Libguestfs - May 2019 - [nbdkit PATCH 6/9] sh: Implement .cache script callback

I'm still working my way through the filters before this series will
be complete, but this is enough of a start to at least get some
feedback on the idea of implementing another NBD protocol extension.

Eric Blake (9):
  server: Internal hooks for implementing NBD_CMD_CACHE
  plugins: Add .cache callback
  file, split: Implement .cache with posix_fadvise
  nbd: Implement NBD_CMD_CACHE passthrough
  plugins: Implement no-op .cache for in-memory plugins
  sh: Implement .cache script callback
  filters: Add .cache callback
  test-layers: Test .cache usage
  blocksize: Implement .cache rounding

 docs/nbdkit-filter.pod          | 27 ++++++++++++++-
 docs/nbdkit-plugin.pod          | 52 ++++++++++++++++++++++++++++
 docs/nbdkit-protocol.pod        | 10 ++++++
 plugins/sh/nbdkit-sh-plugin.pod | 12 ++++++-
 configure.ac                    |  3 +-
 common/protocol/protocol.h      |  2 ++
 include/nbdkit-filter.h         |  8 +++++
 include/nbdkit-plugin.h         |  2 ++
 server/internal.h               |  4 +++
 filters/blocksize/blocksize.c   | 31 ++++++++++++++++-
 plugins/data/data.c             | 10 ++++++
 plugins/file/file.c             | 22 ++++++++++++
 plugins/full/full.c             | 12 ++++++-
 plugins/memory/memory.c         | 10 ++++++
 plugins/nbd/nbd.c               | 22 ++++++++++++
 plugins/null/null.c             | 12 ++++++-
 plugins/pattern/pattern.c       | 12 ++++++-
 plugins/random/random.c         | 12 ++++++-
 plugins/sh/sh.c                 | 40 ++++++++++++++++++++++
 plugins/split/split.c           | 37 +++++++++++++++++++-
 plugins/streaming/streaming.c   | 11 ++++++
 plugins/zero/zero.c             | 12 ++++++-
 server/filters.c                | 60 ++++++++++++++++++++++++++++++++-
 server/plugins.c                | 39 +++++++++++++++++++++
 server/protocol-handshake.c     | 10 +++++-
 server/protocol.c               | 25 ++++++++++++++
 tests/test-layers-filter.c      | 22 +++++++++++-
 tests/test-layers-plugin.c      | 17 ++++++++++
 tests/test-layers.c             | 36 ++++++++++++++++++++
 tests/test-eflags.sh            | 53 +++++++++++++++--------------
 30 files changed, 587 insertions(+), 38 deletions(-)

-- 
2.20.1

Until the next few patches expose and implement new callbacks for
filters and plugins, our initial implementation for NBD_CMD_CACHE is
to just blindly advertise the feature, and call into .pread with an
ignored buffer.

Note that for bisection reasons, this patch treats any use of a filter
as a forced .can_cache of false to bypass the filter's caching; once
all affected filters are patched to handle cache requests correctly, a
later patch will then switch the filter default to passthrough for the
sake of remaining filters.

Signed-off-by: Eric Blake <eblake@redhat.com>

---

Note: we could also choose to always advertise caching, but make the
nbdkit default version be a no-op rather than call out to .pread. For
that matter, maybe a plugin's .can_cache should allow the plugin to
choose between a tri-state of no-op, fallback to .pread, or call
.cache; I'm not sure which is the saner default for a random plugin
compiled against older nbdkit. At least it's fairly easy to write a
filter that changes the default from no-op to .pread or from .pread to
no-op. Be thinking about that as you review the rest of the series.
---
 docs/nbdkit-protocol.pod    | 10 +++++++
 common/protocol/protocol.h  |  2 ++
 server/internal.h           |  4 +++
 server/filters.c            | 33 ++++++++++++++++++++++-
 server/plugins.c            | 34 ++++++++++++++++++++++++
 server/protocol-handshake.c | 10 ++++++-
 server/protocol.c           | 25 +++++++++++++++++
 tests/test-eflags.sh        | 53 ++++++++++++++++++++-----------------
 8 files changed, 144 insertions(+), 27 deletions(-)

diff --git a/docs/nbdkit-protocol.pod b/docs/nbdkit-protocol.pod
index f706cfd..6b85c14 100644
--- a/docs/nbdkit-protocol.pod
+++ b/docs/nbdkit-protocol.pod
@@ -152,6 +152,16 @@ when structured replies are in effect. However, the flag is
a no-op
 until we extend the plugin API to allow a fragmented read in the first
 place.

