if (r == 0) {
+      if (file_debug_zero)
+        nbdkit_debug ("fops->can_zeroout && IS_ALIGNED: "
+                      "zero succeeded using BLKZEROOUT");
+      goto out;
+    }
+
+    if (errno != ENOTTY) {
+      nbdkit_error ("zero: %m");
+      return -1;
+    }
+
+    fops->can_zeroout = false;
+  }
+#endif
+
+  /* Trigger a fall back to writing */
+  if (file_debug_zero)
+    nbdkit_debug ("zero falling back to writing");
+  errno = EOPNOTSUPP;
+  return -1;
+
+ out:
+  if ((flags & NBDKIT_FLAG_FUA) && fileops_flush (handle, 0) == -1)
+    return -1;
+  return 0;
+}
+
+/* Punch a hole in the file. */
+int
+fileops_trim (void *handle, uint32_t count, uint64_t offset, uint32_t flags)
+{
+#ifdef FALLOC_FL_PUNCH_HOLE
+  struct fileops *fops = handle;
+  int r;
+
+  if (fops->can_punch_hole) {
+    r = do_fallocate (fops->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
+                      offset, count);
+    if (r == -1) {
+      /* Trim is advisory; we don't care if it fails for anything other
+       * than EIO or EPERM. */
+      if (errno == EPERM || errno == EIO) {
+        nbdkit_error ("fallocate: %m");
+        return -1;
+      }
+
+      if (is_enotsup (EOPNOTSUPP))
+        fops->can_punch_hole = false;
+
+      nbdkit_debug ("ignoring failed fallocate during trim: %m");
+    }
+  }
+#endif
+
+  if ((flags & NBDKIT_FLAG_FUA) && fileops_flush (handle, 0) == -1)
+    return -1;
+
+  return 0;
+}
+
+#ifdef SEEK_HOLE
+/* Extents. */
+
+int
+fileops_can_extents (void *handle)
+{
+  struct fileops *fops = handle;
+  off_t r;
+
+  /* A simple test to see whether SEEK_HOLE etc is likely to work on
+   * the current filesystem.
+   */
+  ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&lseek_lock);
+  r = lseek (fops->fd, 0, SEEK_HOLE);
+  if (r == -1) {
+    nbdkit_debug ("extents disabled: lseek: SEEK_HOLE: %m");
+    return 0;
+  }
+  return 1;
+}
+
+static int
+do_extents (void *handle, uint32_t count, uint64_t offset,
+            uint32_t flags, struct nbdkit_extents *extents)
+{
+  struct fileops *fops = handle;
+  const bool req_one = flags & NBDKIT_FLAG_REQ_ONE;
+  uint64_t end = offset + count;
+
+  do {
+    off_t pos;
+
+    pos = lseek (fops->fd, offset, SEEK_DATA);
+    if (pos == -1) {
+      if (errno == ENXIO) {
+        /* The current man page does not describe this situation well,
+         * but a proposed change to POSIX adds these words for ENXIO:
+         * "or the whence argument is SEEK_DATA and the offset falls
+         * within the final hole of the file."
+         */
+        pos = end;
+      }
+      else {
+        nbdkit_error ("lseek: SEEK_DATA: %" PRIu64 ": %m",
offset);
+        return -1;
+      }
+    }
+
+    /* We know there is a hole from offset to pos-1. */
+    if (pos > offset) {
+      if (nbdkit_add_extent (extents, offset, pos - offset,
+                             NBDKIT_EXTENT_HOLE | NBDKIT_EXTENT_ZERO) == -1)
+        return -1;
+      if (req_one)
+        break;
+    }
+
+    offset = pos;
+    if (offset >= end)
+      break;
+
+    pos = lseek (fops->fd, offset, SEEK_HOLE);
+    if (pos == -1) {
+      nbdkit_error ("lseek: SEEK_HOLE: %" PRIu64 ": %m",
offset);
+      return -1;
+    }
+
+    /* We know there is data from offset to pos-1. */
+    if (pos > offset) {
+      if (nbdkit_add_extent (extents, offset, pos - offset,
+                             0 /* allocated data */) == -1)
+        return -1;
+      if (req_one)
+        break;
+    }
+
+    offset = pos;
+  } while (offset < end);
+
+  return 0;
+}
+
+int
+fileops_extents (void *handle, uint32_t count, uint64_t offset,
+                 uint32_t flags, struct nbdkit_extents *extents)
+{
+  ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&lseek_lock);
+  return do_extents (handle, count, offset, flags, extents);
+}
+#endif /* SEEK_HOLE */
+
+#if HAVE_POSIX_FADVISE
+/* Caching. */
+int
+fileops_cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags)
+{
+  struct fileops *fops = handle;
+  int r;
+
+  /* Cache is advisory, we don't care if this fails */
+  r = posix_fadvise (fops->fd, offset, count, POSIX_FADV_WILLNEED);
+  if (r) {
+    errno = r;
+    nbdkit_error ("posix_fadvise: %m");
+    return -1;
+  }
+  return 0;
+}
+#endif /* HAVE_POSIX_FADVISE */
diff --git a/plugins/file/file.c b/plugins/file/file.c
index 8c2ea077..3dd6c944 100644
--- a/plugins/file/file.c
+++ b/plugins/file/file.c
@@ -34,51 +34,18 @@
 
 #include <stdio.h>
 #include <stdlib.h>
-#include <stdbool.h>
+#include <stdint.h>
 #include <string.h>
-#include <inttypes.h>
 #include <fcntl.h>
-#include <unistd.h>
-#include <sys/types.h>
-#include <sys/stat.h>
-#include <sys/ioctl.h>
 #include <errno.h>
 
-#include <pthread.h>
-
-#if defined (__linux__) && !defined (FALLOC_FL_PUNCH_HOLE)
-#include <linux/falloc.h>   /* For FALLOC_FL_*, glibc < 2.18 */
-#endif
-
-#if defined (__linux__)
-#include <linux/fs.h>       /* For BLKZEROOUT */
-#endif
-
 #define NBDKIT_API_VERSION 2
-
 #include <nbdkit-plugin.h>
 
-#include "cleanup.h"
-#include "isaligned.h"
-
-#ifndef HAVE_FDATASYNC
-#define fdatasync fsync
-#endif
+#include "fileops.h"
 
 static char *filename = NULL;
 
-/* Any callbacks using lseek must be protected by this lock. */
-static pthread_mutex_t lseek_lock = PTHREAD_MUTEX_INITIALIZER;
-
-/* to enable: -D file.zero=1 */
-int file_debug_zero;
-
-static bool
-is_enotsup (int err)
-{
-  return err == ENOTSUP || err == EOPNOTSUPP;
-}
-
 static void
 file_unload (void)
 {
@@ -131,41 +98,18 @@ file_config_complete (void)
 static void
 file_dump_plugin (void)
 {
-#ifdef BLKSSZGET
-  printf ("file_blksszget=yes\n");
-#endif
-#ifdef BLKZEROOUT
-  printf ("file_blkzeroout=yes\n");
-#endif
-#ifdef FALLOC_FL_PUNCH_HOLE
-  printf ("file_falloc_fl_punch_hole=yes\n");
-#endif
-#ifdef FALLOC_FL_ZERO_RANGE
-  printf ("file_falloc_fl_zero_range=yes\n");
-#endif
+  fileops_dump_plugin ();
 }
 
-/* The per-connection handle. */
-struct handle {
-  int fd;
-  bool is_block_device;
-  int sector_size;
-  bool can_punch_hole;
-  bool can_zero_range;
-  bool can_fallocate;
-  bool can_zeroout;
-};
-
 /* Create the per-connection handle. */
 static void *
 file_open (int readonly)
 {
-  struct handle *h;
-  struct stat statbuf;
-  int flags;
+  struct fileops *fops;
+  int fd, flags;
 
-  h = malloc (sizeof *h);
-  if (h == NULL) {
+  fops = malloc (sizeof *fops);
+  if (fops == NULL) {
     nbdkit_error ("malloc: %m");
     return NULL;
   }
@@ -176,98 +120,33 @@ file_open (int readonly)
   else
     flags |= O_RDWR;
 
-  h->fd = open (filename, flags);
-  if (h->fd == -1) {
+  fd = open (filename, flags);
+  if (fd == -1) {
     nbdkit_error ("open: %s: %m", filename);
-    free (h);
+    free (fops);
     return NULL;
   }
 
-  if (fstat (h->fd, &statbuf) == -1) {
-    nbdkit_error ("fstat: %s: %m", filename);
-    free (h);
+  if (init_fileops (fd, fops) == -1) {
+    free (fops);
     return NULL;
   }
 
-  h->is_block_device = S_ISBLK (statbuf.st_mode);
-  h->sector_size = 4096;  /* Start with safe guess */
-
-#ifdef BLKSSZGET
-  if (h->is_block_device) {
-    if (ioctl (h->fd, BLKSSZGET, &h->sector_size))
-      nbdkit_debug ("cannot get sector size: %s: %m", filename);
-  }
-#endif
-
-#ifdef FALLOC_FL_PUNCH_HOLE
-  h->can_punch_hole = true;
-#else
-  h->can_punch_hole = false;
-#endif
-
-#ifdef FALLOC_FL_ZERO_RANGE
-  h->can_zero_range = true;
-#else
-  h->can_zero_range = false;
-#endif
-
-  h->can_fallocate = true;
-  h->can_zeroout = h->is_block_device;
-
-  return h;
+  return fops;
 }
 
 /* Free up the per-connection handle. */
 static void
 file_close (void *handle)
 {
-  struct handle *h = handle;
+  struct fileops *fops = handle;
 
-  close (h->fd);
-  free (h);
+  close_fileops (fops);
+  free (fops);
 }
 
 #define THREAD_MODEL NBDKIT_THREAD_MODEL_PARALLEL
 
-/* For block devices, stat->st_size is not the true size.  The caller
- * grabs the lseek_lock.
- */
-static int64_t
-block_device_size (int fd)
-{
-  off_t size;
-
-  size = lseek (fd, 0, SEEK_END);
-  if (size == -1) {
-    nbdkit_error ("lseek (to find device size): %m");
-    return -1;
-  }
-
-  return size;
-}
-
-/* Get the file size. */
-static int64_t
-file_get_size (void *handle)
-{
-  struct handle *h = handle;
-
-  if (h->is_block_device) {
-    ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&lseek_lock);
-    return block_device_size (h->fd);
-  } else {
-    /* Regular file. */
-    struct stat statbuf;
-
-    if (fstat (h->fd, &statbuf) == -1) {
-      nbdkit_error ("fstat: %m");
-      return -1;
-    }
-
-    return statbuf.st_size;
-  }
-}
-
 /* Allow multiple parallel connections from a single client. */
 static int
 file_can_multi_conn (void *handle)
@@ -275,374 +154,6 @@ file_can_multi_conn (void *handle)
   return 1;
 }
 