+=item C<NBD_CMD_CACHE>
+
+Supported in nbdkit E<ge> 1.13.4.
+
+This protocol extension allows a client to inform the server about
+intent to access a portion of the export, to allow the server an
+opportunity to cache things appropriately. For plugins that do not
+support a particular form of caching, nbdkit performs pread then
+ignores the results.
+
 =item Resize Extension

 I<Not supported>.
diff --git a/common/protocol/protocol.h b/common/protocol/protocol.h
index c27104c..e938643 100644
--- a/common/protocol/protocol.h
+++ b/common/protocol/protocol.h
@@ -95,6 +95,7 @@ extern const char *name_of_nbd_flag (int);
 #define NBD_FLAG_SEND_WRITE_ZEROES (1 << 6)
 #define NBD_FLAG_SEND_DF           (1 << 7)
 #define NBD_FLAG_CAN_MULTI_CONN    (1 << 8)
+#define NBD_FLAG_SEND_CACHE        (1 << 10)

 /* NBD options (new style handshake only). */
 extern const char *name_of_nbd_opt (int);
@@ -217,6 +218,7 @@ extern const char *name_of_nbd_cmd (int);
 #define NBD_CMD_DISC              2 /* Disconnect. */
 #define NBD_CMD_FLUSH             3
 #define NBD_CMD_TRIM              4
+#define NBD_CMD_CACHE             5
 #define NBD_CMD_WRITE_ZEROES      6
 #define NBD_CMD_BLOCK_STATUS      7

diff --git a/server/internal.h b/server/internal.h
index 67fccfc..6414a78 100644
--- a/server/internal.h
+++ b/server/internal.h
@@ -170,6 +170,7 @@ struct connection {
   bool can_zero;
   bool can_fua;
   bool can_multi_conn;
+  bool can_cache;
   bool can_extents;
   bool using_tls;
   bool structured_replies;
@@ -276,6 +277,7 @@ struct backend {
   int (*can_extents) (struct backend *, struct connection *conn);
   int (*can_fua) (struct backend *, struct connection *conn);
   int (*can_multi_conn) (struct backend *, struct connection *conn);
+  int (*can_cache) (struct backend *, struct connection *conn);

   int (*pread) (struct backend *, struct connection *conn, void *buf,
                 uint32_t count, uint64_t offset, uint32_t flags, int *err);
@@ -290,6 +292,8 @@ struct backend {
   int (*extents) (struct backend *, struct connection *conn, uint32_t count,
                   uint64_t offset, uint32_t flags,
                   struct nbdkit_extents *extents, int *err);
+  int (*cache) (struct backend *, struct connection *conn, uint32_t count,
+                uint64_t offset, uint32_t flags, int *err);
 };

 /* plugins.c */
diff --git a/server/filters.c b/server/filters.c
index b73e74f..c619fd6 100644
--- a/server/filters.c
+++ b/server/filters.c
@@ -1,5 +1,5 @@
 /* nbdkit
- * Copyright (C) 2013-2018 Red Hat Inc.
+ * Copyright (C) 2013-2019 Red Hat Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
@@ -573,6 +573,18 @@ filter_can_multi_conn (struct backend *b, struct connection
*conn)
     return f->backend.next->can_multi_conn (f->backend.next, conn);
 }

+static int
+filter_can_cache (struct backend *b, struct connection *conn)
+{
+  struct backend_filter *f = container_of (b, struct backend_filter, backend);
+
+  debug ("%s: can_cache", f->name);
+
+  /* FIXME: Default to f->backend.next->can_cache, once all filters
+     have been audited */
+  return 0;
+}
+
 static int
 filter_pread (struct backend *b, struct connection *conn,
               void *buf, uint32_t count, uint64_t offset,
@@ -702,6 +714,23 @@ filter_extents (struct backend *b, struct connection *conn,
                                      extents, err);
 }

+static int
+filter_cache (struct backend *b, struct connection *conn,
+              uint32_t count, uint64_t offset,
+              uint32_t flags, int *err)
+{
+  struct backend_filter *f = container_of (b, struct backend_filter, backend);
+
+  assert (flags == 0);
+
+  debug ("%s: cache count=%" PRIu32 " offset=%" PRIu64
" flags=0x%" PRIx32,
+         f->name, count, offset, flags);
+
+  /* FIXME: Allow filter to rewrite request */
+  return f->backend.next->cache (f->backend.next, conn,
+                                 count, offset, flags, err);
+}
+
 static struct backend filter_functions = {
   .free = filter_free,
   .thread_model = filter_thread_model,
@@ -726,12 +755,14 @@ static struct backend filter_functions = {
   .can_extents = filter_can_extents,
   .can_fua = filter_can_fua,
   .can_multi_conn = filter_can_multi_conn,
+  .can_cache = filter_can_cache,
   .pread = filter_pread,
   .pwrite = filter_pwrite,
   .flush = filter_flush,
   .trim = filter_trim,
   .zero = filter_zero,
   .extents = filter_extents,
+  .cache = filter_cache,
 };

 /* Register and load a filter. */
diff --git a/server/plugins.c b/server/plugins.c
index e26d133..cabf543 100644
--- a/server/plugins.c
+++ b/server/plugins.c
@@ -448,6 +448,17 @@ plugin_can_multi_conn (struct backend *b, struct connection
*conn)
     return 0; /* assume false */
 }