-static int
-file_can_trim (void *handle)
-{
-  /* Trim is advisory, but we prefer to advertise it only when we can
-   * actually (attempt to) punch holes.  Since not all filesystems
-   * support all fallocate modes, it would be nice if we had a way
-   * from fpathconf() to definitively learn what will work on a given
-   * fd for a more precise answer; oh well.  */
-#ifdef FALLOC_FL_PUNCH_HOLE
-  return 1;
-#else
-  return 0;
-#endif
-}
-
-static int
-file_can_fua (void *handle)
-{
-  return NBDKIT_FUA_NATIVE;
-}
-
-static int
-file_can_cache (void *handle)
-{
-  /* Prefer posix_fadvise(), but letting nbdkit call .pread on our
-   * behalf also tends to work well for the local file system
-   * cache.
-   */
-#if HAVE_POSIX_FADVISE
-  return NBDKIT_FUA_NATIVE;
-#else
-  return NBDKIT_FUA_EMULATE;
-#endif
-}
-
-/* Flush the file to disk. */
-static int
-file_flush (void *handle, uint32_t flags)
-{
-  struct handle *h = handle;
-
-  if (fdatasync (h->fd) == -1) {
-    nbdkit_error ("fdatasync: %m");
-    return -1;
-  }
-
-  return 0;
-}
-
-/* Read data from the file. */
-static int
-file_pread (void *handle, void *buf, uint32_t count, uint64_t offset,
-            uint32_t flags)
-{
-  struct handle *h = handle;
-
-  while (count > 0) {
-    ssize_t r = pread (h->fd, buf, count, offset);
-    if (r == -1) {
-      nbdkit_error ("pread: %m");
-      return -1;
-    }
-    if (r == 0) {
-      nbdkit_error ("pread: unexpected end of file");
-      return -1;
-    }
-    buf += r;
-    count -= r;
-    offset += r;
-  }
-
-  return 0;
-}
-
-/* Write data to the file. */
-static int
-file_pwrite (void *handle, const void *buf, uint32_t count, uint64_t offset,
-             uint32_t flags)
-{
-  struct handle *h = handle;
-
-  while (count > 0) {
-    ssize_t r = pwrite (h->fd, buf, count, offset);
-    if (r == -1) {
-      nbdkit_error ("pwrite: %m");
-      return -1;
-    }
-    buf += r;
-    count -= r;
-    offset += r;
-  }
-
-  if ((flags & NBDKIT_FLAG_FUA) && file_flush (handle, 0) == -1)
-    return -1;
-
-  return 0;
-}
-
-#if defined (FALLOC_FL_PUNCH_HOLE) || defined (FALLOC_FL_ZERO_RANGE)
-static int
-do_fallocate (int fd, int mode, off_t offset, off_t len)
-{
-  int r = fallocate (fd, mode, offset, len);
-  if (r == -1 && errno == ENODEV) {
-    /* kernel 3.10 fails with ENODEV for block device. Kernel >= 4.9 fails
-       with EOPNOTSUPP in this case. Normalize errno to simplify callers. */
-    errno = EOPNOTSUPP;
-  }
-  return r;
-}
-#endif
-
-/* Write zeroes to the file. */
-static int
-file_zero (void *handle, uint32_t count, uint64_t offset, uint32_t flags)
-{
-  struct handle *h = handle;
-  int r;
-
-#ifdef FALLOC_FL_PUNCH_HOLE
-  if (h->can_punch_hole && (flags & NBDKIT_FLAG_MAY_TRIM)) {
-    r = do_fallocate (h->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
-                      offset, count);
-    if (r == 0) {
-      if (file_debug_zero)
-        nbdkit_debug ("h->can_punch_hole && may_trim: "
-                      "zero succeeded using fallocate");
-      goto out;
-    }
-
-    if (!is_enotsup (errno)) {
-      nbdkit_error ("zero: %m");
-      return -1;
-    }
-
-    h->can_punch_hole = false;
-  }
-#endif
-
-#ifdef FALLOC_FL_ZERO_RANGE
-  if (h->can_zero_range) {
-    r = do_fallocate (h->fd, FALLOC_FL_ZERO_RANGE, offset, count);
-    if (r == 0) {
-      if (file_debug_zero)
-        nbdkit_debug ("h->can_zero-range: "
-                      "zero succeeded using fallocate");
-      goto out;
-    }
-
-    if (!is_enotsup (errno)) {
-      nbdkit_error ("zero: %m");
-      return -1;
-    }
-
-    h->can_zero_range = false;
-  }
-#endif
-
-#ifdef FALLOC_FL_PUNCH_HOLE
-  /* If we can punch hole but may not trim, we can combine punching hole and
-   * fallocate to zero a range. This is expected to be more efficient than
-   * writing zeroes manually. */
-  if (h->can_punch_hole && h->can_fallocate) {
-    r = do_fallocate (h->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
-                      offset, count);
-    if (r == 0) {
-      r = do_fallocate (h->fd, 0, offset, count);
-      if (r == 0) {
-        if (file_debug_zero)
-          nbdkit_debug ("h->can_punch_hole &&
h->can_fallocate: "
-                        "zero succeeded using fallocate");
-        goto out;
-      }
-
-      if (!is_enotsup (errno)) {
-        nbdkit_error ("zero: %m");
-        return -1;
-      }
-
-      h->can_fallocate = false;
-    } else {
-      if (!is_enotsup (errno)) {
-        nbdkit_error ("zero: %m");
-        return -1;
-      }
-
-      h->can_punch_hole = false;
-    }
-  }
-#endif
-
-#ifdef BLKZEROOUT
-  /* For aligned range and block device, we can use BLKZEROOUT. */
-  if (h->can_zeroout && IS_ALIGNED (offset | count,
h->sector_size)) {
-    uint64_t range[2] = {offset, count};
-
-    r = ioctl (h->fd, BLKZEROOUT, &range);
-    if (r == 0) {
-      if (file_debug_zero)
-        nbdkit_debug ("h->can_zeroout && IS_ALIGNED: "
-                      "zero succeeded using BLKZEROOUT");
-      goto out;
-    }
-
-    if (errno != ENOTTY) {
-      nbdkit_error ("zero: %m");
-      return -1;
-    }
-
-    h->can_zeroout = false;
-  }
-#endif
-
-  /* Trigger a fall back to writing */
-  if (file_debug_zero)
-    nbdkit_debug ("zero falling back to writing");
-  errno = EOPNOTSUPP;
-  return -1;
-
- out:
-  if ((flags & NBDKIT_FLAG_FUA) && file_flush (handle, 0) == -1)
-    return -1;
-  return 0;
-}
-
-/* Punch a hole in the file. */
-static int
-file_trim (void *handle, uint32_t count, uint64_t offset, uint32_t flags)
-{
-#ifdef FALLOC_FL_PUNCH_HOLE
-  struct handle *h = handle;
-  int r;
-
-  if (h->can_punch_hole) {
-    r = do_fallocate (h->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
-                      offset, count);
-    if (r == -1) {
-      /* Trim is advisory; we don't care if it fails for anything other
-       * than EIO or EPERM. */
-      if (errno == EPERM || errno == EIO) {
-        nbdkit_error ("fallocate: %m");
-        return -1;
-      }
-
-      if (is_enotsup (EOPNOTSUPP))
-        h->can_punch_hole = false;
-
-      nbdkit_debug ("ignoring failed fallocate during trim: %m");
-    }
-  }
-#endif
-
-  if ((flags & NBDKIT_FLAG_FUA) && file_flush (handle, 0) == -1)
-    return -1;
-
-  return 0;
-}
-
-#ifdef SEEK_HOLE
-/* Extents. */
-
-static int
-file_can_extents (void *handle)
-{
-  struct handle *h = handle;
-  off_t r;
-
-  /* A simple test to see whether SEEK_HOLE etc is likely to work on
-   * the current filesystem.
-   */
-  ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&lseek_lock);
-  r = lseek (h->fd, 0, SEEK_HOLE);
-  if (r == -1) {
-    nbdkit_debug ("extents disabled: lseek: SEEK_HOLE: %m");
-    return 0;
-  }
-  return 1;
-}
-
-static int
-do_extents (void *handle, uint32_t count, uint64_t offset,
-            uint32_t flags, struct nbdkit_extents *extents)
-{
-  struct handle *h = handle;
-  const bool req_one = flags & NBDKIT_FLAG_REQ_ONE;
-  uint64_t end = offset + count;
-
-  do {
-    off_t pos;
-
-    pos = lseek (h->fd, offset, SEEK_DATA);
-    if (pos == -1) {
-      if (errno == ENXIO) {
-        /* The current man page does not describe this situation well,
-         * but a proposed change to POSIX adds these words for ENXIO:
-         * "or the whence argument is SEEK_DATA and the offset falls
-         * within the final hole of the file."
-         */
-        pos = end;
-      }
-      else {
-        nbdkit_error ("lseek: SEEK_DATA: %" PRIu64 ": %m",
offset);
-        return -1;
-      }
-    }
-
-    /* We know there is a hole from offset to pos-1. */
-    if (pos > offset) {
-      if (nbdkit_add_extent (extents, offset, pos - offset,
-                             NBDKIT_EXTENT_HOLE | NBDKIT_EXTENT_ZERO) == -1)
-        return -1;
-      if (req_one)
-        break;
-    }
-
-    offset = pos;
-    if (offset >= end)
-      break;
-
-    pos = lseek (h->fd, offset, SEEK_HOLE);
-    if (pos == -1) {
-      nbdkit_error ("lseek: SEEK_HOLE: %" PRIu64 ": %m",
offset);
-      return -1;
-    }
-
-    /* We know there is data from offset to pos-1. */
-    if (pos > offset) {
-      if (nbdkit_add_extent (extents, offset, pos - offset,
-                             0 /* allocated data */) == -1)
-        return -1;
-      if (req_one)
-        break;
-    }
-
-    offset = pos;
-  } while (offset < end);
-
-  return 0;
-}
-
-static int
-file_extents (void *handle, uint32_t count, uint64_t offset,
-              uint32_t flags, struct nbdkit_extents *extents)
-{
-  ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&lseek_lock);
-  return do_extents (handle, count, offset, flags, extents);
-}
-#endif /* SEEK_HOLE */
-
-#if HAVE_POSIX_FADVISE
-/* Caching. */
-static int
-file_cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags)
-{
-  struct handle *h = handle;
-  int r;
-
-  /* Cache is advisory, we don't care if this fails */
-  r = posix_fadvise (h->fd, offset, count, POSIX_FADV_WILLNEED);
-  if (r) {
-    errno = r;
-    nbdkit_error ("posix_fadvise: %m");
-    return -1;
-  }
-  return 0;
-}
-#endif /* HAVE_POSIX_FADVISE */
-
 static struct nbdkit_plugin plugin = {
   .name              = "file",
   .longname          = "nbdkit file plugin",
@@ -655,24 +166,10 @@ static struct nbdkit_plugin plugin = {
   .dump_plugin       = file_dump_plugin,
   .open              = file_open,
   .close             = file_close,
-  .get_size          = file_get_size,
   .can_multi_conn    = file_can_multi_conn,
-  .can_trim          = file_can_trim,
-  .can_fua           = file_can_fua,
-  .can_cache         = file_can_cache,
-  .pread             = file_pread,
-  .pwrite            = file_pwrite,
-  .flush             = file_flush,
-  .trim              = file_trim,
-  .zero              = file_zero,
-#ifdef SEEK_HOLE
-  .can_extents       = file_can_extents,
-  .extents           = file_extents,
-#endif
-#if HAVE_POSIX_FADVISE
-  .cache             = file_cache,
-#endif
-  .errno_is_preserved = 1,
+
+  /* This bulk of this plugin is implemented in common/fileops/fileops.c */
+  FILEOPS_READ_WRITE_CALLBACKS
 };
 
 NBDKIT_REGISTER_PLUGIN(plugin)
-- 
2.25.0

The plugin should now support pre-fetch and extents, although most ISO
images will be non-sparse so extents probably isn't that useful.
---
 plugins/iso/Makefile.am |  4 +-
 plugins/iso/iso.c       | 99 +++++++++++++++++++----------------------
 2 files changed, 48 insertions(+), 55 deletions(-)

diff --git a/plugins/iso/Makefile.am b/plugins/iso/Makefile.am
index a0fd337a..44ecbc8a 100644
--- a/plugins/iso/Makefile.am
+++ b/plugins/iso/Makefile.am
@@ -43,8 +43,9 @@ nbdkit_iso_plugin_la_SOURCES = \
 	$(NULL)
 
 nbdkit_iso_plugin_la_CPPFLAGS = \
-	-I$(top_srcdir)/common/utils \
 	-I$(top_srcdir)/include \
+	-I$(top_srcdir)/common/fileops \
+	-I$(top_srcdir)/common/utils \
 	-I. \
 	$(NULL)
 nbdkit_iso_plugin_la_CFLAGS = $(WARNINGS_CFLAGS)
@@ -53,6 +54,7 @@ nbdkit_iso_plugin_la_LDFLAGS = \
 	-Wl,--version-script=$(top_srcdir)/plugins/plugins.syms \
 	$(NULL)
 nbdkit_iso_plugin_la_LIBADD = \
+	$(top_builddir)/common/fileops/libfileops.la \
 	$(top_builddir)/common/utils/libutils.la \
 	$(NULL)
 
diff --git a/plugins/iso/iso.c b/plugins/iso/iso.c
index 92554ace..b8a4b5e3 100644
--- a/plugins/iso/iso.c
+++ b/plugins/iso/iso.c
@@ -39,9 +39,11 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 
+#define NBDKIT_API_VERSION 2
 #include <nbdkit-plugin.h>
 
 #include "cleanup.h"
+#include "fileops.h"
 #include "utils.h"
 
 /* List of directories parsed from the command line. */
@@ -57,7 +59,7 @@ static const char *isoprog = ISOPROG;
 static const char *params = NULL;
 
 /* The temporary ISO. */
-static int fd = -1;
+static int iso_fd = -1;
 
 /* Construct the temporary ISO. */
 static int
@@ -80,8 +82,8 @@ make_iso (void)
     return -1;
   }
 
-  fd = mkstemp (template);
-  if (fd == -1) {
+  iso_fd = mkstemp (template);
+  if (iso_fd == -1) {
     nbdkit_error ("mkstemp: %s: %m", template);
     return -1;
   }
@@ -103,7 +105,7 @@ make_iso (void)
     shell_quote (dirs[i], fp);
   }
   /* Redirect output to the temporary file. */
-  fprintf (fp, " >&%d", fd);
+  fprintf (fp, " >&%d", iso_fd);
 
   if (fclose (fp) == EOF) {
     nbdkit_error ("memstream failed: %m");
@@ -128,8 +130,8 @@ iso_unload (void)
     free (dirs[i]);
   free (dirs);
 
-  if (fd >= 0)
-    close (fd);
+  if (iso_fd >= 0)
+    close (iso_fd);
 }
 
 static int
@@ -193,25 +195,43 @@ iso_get_ready (void)
 static void *
 iso_open (int readonly)
 {
-  return NBDKIT_HANDLE_NOT_NEEDED;
+  struct fileops *fops;
+  int fd;
+
+  fops = malloc (sizeof *fops);
+  if (fops == NULL) {
+    nbdkit_error ("malloc: %m");
+    return NULL;
+  }
+
+  /* Copy the fd because fileops needs to have its own file descriptor. */
+  fd = dup (iso_fd);
+  if (fd == -1) {
+    nbdkit_error ("dup: %m");
+    free (fops);
+    return NULL;
+  }
+
+  if (init_fileops (fd, fops) == -1) {
+    free (fops);
+    return NULL;
+  }
+
+  return fops;
+}
+
+/* Free up the per-connection handle. */
+static void
+iso_close (void *handle)
+{
+  struct fileops *fops = handle;
+
+  close_fileops (fops);
+  free (fops);
 }
 
 #define THREAD_MODEL NBDKIT_THREAD_MODEL_PARALLEL
 
-/* Get the file size. */
-static int64_t
-iso_get_size (void *handle)
-{
-  struct stat statbuf;
-
-  if (fstat (fd, &statbuf) == -1) {
-    nbdkit_error ("fstat: %m");
-    return -1;
-  }
-
-  return statbuf.st_size;
-}
-
 /* Serves the same data over multiple connections. */
 static int
 iso_can_multi_conn (void *handle)
@@ -219,35 +239,6 @@ iso_can_multi_conn (void *handle)
   return 1;
 }
 
-static int
-iso_can_cache (void *handle)
-{
-  /* Let nbdkit call pread to populate the file system cache. */
-  return NBDKIT_CACHE_EMULATE;
-}
-
-/* Read data from the file. */
-static int
-iso_pread (void *handle, void *buf, uint32_t count, uint64_t offset)
-{
-  while (count > 0) {
-    ssize_t r = pread (fd, buf, count, offset);
-    if (r == -1) {
-      nbdkit_error ("pread: %m");
-      return -1;
-    }
-    if (r == 0) {
-      nbdkit_error ("pread: unexpected end of file");
-      return -1;
-    }
-    buf += r;
-    count -= r;
-    offset += r;
-  }
-
-  return 0;
-}
-
 static struct nbdkit_plugin plugin = {
   .name              = "iso",
   .longname          = "nbdkit iso plugin",
@@ -259,11 +250,11 @@ static struct nbdkit_plugin plugin = {
   .magic_config_key  = "dir",
   .get_ready         = iso_get_ready,
   .open              = iso_open,
-  .get_size          = iso_get_size,
+  .close             = iso_close,
   .can_multi_conn    = iso_can_multi_conn,
-  .can_cache         = iso_can_cache,
-  .pread             = iso_pread,
-  .errno_is_preserved = 1,
+
+  /* This bulk of this plugin is implemented in common/fileops/fileops.c */
+  FILEOPS_READ_ONLY_CALLBACKS
 };
 
 NBDKIT_REGISTER_PLUGIN(plugin)
-- 
2.25.0

This plugin now implements efficient zeroing, pre-fetch and extents,
which should give quite a performance boost.