+static int
+plugin_can_cache (struct backend *b, struct connection *conn)
+{
+  assert (connection_get_handle (conn, 0));
+
+  debug ("can_cache");
+
+  /* FIXME: return default based on plugin->cache */
+  return 0;
+}
+
 /* Plugins and filters can call this to set the true errno, in cases
  * where !errno_is_preserved.
  */
@@ -693,6 +704,27 @@ plugin_extents (struct backend *b, struct connection *conn,
   return r;
 }

+static int
+plugin_cache (struct backend *b, struct connection *conn,
+              uint32_t count, uint64_t offset, uint32_t flags,
+              int *err)
+{
+  struct backend_plugin *p = container_of (b, struct backend_plugin, backend);
+  int r = -1;
+
+  assert (connection_get_handle (conn, 0));
+  assert (!flags);
+
+  debug ("cache count=%" PRIu32 " offset=%" PRIu64, count,
offset);
+
+  /* FIXME: assert plugin->cache and call it */
+  assert (false);
+
+  if (r == -1)
+    *err = get_error (p);
+  return r;
+}
+
 static struct backend plugin_functions = {
   .free = plugin_free,
   .thread_model = plugin_thread_model,
@@ -717,12 +749,14 @@ static struct backend plugin_functions = {
   .can_extents = plugin_can_extents,
   .can_fua = plugin_can_fua,
   .can_multi_conn = plugin_can_multi_conn,
+  .can_cache = plugin_can_cache,
   .pread = plugin_pread,
   .pwrite = plugin_pwrite,
   .flush = plugin_flush,
   .trim = plugin_trim,
   .zero = plugin_zero,
   .extents = plugin_extents,
+  .cache = plugin_cache,
 };

 /* Register and load a plugin. */
diff --git a/server/protocol-handshake.c b/server/protocol-handshake.c
index 03377a9..af1cd18 100644
--- a/server/protocol-handshake.c
+++ b/server/protocol-handshake.c
@@ -109,7 +109,7 @@ protocol_compute_eflags (struct connection *conn, uint16_t
*flags)
     conn->can_multi_conn = true;
   }

-  /* The result of this is not returned to callers here (or at any
+  /* The results of the next two are not returned to callers here (or at any
    * time during the handshake).  However it makes sense to do it once
    * per connection and store the result in the handle anyway.  This
    * protocol_compute_eflags function is a bit misnamed XXX.
@@ -120,6 +120,14 @@ protocol_compute_eflags (struct connection *conn, uint16_t
*flags)
   if (fl)
     conn->can_extents = true;

+  fl = backend->can_cache (backend, conn);
+  if (fl == -1)
+    return -1;
+  if (fl)
+    conn->can_cache = true;
+  /* We emulate cache even when plugin doesn't support it */
+  eflags |= NBD_FLAG_SEND_CACHE;
+
   if (conn->structured_replies)
     eflags |= NBD_FLAG_SEND_DF;

diff --git a/server/protocol.c b/server/protocol.c
index 01d4c71..69227e5 100644
--- a/server/protocol.c
+++ b/server/protocol.c
@@ -76,6 +76,7 @@ validate_request (struct connection *conn,
   /* Validate cmd, offset, count. */
   switch (cmd) {
   case NBD_CMD_READ:
+  case NBD_CMD_CACHE:
   case NBD_CMD_WRITE:
   case NBD_CMD_TRIM:
   case NBD_CMD_WRITE_ZEROES:
@@ -254,6 +255,30 @@ handle_request (struct connection *conn,
       return err;
     break;

+  case NBD_CMD_CACHE:
+    /* The other backend methods don't check can_*.  That is because
+     * those methods are implicitly suppressed by returning fewer
+     * eflags to the client.  However the eflag for cache is always
+     * sent, because we emulate it here when the plugin lacks it.
+     */
+    if (conn->can_cache) {
+      if (backend->cache (backend, conn, count, offset, 0, &err) == -1)
+        return err;
+    }
+    else {
+      static char buf[MAX_REQUEST_SIZE]; /* data sink, never read */
+      uint32_t limit;
+
+      while (count) {
+        limit = MIN (count, sizeof buf);
+        if (backend->pread (backend, conn, buf, limit, offset, flags,
+                            &err) == -1)
+          return err;
+        count -= limit;
+      }
+    }
+    break;
+
   case NBD_CMD_WRITE_ZEROES:
     if (!(flags & NBD_CMD_FLAG_NO_HOLE))
       f |= NBDKIT_FLAG_MAY_TRIM;
diff --git a/tests/test-eflags.sh b/tests/test-eflags.sh
index 84dcfe8..0234aca 100755
--- a/tests/test-eflags.sh
+++ b/tests/test-eflags.sh
@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 # nbdkit
-# Copyright (C) 2018 Red Hat Inc.
+# Copyright (C) 2018-2019 Red Hat Inc.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
@@ -94,8 +94,8 @@ case "$1" in
 esac
 EOF

-[ $eflags -eq $(( HAS_FLAGS|READ_ONLY|SEND_DF )) ] ||
-    fail "expected HAS_FLAGS|READ_ONLY|SEND_DF"
+[ $eflags -eq $(( HAS_FLAGS|READ_ONLY|SEND_DF|SEND_CACHE )) ] ||
+    fail "expected HAS_FLAGS|READ_ONLY|SEND_DF|SEND_CACHE"

 #----------------------------------------------------------------------
 # -r
@@ -108,8 +108,8 @@ case "$1" in
 esac
 EOF

-[ $eflags -eq $(( HAS_FLAGS|READ_ONLY|SEND_DF )) ] ||
-    fail "expected HAS_FLAGS|READ_ONLY|SEND_DF"
+[ $eflags -eq $(( HAS_FLAGS|READ_ONLY|SEND_DF|SEND_CACHE )) ] ||
+    fail "expected HAS_FLAGS|READ_ONLY|SEND_DF|SEND_CACHE"

 #----------------------------------------------------------------------
 # can_write=true
@@ -126,8 +126,8 @@ case "$1" in
 esac
 EOF

-[ $eflags -eq $(( HAS_FLAGS|SEND_WRITE_ZEROES|SEND_DF )) ] ||
-    fail "expected HAS_FLAGS|SEND_WRITE_ZEROES|SEND_DF"
+[ $eflags -eq $(( HAS_FLAGS|SEND_WRITE_ZEROES|SEND_DF|SEND_CACHE )) ] ||
+    fail "expected HAS_FLAGS|SEND_WRITE_ZEROES|SEND_DF|SEND_CACHE"

 #----------------------------------------------------------------------
 # -r
@@ -144,8 +144,8 @@ case "$1" in
 esac
 EOF

-[ $eflags -eq $(( HAS_FLAGS|READ_ONLY|SEND_DF )) ] ||
-    fail "expected HAS_FLAGS|READ_ONLY|SEND_DF"
+[ $eflags -eq $(( HAS_FLAGS|READ_ONLY|SEND_DF|SEND_CACHE )) ] ||
+    fail "expected HAS_FLAGS|READ_ONLY|SEND_DF|SEND_CACHE"

 #----------------------------------------------------------------------
 # can_write=false
@@ -164,8 +164,8 @@ case "$1" in
 esac
 EOF

-[ $eflags -eq $(( HAS_FLAGS|READ_ONLY|SEND_DF )) ] ||
-    fail "expected HAS_FLAGS|READ_ONLY|SEND_DF"
+[ $eflags -eq $(( HAS_FLAGS|READ_ONLY|SEND_DF|SEND_CACHE )) ] ||
+    fail "expected HAS_FLAGS|READ_ONLY|SEND_DF|SEND_CACHE"

 #----------------------------------------------------------------------
 # -r
@@ -185,8 +185,8 @@ case "$1" in
 esac
 EOF

-[ $eflags -eq $(( HAS_FLAGS|READ_ONLY|SEND_DF )) ] ||
-    fail "expected HAS_FLAGS|READ_ONLY|SEND_DF"
+[ $eflags -eq $(( HAS_FLAGS|READ_ONLY|SEND_DF|SEND_CACHE )) ] ||
+    fail "expected HAS_FLAGS|READ_ONLY|SEND_DF|SEND_CACHE"

 #----------------------------------------------------------------------
 # can_write=true
@@ -201,8 +201,8 @@ case "$1" in
 esac
 EOF

-[ $eflags -eq $(( HAS_FLAGS|SEND_TRIM|SEND_WRITE_ZEROES|SEND_DF )) ] ||
-    fail "expected HAS_FLAGS|SEND_TRIM|SEND_WRITE_ZEROES|SEND_DF"
+[ $eflags -eq $(( HAS_FLAGS|SEND_TRIM|SEND_WRITE_ZEROES|SEND_DF|SEND_CACHE )) ]
||
+    fail "expected
HAS_FLAGS|SEND_TRIM|SEND_WRITE_ZEROES|SEND_DF|SEND_CACHE"

 #----------------------------------------------------------------------
 # can_write=true
@@ -217,8 +217,8 @@ case "$1" in
 esac
 EOF

-[ $eflags -eq $(( HAS_FLAGS|ROTATIONAL|SEND_WRITE_ZEROES|SEND_DF )) ] ||
-    fail "expected HAS_FLAGS|ROTATIONAL|SEND_WRITE_ZEROES|SEND_DF"
+[ $eflags -eq $(( HAS_FLAGS|ROTATIONAL|SEND_WRITE_ZEROES|SEND_DF|SEND_CACHE ))
] ||
+    fail "expected
HAS_FLAGS|ROTATIONAL|SEND_WRITE_ZEROES|SEND_DF|SEND_CACHE"

 #----------------------------------------------------------------------
 # -r
@@ -234,8 +234,8 @@ case "$1" in
 esac
 EOF

-[ $eflags -eq $(( HAS_FLAGS|READ_ONLY|ROTATIONAL|SEND_DF )) ] ||
-    fail "expected HAS_FLAGS|READ_ONLY|ROTATIONAL|SEND_DF"
+[ $eflags -eq $(( HAS_FLAGS|READ_ONLY|ROTATIONAL|SEND_DF|SEND_CACHE )) ] ||
+    fail "expected HAS_FLAGS|READ_ONLY|ROTATIONAL|SEND_DF|SEND_CACHE"