XXX
On the flip side, it no longer ignores flush and FUA (which we can
ignore because these are temporary disks), which very likely has a
large negative impact on performance.  Fixing this would involve
generalising fileops a little.
---
 plugins/tmpdisk/Makefile.am |   2 +
 plugins/tmpdisk/tmpdisk.c   | 247 +++++-------------------------------
 2 files changed, 32 insertions(+), 217 deletions(-)

diff --git a/plugins/tmpdisk/Makefile.am b/plugins/tmpdisk/Makefile.am
index 20aeb05f..471a0f6a 100644
--- a/plugins/tmpdisk/Makefile.am
+++ b/plugins/tmpdisk/Makefile.am
@@ -57,6 +57,7 @@ nbdkit_tmpdisk_plugin_la_SOURCES = \
 
 nbdkit_tmpdisk_plugin_la_CPPFLAGS = \
 	-I$(top_srcdir)/include \
+	-I$(top_srcdir)/common/fileops \
 	-I$(top_srcdir)/common/utils \
 	$(NULL)
 nbdkit_tmpdisk_plugin_la_CFLAGS = $(WARNINGS_CFLAGS)
@@ -65,6 +66,7 @@ nbdkit_tmpdisk_plugin_la_LDFLAGS = \
 	-Wl,--version-script=$(top_srcdir)/plugins/plugins.syms \
 	$(NULL)
 nbdkit_tmpdisk_plugin_la_LIBADD = \
+	$(top_builddir)/common/fileops/libfileops.la \
 	$(top_builddir)/common/utils/libutils.la \
 	$(NULL)
 
diff --git a/plugins/tmpdisk/tmpdisk.c b/plugins/tmpdisk/tmpdisk.c
index 6d8efc08..23f335ac 100644
--- a/plugins/tmpdisk/tmpdisk.c
+++ b/plugins/tmpdisk/tmpdisk.c
@@ -43,13 +43,13 @@
 #include <errno.h>
 #include <assert.h>
 #include <sys/types.h>
-#include <sys/stat.h>
 #include <sys/wait.h>
 
 #define NBDKIT_API_VERSION 2
 #include <nbdkit-plugin.h>
 
 #include "cleanup.h"
+#include "fileops.h"
 #include "utils.h"
 
 static const char *tmpdir = "/var/tmp";
@@ -153,12 +153,6 @@ tmpdisk_config_complete (void)
   "type=ext4|...               The filesystem type.\n" \
   "command=<COMMAND>           Alternate command instead of
mkfs."
 
-struct handle {
-  int fd;
-  int64_t size;
-  bool can_punch_hole;
-};
-
 /* Multi-conn is absolutely unsafe!  In this callback it is simply
  * returning the default value (no multi-conn), that's to make it
  * clear for future authors.
@@ -169,33 +163,6 @@ tmpdisk_can_multi_conn (void *handle)
   return 0;
 }
 
-static int
-tmpdisk_can_trim (void *handle)
-{
-#ifdef FALLOC_FL_PUNCH_HOLE
-  return 1;
-#else
-  return 0;
-#endif
-}
-
-/* Pretend we have native FUA support, but actually because all disks
- * are temporary we will deliberately ignore flush/FUA operations.
- */
-static int
-tmpdisk_can_fua (void *handle)
-{
-  return NBDKIT_FUA_NATIVE;
-}
-
-static int64_t
-tmpdisk_get_size (void *handle)
-{
-  struct handle *h = handle;
-
-  return h->size;
-}
-
 /* This creates and runs the full "mkfs" (or whatever) command. */
 static int
 run_command (const char *disk)
@@ -263,37 +230,18 @@ run_command (const char *disk)
   return 0;
 }
 
-/* For block devices, stat->st_size is not the true size. */
-static int64_t
-block_device_size (int fd)
-{
-  off_t size;
-
-  size = lseek (fd, 0, SEEK_END);
-  if (size == -1) {
-    nbdkit_error ("lseek: %m");
-    return -1;
-  }
-
-  return size;
-}
-
 static void *
 tmpdisk_open (int readonly)
 {
-  struct handle *h;
+  struct fileops *fops;
   CLEANUP_FREE char *dir = NULL, *disk = NULL;
-  int flags;
-  struct stat statbuf;
+  int fd, flags;
 
-  h = malloc (sizeof *h);
-  if (h == NULL) {
+  fops = malloc (sizeof *fops);
+  if (fops == NULL) {
     nbdkit_error ("malloc: %m");
-    goto error;
+    return NULL;
   }
-  h->fd = -1;
-  h->size = -1;
-  h->can_punch_hole = true;
 
   /* For security reasons we have to create a temporary directory
    * under tmpdir that only the current user can access.  If we
@@ -303,20 +251,20 @@ tmpdisk_open (int readonly)
    */
   if (asprintf (&dir, "%s/tmpdiskXXXXXX", tmpdir) == -1) {
     nbdkit_error ("asprintf: %m");
-    goto error;
+    goto error0;
   }
   if (mkdtemp (dir) == NULL) {
     nbdkit_error ("%s: %m", dir);
-    goto error;
+    goto error0;
   }
   if (asprintf (&disk, "%s/disk", dir) == -1) {
     nbdkit_error ("asprintf: %m");
-    goto error;
+    goto error1;
   }
 
   /* Now run the mkfs command. */
   if (run_command (disk) == -1)
-    goto error;
+    goto error2;
 
   /* The external command must have created the disk, and then we must
    * find the true size.
@@ -325,29 +273,14 @@ tmpdisk_open (int readonly)
     flags = O_RDONLY | O_CLOEXEC;
   else
     flags = O_RDWR | O_CLOEXEC;
-  h->fd = open (disk, flags);
-  if (h->fd == -1) {
+  fd = open (disk, flags);
+  if (fd == -1) {
     nbdkit_error ("open: %s: %m", disk);
-    goto error;
+    goto error2;
   }
 
-  if (fstat (h->fd, &statbuf) == -1) {
-    nbdkit_error ("fstat: %s: %m", disk);
-    goto error;
-  }
-
-  /* The command could set $disk to a regular file or a block device
-   * (or a symlink to either), so we must check that here.
-   */
-  if (S_ISBLK (statbuf.st_mode)) {
-    h->size = block_device_size (h->fd);
-    if (h->size == -1)
-      goto error;
-  }
-  else                          /* Regular file. */
-    h->size = statbuf.st_size;
-  nbdkit_debug ("tmpdisk: requested_size = %" PRIi64 ", size =
%" PRIi64,
-                requested_size, h->size);
+  if (init_fileops (fd, fops) == -1)
+    goto error3;
 
   /* We don't need the disk to appear in the filesystem since we hold
    * a file descriptor and access it through that, so unlink the disk.
@@ -357,139 +290,26 @@ tmpdisk_open (int readonly)
   rmdir (dir);
 
   /* Return the handle. */
-  return h;
+  return fops;
 