 #----------------------------------------------------------------------
 # can_write=true
@@ -250,8 +250,8 @@ case "$1" in
 esac
 EOF

-[ $eflags -eq $(( HAS_FLAGS|SEND_FUA|SEND_WRITE_ZEROES|SEND_DF )) ] ||
-    fail "expected HAS_FLAGS|SEND_FUA|SEND_WRITE_ZEROES|SEND_DF"
+[ $eflags -eq $(( HAS_FLAGS|SEND_FUA|SEND_WRITE_ZEROES|SEND_DF|SEND_CACHE )) ]
||
+    fail "expected
HAS_FLAGS|SEND_FUA|SEND_WRITE_ZEROES|SEND_DF|SEND_CACHE"

 #----------------------------------------------------------------------
 # -r
@@ -269,8 +269,8 @@ case "$1" in
 esac
 EOF

-[ $eflags -eq $(( HAS_FLAGS|READ_ONLY|SEND_DF )) ] ||
-    fail "expected HAS_FLAGS|READ_ONLY|SEND_DF"
+[ $eflags -eq $(( HAS_FLAGS|READ_ONLY|SEND_DF|SEND_CACHE )) ] ||
+    fail "expected HAS_FLAGS|READ_ONLY|SEND_DF|SEND_CACHE"

 #----------------------------------------------------------------------
 # -r
@@ -284,5 +284,8 @@ case "$1" in
 esac
 EOF

-[ $eflags -eq $(( HAS_FLAGS|READ_ONLY|SEND_DF|CAN_MULTI_CONN )) ] ||
-    fail "expected HAS_FLAGS|READ_ONLY|SEND_DF|CAN_MULTI_CONN"
+[ $eflags -eq $(( HAS_FLAGS|READ_ONLY|SEND_DF|CAN_MULTI_CONN|SEND_CACHE )) ] ||
+    fail "expected
HAS_FLAGS|READ_ONLY|SEND_DF|CAN_MULTI_CONN|SEND_CACHE"
+
+# NBD_FLAG_SEND_CACHE is always set even if can_cache returns false, because
+# nbdkit reckons it can emulate caching using pread.
-- 
2.20.1

Make it possible for plugins to provide an alternative behavior for
NBD_CMD_CACHE rather than just discarding the buffer of .pread.

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 docs/nbdkit-plugin.pod  | 52 +++++++++++++++++++++++++++++++++++++++++
 include/nbdkit-plugin.h |  2 ++
 server/plugins.c        | 17 +++++++++-----
 3 files changed, 65 insertions(+), 6 deletions(-)

diff --git a/docs/nbdkit-plugin.pod b/docs/nbdkit-plugin.pod
index 318be4c..0657151 100644
--- a/docs/nbdkit-plugin.pod
+++ b/docs/nbdkit-plugin.pod
@@ -620,6 +620,31 @@ with an error message and return C<-1>.

 This callback is not required.  If omitted, then we return false.

+=head2 C<.can_cache>
+
+ int can_cache (void *handle);
+
+This is called during the option negotiation phase to find out if the
+plugin supports a cache operation. The nature of the caching is
+unspecified (including whether there are limits on how much can be
+cached at once, and whether writes to a cached region have
+write-through or write-back semantics), but the command exists to let
+clients issue a hint to the server that they will be accessing that
+region of the export.
+
+If there is an error, C<.can_cache> should call C<nbdkit_error>
with
+an error message and return C<-1>.  Since the NBD protocol has
+reserved the right to add future flags to mandate specific caching
+capabilities, this function should only return C<1> on success (in the
+future, other positive results may be treated as a bitmask of
+supported flags).
+
+This callback is not required.  If omitted, then we return true iff a
+C<.cache> callback has been defined. Note that nbdkit always
+advertises caching capabilities to the client regardless of this
+callback; if this callback returns false, nbdkit handles a client
+cache request by calling C<.pread> and ignoring the resulting data.
+
 =head2 C<.pread>

  int pread (void *handle, void *buf, uint32_t count, uint64_t offset,
@@ -814,6 +839,33 @@ C<nbdkit_extent_add> returns C<0> on success or
C<-1> on failure.  On
 failure C<nbdkit_error> and/or C<nbdkit_set_error> has already been
 called.  C<errno> will be set to a suitable value.

+=head2 C<.cache>
+
+ int cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags);
+
+During the data serving phase, this callback is used to give the
+plugin a hint that the client intends to make further accesses to the
+given region of the export.  The nature of caching is not specified
+further by the NBD specification (for example, a server may place
+limits on how much may be cached at once, and there is no way to
+control if writes to a cached area have write-through or write-back
+semantics).  In fact, the cache command can always fail and still be
+compliant, and success might not guarantee a performance gain.  If
+this callback is omitted, nbdkit defaults to providing cache semantics
+obtained by calling C<.pread> over the same region and ignoring the
+results.
+
+This function will not be called if C<.can_cache> returned false.  The
+parameter C<flags> exists in case of future NBD protocol extensions;
+at this time, it will be 0 on input. A plugin must fail this function
+if C<flags> includes an unrecognized flag, as that may indicate a
+requirement that the plugin comply must with a specific caching
+semantic.
+
+If there is an error, C<.cache> should call C<nbdkit_error> with an
+error message, and C<nbdkit_set_error> to record an appropriate error
+(unless C<errno> is sufficient), then return C<-1>.
+
 =head1 THREADS

 Each nbdkit plugin must declare its thread safety model by defining
diff --git a/include/nbdkit-plugin.h b/include/nbdkit-plugin.h
index 54b4ce2..e9b1808 100644
--- a/include/nbdkit-plugin.h
+++ b/include/nbdkit-plugin.h
@@ -128,6 +128,8 @@ struct nbdkit_plugin {
   int (*can_extents) (void *handle);
   int (*extents) (void *handle, uint32_t count, uint64_t offset, uint32_t
flags,
                   struct nbdkit_extents *extents);
+  int (*can_cache) (void *handle);
+  int (*cache) (void *handle, uint32_t count, uint64_t offset, uint32_t flags);
 };

 extern void nbdkit_set_error (int err);
diff --git a/server/plugins.c b/server/plugins.c
index cabf543..947fe79 100644
--- a/server/plugins.c
+++ b/server/plugins.c
@@ -201,6 +201,8 @@ plugin_dump_fields (struct backend *b)
   HAS (can_multi_conn);
   HAS (can_extents);
   HAS (extents);
+  HAS (can_cache);
+  HAS (cache);
 #undef HAS

   /* Custom fields. */
@@ -451,12 +453,16 @@ plugin_can_multi_conn (struct backend *b, struct
connection *conn)
 static int
 plugin_can_cache (struct backend *b, struct connection *conn)
 {
+  struct backend_plugin *p = container_of (b, struct backend_plugin, backend);
+
   assert (connection_get_handle (conn, 0));

   debug ("can_cache");

-  /* FIXME: return default based on plugin->cache */
-  return 0;
+  if (p->plugin.can_cache)
+    return p->plugin.can_cache (connection_get_handle (conn, 0));
+  else
+    return p->plugin.cache != NULL;
 }

 /* Plugins and filters can call this to set the true errno, in cases
@@ -710,16 +716,15 @@ plugin_cache (struct backend *b, struct connection *conn,
               int *err)
 {
   struct backend_plugin *p = container_of (b, struct backend_plugin, backend);
-  int r = -1;
+  int r;

   assert (connection_get_handle (conn, 0));
   assert (!flags);
+  assert (p->plugin.cache);

   debug ("cache count=%" PRIu32 " offset=%" PRIu64, count,
offset);

-  /* FIXME: assert plugin->cache and call it */
-  assert (false);
-
+  r = p->plugin.cache (connection_get_handle (conn, 0), count, offset,
flags);
   if (r == -1)
     *err = get_error (p);
   return r;
-- 
2.20.1

Since NBD_CMD_CACHE is already advisory, let's use an advisory kernel
interface to implement it ;) When posix_fadvise() is not present, it
is likely that nbdkit's fallback to .pread will actually have a
similar benefit in populating the filesystem cache, since we aren't
using O_DIRECT to avoid that cache.

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 configure.ac          |  3 ++-
 plugins/file/file.c   | 22 ++++++++++++++++++++++
 plugins/split/split.c | 37 ++++++++++++++++++++++++++++++++++++-
 3 files changed, 60 insertions(+), 2 deletions(-)

diff --git a/configure.ac b/configure.ac
index 58031f3..06124c5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -195,7 +195,8 @@ dnl Check for functions in libc, all optional.
 AC_CHECK_FUNCS([\
 	fdatasync \
 	get_current_dir_name \
-	mkostemp])
+	mkostemp \
+	posix_fadvise])

 dnl Check whether printf("%m") works
 AC_CACHE_CHECK([whether the printf family supports %m],
diff --git a/plugins/file/file.c b/plugins/file/file.c
index f0ac23b..85b033b 100644
--- a/plugins/file/file.c
+++ b/plugins/file/file.c
@@ -608,6 +608,25 @@ file_extents (void *handle, uint32_t count, uint64_t
offset,
 }
 #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",
@@ -632,6 +651,9 @@ static struct nbdkit_plugin plugin = {
 #ifdef SEEK_HOLE
   .can_extents       = file_can_extents,
   .extents           = file_extents,
+#endif
+#if HAVE_POSIX_FADVISE
+  .cache             = file_cache,
 #endif
   .errno_is_preserved = 1,
 };
diff --git a/plugins/split/split.c b/plugins/split/split.c
index cf2b2c7..cef2d51 100644
--- a/plugins/split/split.c
+++ b/plugins/split/split.c
@@ -1,5 +1,5 @@
 /* nbdkit
- * Copyright (C) 2017-2018 Red Hat Inc.
+ * Copyright (C) 2017-2019 Red Hat Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
@@ -277,6 +277,38 @@ split_pwrite (void *handle, const void *buf, uint32_t
count, uint64_t offset)
   return 0;
 }