- error:
-  if (h) {
-    if (h->fd >= 0) {
-      close (h->fd);
-      unlink (disk);
-    }
-    free (h);
-  }
+ error3:
+  close (fd);
+ error2:
+  unlink (disk);
+ error1:
+  rmdir (dir);
+ error0:
+  free (fops);
   return NULL;
 }
 
 static void
 tmpdisk_close (void *handle)
 {
-  struct handle *h = handle;
+  struct fileops *fops = handle;
 
-  close (h->fd);
-  free (h);
-}
-
-/* Read data from the file. */
-static int
-tmpdisk_pread (void *handle, void *buf,
-               uint32_t count, uint64_t offset,
-               uint32_t flags)
-{
-  struct handle *h = handle;
-
-  while (count > 0) {
-    ssize_t r = pread (h->fd, buf, count, offset);
-    if (r == -1) {
-      nbdkit_error ("pread: %m");
-      return -1;
-    }
-    if (r == 0) {
-      nbdkit_error ("pread: unexpected end of file");
-      return -1;
-    }
-    buf += r;
-    count -= r;
-    offset += r;
-  }
-
-  return 0;
-}
-
-/* Write data to the file. */
-static int
-tmpdisk_pwrite (void *handle, const void *buf,
-                uint32_t count, uint64_t offset,
-                uint32_t flags)
-{
-  struct handle *h = handle;
-
-  while (count > 0) {
-    ssize_t r = pwrite (h->fd, buf, count, offset);
-    if (r == -1) {
-      nbdkit_error ("pwrite: %m");
-      return -1;
-    }
-    buf += r;
-    count -= r;
-    offset += r;
-  }
-
-  /* Deliberately ignore FUA if present in flags. */
-
-  return 0;
-}
-
-/* This plugin deliberately provides a null flush operation, because
- * all of the disks created are temporary.
- */
-static int
-tmpdisk_flush (void *handle, uint32_t flags)
-{
-  return 0;
-}
-
-#if defined (FALLOC_FL_PUNCH_HOLE)
-static int
-do_fallocate (int fd, int mode, off_t offset, off_t len)
-{
-  int r = fallocate (fd, mode, offset, len);
-  if (r == -1 && errno == ENODEV) {
-    /* kernel 3.10 fails with ENODEV for block device. Kernel >= 4.9 fails
-     * with EOPNOTSUPP in this case. Normalize errno to simplify callers.
-     */
-    errno = EOPNOTSUPP;
-  }
-  return r;
-}
-
-static bool
-is_enotsup (int err)
-{
-  return err == ENOTSUP || err == EOPNOTSUPP;
-}
-#endif
-
-/* Punch a hole in the file. */
-static int
-tmpdisk_trim (void *handle, uint32_t count, uint64_t offset, uint32_t flags)
-{
-#ifdef FALLOC_FL_PUNCH_HOLE
-  struct handle *h = handle;
-  int r;
-
-  if (h->can_punch_hole) {
-    r = do_fallocate (h->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
-                      offset, count);
-    if (r == -1) {
-      /* Trim is advisory; we don't care if it fails for anything other
-       * than EIO or EPERM.
-       */
-      if (errno == EPERM || errno == EIO) {
-        nbdkit_error ("fallocate: %m");
-        return -1;
-      }
-
-      if (is_enotsup (EOPNOTSUPP))
-        h->can_punch_hole = false;
-
-      nbdkit_debug ("ignoring failed fallocate during trim: %m");
-    }
-  }
-#endif
-
-  /* Deliberately ignore FUA if present in flags. */
-
-  return 0;
+  close_fileops (fops);
+  free (fops);
 }
 
 #define THREAD_MODEL NBDKIT_THREAD_MODEL_PARALLEL
@@ -505,19 +325,12 @@ static struct nbdkit_plugin plugin = {
   .config_help       = tmpdisk_config_help,
   .magic_config_key  = "size",
 
-  .can_multi_conn    = tmpdisk_can_multi_conn,
-  .can_trim          = tmpdisk_can_trim,
-  .can_fua           = tmpdisk_can_fua,
-  .get_size          = tmpdisk_get_size,
-
   .open              = tmpdisk_open,
   .close             = tmpdisk_close,
-  .pread             = tmpdisk_pread,
-  .pwrite            = tmpdisk_pwrite,
-  .flush             = tmpdisk_flush,
-  .trim              = tmpdisk_trim,
+  .can_multi_conn    = tmpdisk_can_multi_conn,
 
-  .errno_is_preserved = 1,
+  /* Data serving is implemented in common/fileops/fileops.c */
+  FILEOPS_READ_WRITE_CALLBACKS
 };
 
 NBDKIT_REGISTER_PLUGIN(plugin)
-- 
2.25.0

On 4/9/20 3:36 AM, Richard W.M. Jones wrote:
> Writing "file-like" plugins is hard because you have to implement
your
> own .zero, .trim, .extents, etc, and that is very complicated.
> However implementations of these functions already exist in the file
> plugin.  By factoring out the file plugin into a separate
"fileops"
> mini-library we can reuse these implementations in other plugins.
> 
> This refactoring commit creates a new mini-library called fileops, and
> uses it to implement the file plugin.
> 
> Note that the name or prefix "file" leaks into fileops in a few
> places: the debug option is still called ‘-D file.zero=1’ and the
> nbdkit --dump-plugin output will still contain ‘file_blksszget’ etc.
> However I think that is fine as it should make usage more consistent
> across future file-like plugins.
Agreed.
> ---
>   configure.ac               |   1 +
>   Makefile.am                |   1 +
>   common/fileops/Makefile.am |  45 +++
>   plugins/file/Makefile.am   |   2 +
>   common/fileops/fileops.h   | 158 +++++++++++
>   common/fileops/fileops.c   | 547 +++++++++++++++++++++++++++++++++++++
>   plugins/file/file.c        | 541 ++----------------------------------
>   7 files changed, 773 insertions(+), 522 deletions(-)
> 
> +++ b/common/fileops/fileops.h
> +#ifndef NBDKIT_FILEOPS_H
> +#define NBDKIT_FILEOPS_H
> +
> +#include <config.h>
Unnecessary.  Since all of our .h files will be included by another .c 
file that has <config.h> as its first include, we don't need .h files
to
try the include a second time (even if it is idempotent).