+#if HAVE_POSIX_FADVISE
+/* Caching. */
+static int
+split_cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags)
+{
+  struct handle *h = handle;
+
+  /* Cache is advisory, we don't care if this fails */
+  while (count > 0) {
+    struct file *file = get_file (h, offset);
+    uint64_t foffs = offset - file->offset;
+    uint64_t max;
+    int r;
+
+    max = file->size - foffs;
+    if (max > count)
+      max = count;
+
+    r = posix_fadvise (file->fd, offset, max, POSIX_FADV_WILLNEED);
+    if (r) {
+      errno = r;
+      nbdkit_error ("posix_fadvise: %m");
+      return -1;
+    }
+    count -= r;
+    offset += r;
+  }
+
+  return 0;
+}
+#endif /* HAVE_POSIX_FADVISE */
+
 static struct nbdkit_plugin plugin = {
   .name              = "split",
   .version           = PACKAGE_VERSION,
@@ -289,6 +321,9 @@ static struct nbdkit_plugin plugin = {
   .get_size          = split_get_size,
   .pread             = split_pread,
   .pwrite            = split_pwrite,
+#if HAVE_POSIX_FADVISE
+  .cache             = split_cache,
+#endif
   /* In this plugin, errno is preserved properly along error return
    * paths from failed system calls.
    */
-- 
2.20.1

In the nbd plugin, if the remote server supports any form of caching,
we should utilize that rather than nbdkit's fallback to .pread.

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 plugins/nbd/nbd.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/plugins/nbd/nbd.c b/plugins/nbd/nbd.c
index 821f256..71549d7 100644
--- a/plugins/nbd/nbd.c
+++ b/plugins/nbd/nbd.c
@@ -1151,6 +1151,14 @@ nbd_can_multi_conn (void *handle)
   return h->flags & NBD_FLAG_CAN_MULTI_CONN;
 }

+static int
+nbd_can_cache (void *handle)
+{
+  struct handle *h = handle;
+
+  return h->flags & NBD_FLAG_SEND_CACHE;
+}
+
 static int
 nbd_can_extents (void *handle)
 {
@@ -1245,6 +1253,18 @@ nbd_extents (void *handle, uint32_t count, uint64_t
offset,
   return c < 0 ? c : nbd_reply (h, c);
 }

+/* Cache a portion of the file. */
+static int
+nbd_cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags)
+{
+  struct handle *h = handle;
+  int c;
+
+  assert (!flags);
+  c = nbd_request (h, 0, NBD_CMD_CACHE, offset, count);
+  return c < 0 ? c : nbd_reply (h, c);
+}
+
 static struct nbdkit_plugin plugin = {
   .name               = "nbd",
   .longname           = "nbdkit nbd plugin",
@@ -1264,12 +1284,14 @@ static struct nbdkit_plugin plugin = {
   .can_fua            = nbd_can_fua,
   .can_multi_conn     = nbd_can_multi_conn,
   .can_extents        = nbd_can_extents,
+  .can_cache          = nbd_can_cache,
   .pread              = nbd_pread,
   .pwrite             = nbd_pwrite,
   .zero               = nbd_zero,
   .flush              = nbd_flush,
   .trim               = nbd_trim,
   .extents            = nbd_extents,
+  .cache              = nbd_cache,
   .errno_is_preserved = 1,
 };

-- 
2.20.1

For our plugins which have no backing file but generate everything on
the fly or store things in memory, falling back to .pread on a cache
request is just wasted work. Implement a no-op .cache callback for the
drivers where there is no benefit to trying to cache anything.

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 plugins/data/data.c           | 10 ++++++++++
 plugins/full/full.c           | 12 +++++++++++-
 plugins/memory/memory.c       | 10 ++++++++++
 plugins/null/null.c           | 12 +++++++++++-
 plugins/pattern/pattern.c     | 12 +++++++++++-
 plugins/random/random.c       | 12 +++++++++++-
 plugins/streaming/streaming.c | 11 +++++++++++
 plugins/zero/zero.c           | 12 +++++++++++-
 8 files changed, 86 insertions(+), 5 deletions(-)

diff --git a/plugins/data/data.c b/plugins/data/data.c
index 55380c6..aaa3d2d 100644
--- a/plugins/data/data.c
+++ b/plugins/data/data.c
@@ -377,6 +377,15 @@ data_extents (void *handle, uint32_t count, uint64_t
offset,
   return sparse_array_extents (sa, count, offset, extents);
 }