I'll clean up the existing places where we don't need it, as a separate 
cleanup patch.
> +/* This mini-library helps when writing plugins which are like
> + * nbdkit-file-plugin.  It is also used to implement
> + * nbdkit-file-plugin itself.  What this means is if your plugin
> + * (after perhaps some custom setup) serves a single, whole, local
> + * file or block device then you can implement most of the data
> + * serving part of the plugin using these generic fileops.  The
> + * advantage is that this mini-library deals with the complexity of
> + * implementing callbacks such as .zero and .trim efficiently.
Even if your plugin serves the concatenation of several fds as if one 
single image, fileops may still prove useful; in those situations, 
you'll have to write your own callbacks that then call into specific 
fileops helper functions, rather than being able to use the convenience 
macro that directly sets callbacks to fileops functions.  But that can 
be a later patch; for now, this wording is fine.
> + *
> + * To use it:
> + *
> + * - your plugin must define and use NBDKIT_API_VERSION == 2
> + *
> + * - your plugin per-connection handle must either have type ‘struct
> + *   fileops *’, or include ‘struct fileops’ as the first member
> + *
> + * - add FILEOPS_READ_WRITE_CALLBACKS or
> + *   FILEOPS_READ_ONLY_CALLBACKS to your ‘struct nbdkit_plugin’
This is the step that gets dropped when using fileops for concatenated fds.
> + *
> + * - call init_fileops from your .open callback, and close_fileops
> + *   from your .close callback
> + *
> + * - optionally call fileops_dump_plugin from your .dump_plugin
> + *   callback if you have one
> + *
> + * - the fileops mini-library is linked statically into the plugin
> + *
> + * This mini-library is safe to use from any thread model, including
> + * NBDKIT_THREAD_MODEL_PARALLEL (in fact, parallel is the recommended
> + * thread model).
> + *
> + * See plugins/file/file.c for an example.
> + */
> +
> +
> +/* Initialize the fileops struct.  ‘fd’ is a file descriptor opened on
> + * the local file or block device that you want to serve.  Call this
> + * from your .open callback after allocating the handle and setting up
> + * the file descriptor.
> + */
> +extern int init_fileops (int fd, struct fileops *fops);
Mention that you must not access fd after this call succeeds.  Also, 
decide if that also applies to error situations (does this function 
guarantee to call close(fd) even when returning -1, or does it only take 
over fd on success?).
> +
> +/* Close the file descriptor and perform any other cleanup (but it
> + * doesn’t free the struct or handle).  Use this in your .close
> + * callback.
> + */
> +extern void close_fileops (struct fileops *fops);
> +
> +/* You may optionally define a .dump_plugin callback which calls this. */
> +extern void fileops_dump_plugin (void);
> +
> +/* Use the fileops callbacks by adding either
> + * FILEOPS_READ_WRITE_CALLBACKS or FILEOPS_READ_ONLY_CALLBACKS to your
> + * ‘struct nbdkit_plugin’.
> + */
> +#define FILEOPS_READ_WRITE_CALLBACKS            \
> +  .can_trim           = fileops_can_trim,       \
> +  .can_fua            = fileops_can_fua,        \
> +  .pwrite             = fileops_pwrite,         \
> +  .flush              = fileops_flush,          \
> +  .trim               = fileops_trim,           \
> +  .zero               = fileops_zero,           \
> +  FILEOPS_READ_ONLY_CALLBACKS
Someday, I hope to add .can_fast_zero support; but for now, this matches 
what the file plugin can do.
> +
> +#define FILEOPS_READ_ONLY_CALLBACKS             \
> +  .get_size           = fileops_get_size,       \
> +  .can_cache          = fileops_can_cache,      \
> +  .pread              = fileops_pread,          \
> +  .errno_is_preserved = 1,                      \
> +  FILEOPS_MAYBE_EXTENTS                         \
> +  FILEOPS_MAYBE_CACHE
> +
> +#ifdef SEEK_HOLE
> +#define FILEOPS_MAYBE_EXTENTS                   \
> +  .can_extents        = fileops_can_extents,    \
> +  .extents            = fileops_extents,
> +#else
> +#define FILEOPS_MAYBE_EXTENTS /* nothing */
> +#endif
> +
> +#ifdef HAVE_POSIX_FADVISE
> +#define FILEOPS_MAYBE_CACHE                     \
> +  .cache             = fileops_cache,
> +#else
> +#define FILEOPS_MAYBE_CACHE /* nothing */
> +#endif
> +
> +/* Don’t call these directly. */
Well, don't call them directly if you wrapped a single fd and used the 
macros above.  But for multi-fd clients, calling these after deciding 
which fd to forward to makes sense.  Or maybe we rename these slightly, 
and the versions we export for multi-fd plugins are type-safe with 
struct fileops* instead of void* for the first parameter.
> +extern int64_t fileops_get_size (void *handle);
> +extern int fileops_can_trim (void *handle);
> +++ b/common/fileops/fileops.c
> +
> +int
> +init_fileops (int fd, struct fileops *fops)
> +{
> +  struct stat statbuf;
> +
> +  if (fstat (fd, &statbuf) == -1) {
> +    nbdkit_error ("fstat: %m");
> +    return -1;
> +  }
This does not close fd on failure, but decide if that's the semantics 
you want when tweaking the documentation above.
> +void
> +close_fileops (struct fileops *fops)
> +{
> +  close (fops->fd);
> +}
Do you want to make sure this is safe to call even if init_fileops() 
failed?  If so, I'd recommend that init_fileops set 'fops->fd =
-1'
before returning failure, and that this check for -1 before trying close.
> +/* Get the file size. */
> +int64_t
> +fileops_get_size (void *handle)
> +{
> +  struct fileops *fops = handle;
> +
> +  if (fops->is_block_device) {
> +    ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&lseek_lock);
> +    return block_device_size (fops->fd);
> +  } else {
> +    /* Regular file. */
> +    struct stat statbuf;
> +
> +    if (fstat (fops->fd, &statbuf) == -1) {
This matches what file already did, but should we cache the file size we 
read during init_fileops, rather than having to do a second fstat()?
> +/* Write data to the file. */
> +int
> +fileops_pwrite (void *handle, const void *buf, uint32_t count, uint64_t
offset,
> +                uint32_t flags)
> +{
> +  struct fileops *fops = handle;
> +
> +  while (count > 0) {
> +    ssize_t r = pwrite (fops->fd, buf, count, offset);
> +    if (r == -1) {
> +      nbdkit_error ("pwrite: %m");
> +      return -1;
> +    }
> +    buf += r;
> +    count -= r;
> +    offset += r;
> +  }
> +
> +  if ((flags & NBDKIT_FLAG_FUA) && fileops_flush (handle, 0)
== -1)
> +    return -1;
> +
Yeah, reading ahead to patch 3, I can see parameterizing things (perhaps 
an additional bool parameter to init_fileops) to completely skip 
flushing, in cases where we know that flushing is pointless due to the 
temporary nature of the plugin.

> +
> +#ifdef SEEK_HOLE
> +/* Extents. */
> +
> +int
> +fileops_can_extents (void *handle)
Hmm - this declaration is conditional, and the macros only install the 
callback when the function is defined.  But is that safe enough, or 
should we always compile the entry point even the macros do not install 
it as a callback (where the #if moves inside the function body, with a 
sane fallback for the case where we don't support SEEK_HOLE).

> +#if HAVE_POSIX_FADVISE
> +/* Caching. */
> +int
> +fileops_cache (void *handle, uint32_t count, uint64_t offset, uint32_t
flags)
> +{
And similar.
> +++ b/plugins/file/file.c
> @@ -34,51 +34,18 @@
>   
>   #include <stdio.h>
>   #include <stdlib.h>
> -#include <stdbool.h>
> +#include <stdint.h>
Why the addition?
>   /* Create the per-connection handle. */
>   static void *
>   file_open (int readonly)
>   {
> -  struct handle *h;
> -  struct stat statbuf;
> -  int flags;
> +  struct fileops *fops;
> +  int fd, flags;
>   
> -  h = malloc (sizeof *h);
> -  if (h == NULL) {
> +  fops = malloc (sizeof *fops);
Uninitialized memory...
> +  if (fops == NULL) {
>       nbdkit_error ("malloc: %m");
>       return NULL;
>     }
> @@ -176,98 +120,33 @@ file_open (int readonly)
>     else
>       flags |= O_RDWR;
>   
> -  h->fd = open (filename, flags);
> -  if (h->fd == -1) {
> +  fd = open (filename, flags);
> +  if (fd == -1) {
>       nbdkit_error ("open: %s: %m", filename);
> -    free (h);
> +    free (fops);
>       return NULL;
>     }
>   
> -  if (fstat (h->fd, &statbuf) == -1) {
> -    nbdkit_error ("fstat: %s: %m", filename);
> -    free (h);
> +  if (init_fileops (fd, fops) == -1) {
> +    free (fops);
>       return NULL;
...good. init_fileops() initialized the memory on success, and we don't 
leak the memory on failure.  Bad - we leak fd if init_fileops() fails 
(where you fix it, there or here, determines what you document for the 
contract).
>   /* Allow multiple parallel connections from a single client. */
>   static int
>   file_can_multi_conn (void *handle)
The fileops docs probably ought to mention that .can_multi_conn is the 
responsibility of the plugin, as there is no sane answer that fileops 
can generalize.

>   static struct nbdkit_plugin plugin = {
>     .name              = "file",
>     .longname          = "nbdkit file plugin",
> @@ -655,24 +166,10 @@ static struct nbdkit_plugin plugin = {
>     .dump_plugin       = file_dump_plugin,
>     .open              = file_open,
>     .close             = file_close,
> -  .get_size          = file_get_size,
>     .can_multi_conn    = file_can_multi_conn,
> -  .can_trim          = file_can_trim,
> -  .can_fua           = file_can_fua,
> -  .can_cache         = file_can_cache,
> -  .pread             = file_pread,
> -  .pwrite            = file_pwrite,
> -  .flush             = file_flush,
> -  .trim              = file_trim,
> -  .zero              = file_zero,
> -#ifdef SEEK_HOLE
> -  .can_extents       = file_can_extents,
> -  .extents           = file_extents,
> -#endif
> -#if HAVE_POSIX_FADVISE
> -  .cache             = file_cache,
> -#endif
> -  .errno_is_preserved = 1,
> +
> +  /* This bulk of this plugin is implemented in common/fileops/fileops.c
*/
> +  FILEOPS_READ_WRITE_CALLBACKS
>   };
Overall, I like how this is heading.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

On 4/9/20 3:36 AM, Richard W.M. Jones wrote:
> The plugin should now support pre-fetch and extents, although most ISO
> images will be non-sparse so extents probably isn't that useful.
True, but it can't hurt ;)
> ---
>   plugins/iso/Makefile.am |  4 +-
>   plugins/iso/iso.c       | 99 +++++++++++++++++++----------------------
>   2 files changed, 48 insertions(+), 55 deletions(-)
> 
> @@ -193,25 +195,43 @@ iso_get_ready (void)
>   static void *
>   iso_open (int readonly)
>   {
> -  return NBDKIT_HANDLE_NOT_NEEDED;
> +  struct fileops *fops;
> +  int fd;
> +
> +  fops = malloc (sizeof *fops);
> +  if (fops == NULL) {
> +    nbdkit_error ("malloc: %m");
> +    return NULL;
> +  }
> +
> +  /* Copy the fd because fileops needs to have its own file descriptor. */
> +  fd = dup (iso_fd);
We use system(), but only during .get_ready().  I don't see any places 
where we fork a child after the first .open, so not setting FD_CLOEXEC 
here won't leak into the next client's connection (if we ever add later 
fork()s, we'd have to use fcntl(F_DUPFD_CLOEXEC) instead); but maybe a 
comment to that effect wouldn't hurt (since we already audited which fds 
need CLOEXEC atomically set, seeing the comment will speed up any re-audit).

You are changing from one fd shared among all clients to one fd per 
client, but I don't see that as being a severe problem (the file plugin 
was one fd per client).
>   static struct nbdkit_plugin plugin = {
>     .name              = "iso",
>     .longname          = "nbdkit iso plugin",
> @@ -259,11 +250,11 @@ static struct nbdkit_plugin plugin = {
>     .magic_config_key  = "dir",
>     .get_ready         = iso_get_ready,
>     .open              = iso_open,
> -  .get_size          = iso_get_size,
> +  .close             = iso_close,
>     .can_multi_conn    = iso_can_multi_conn,
Part of me wondered if .can_multi_conn be set for all read-only clients, 
but without making it mandatory on read-write clients.  But I'm fine 
with keeping it as something the plugin must do itself.

Looks good.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

On 4/9/20 3:36 AM, Richard W.M. Jones wrote:
> This plugin now implements efficient zeroing, pre-fetch and extents,
> which should give quite a performance boost.
> 
> XXX
> On the flip side, it no longer ignores flush and FUA (which we can
> ignore because these are temporary disks), which very likely has a
> large negative impact on performance.  Fixing this would involve
> generalising fileops a little.
Indeed. Maybe adding a 'bool ignore_flush' to init_fileops.
>   static void *
>   tmpdisk_open (int readonly)
>   {
> -  struct handle *h;
> +  struct fileops *fops;
> @@ -325,29 +273,14 @@ tmpdisk_open (int readonly)
>       flags = O_RDONLY | O_CLOEXEC;
>     else
>       flags = O_RDWR | O_CLOEXEC;
> -  h->fd = open (disk, flags);
> -  if (h->fd == -1) {
> +  fd = open (disk, flags);
> +  if (fd == -1) {
>       nbdkit_error ("open: %s: %m", disk);
> -    goto error;
> +    goto error2;
>     }
>   
> +  if (init_fileops (fd, fops) == -1)
> +    goto error3;
>   
>     /* We don't need the disk to appear in the filesystem since we hold
>      * a file descriptor and access it through that, so unlink the disk.
> @@ -357,139 +290,26 @@ tmpdisk_open (int readonly)
>     rmdir (dir);
>   
>     /* Return the handle. */
> -  return h;
> +  return fops;
>   
> - error:
> -  if (h) {
> -    if (h->fd >= 0) {
> -      close (h->fd);
> -      unlink (disk);
> -    }
> -    free (h);
> -  }
> + error3:
> +  close (fd);
Here, you are closing the fd; that's a double-close if you change the 
contract of init_fileops() on error.
> @@ -505,19 +325,12 @@ static struct nbdkit_plugin plugin = {
>     .config_help       = tmpdisk_config_help,
>     .magic_config_key  = "size",
>   
> -  .can_multi_conn    = tmpdisk_can_multi_conn,
> -  .can_trim          = tmpdisk_can_trim,
> -  .can_fua           = tmpdisk_can_fua,
> -  .get_size          = tmpdisk_get_size,
> -
>     .open              = tmpdisk_open,
>     .close             = tmpdisk_close,
> -  .pread             = tmpdisk_pread,
> -  .pwrite            = tmpdisk_pwrite,
> -  .flush             = tmpdisk_flush,
> -  .trim              = tmpdisk_trim,
> +  .can_multi_conn    = tmpdisk_can_multi_conn,
>   
> -  .errno_is_preserved = 1,
> +  /* Data serving is implemented in common/fileops/fileops.c */
> +  FILEOPS_READ_WRITE_CALLBACKS
>   };
>   
>   NBDKIT_REGISTER_PLUGIN(plugin)
> 
Otherwise it looks sane.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org