+/* Cache. */
+static int
+data_cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags)
+{
+  /* Everything is already in memory, falling back to .pread is
+     actually slower than treating this as a no-op. */
+  return 0;
+}
+
 static struct nbdkit_plugin plugin = {
   .name              = "data",
   .version           = PACKAGE_VERSION,
@@ -394,6 +403,7 @@ static struct nbdkit_plugin plugin = {
   .zero              = data_zero,
   .trim              = data_trim,
   .extents           = data_extents,
+  .cache             = data_cache,
   /* In this plugin, errno is preserved properly along error return
    * paths from failed system calls.
    */
diff --git a/plugins/full/full.c b/plugins/full/full.c
index 7661856..51a9d67 100644
--- a/plugins/full/full.c
+++ b/plugins/full/full.c
@@ -1,5 +1,5 @@
 /* nbdkit
- * Copyright (C) 2017-2018 Red Hat Inc.
+ * Copyright (C) 2017-2019 Red Hat Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
@@ -137,6 +137,15 @@ full_extents (void *handle, uint32_t count, uint64_t
offset, uint32_t flags,
                             NBDKIT_EXTENT_HOLE | NBDKIT_EXTENT_ZERO);
 }

+/* Cache. */
+static int
+full_cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags)
+{
+  /* Everything is already in memory, falling back to .pread is
+     actually slower than treating this as a no-op. */
+  return 0;
+}
+
 /* Note that we don't need to handle flush: If there has been previous
  * write then we have already returned an error.  If there have been
  * no previous writes then flush can be ignored.
@@ -156,6 +165,7 @@ static struct nbdkit_plugin plugin = {
   .zero              = full_zero,
   .trim              = full_trim,
   .extents           = full_extents,
+  .cache             = full_cache,
   /* In this plugin, errno is preserved properly along error return
    * paths from failed system calls.
    */
diff --git a/plugins/memory/memory.c b/plugins/memory/memory.c
index 90fa99e..baa29e2 100644
--- a/plugins/memory/memory.c
+++ b/plugins/memory/memory.c
@@ -172,6 +172,15 @@ memory_extents (void *handle, uint32_t count, uint64_t
offset,
   return sparse_array_extents (sa, count, offset, extents);
 }

+/* Cache. */
+static int
+memory_cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags)
+{
+  /* Everything is already in memory, falling back to .pread is
+     actually slower than treating this as a no-op. */
+  return 0;
+}
+
 static struct nbdkit_plugin plugin = {
   .name              = "memory",
   .version           = PACKAGE_VERSION,
@@ -189,6 +198,7 @@ static struct nbdkit_plugin plugin = {
   .zero              = memory_zero,
   .trim              = memory_trim,
   .extents           = memory_extents,
+  .cache             = memory_cache,
   /* In this plugin, errno is preserved properly along error return
    * paths from failed system calls.
    */
diff --git a/plugins/null/null.c b/plugins/null/null.c
index 518b63b..5e40868 100644
--- a/plugins/null/null.c
+++ b/plugins/null/null.c
@@ -1,5 +1,5 @@
 /* nbdkit
- * Copyright (C) 2017-2018 Red Hat Inc.
+ * Copyright (C) 2017-2019 Red Hat Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
@@ -140,6 +140,15 @@ null_extents (void *handle, uint32_t count, uint64_t
offset, uint32_t flags,
                             NBDKIT_EXTENT_HOLE | NBDKIT_EXTENT_ZERO);
 }

+/* Cache. */
+static int
+null_cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags)
+{
+  /* Everything is already in memory, falling back to .pread is
+     actually slower than treating this as a no-op. */
+  return 0;
+}
+
 static struct nbdkit_plugin plugin = {
   .name              = "null",
   .version           = PACKAGE_VERSION,
@@ -155,6 +164,7 @@ static struct nbdkit_plugin plugin = {
   .can_fua           = null_can_fua,
   .flush             = null_flush,
   .extents           = null_extents,
+  .cache             = null_cache,
   /* In this plugin, errno is preserved properly along error return
    * paths from failed system calls.
    */
diff --git a/plugins/pattern/pattern.c b/plugins/pattern/pattern.c
index 115bd96..e25da36 100644
--- a/plugins/pattern/pattern.c
+++ b/plugins/pattern/pattern.c
@@ -1,5 +1,5 @@
 /* nbdkit
- * Copyright (C) 2017-2018 Red Hat Inc.
+ * Copyright (C) 2017-2019 Red Hat Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
@@ -117,6 +117,15 @@ pattern_pread (void *handle, void *buf, uint32_t count,
uint64_t offset,
   return 0;
 }

+/* Cache. */
+static int
+pattern_cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags)
+{
+  /* Everything is already in memory, falling back to .pread is
+     actually slower than treating this as a no-op. */
+  return 0;
+}
+
 static struct nbdkit_plugin plugin = {
   .name              = "pattern",
   .version           = PACKAGE_VERSION,
@@ -127,6 +136,7 @@ static struct nbdkit_plugin plugin = {
   .get_size          = pattern_get_size,
   .can_multi_conn    = pattern_can_multi_conn,
   .pread             = pattern_pread,
+  .cache             = pattern_cache,
   /* In this plugin, errno is preserved properly along error return
    * paths from failed system calls.
    */
diff --git a/plugins/random/random.c b/plugins/random/random.c
index 7fb42c8..6219169 100644
--- a/plugins/random/random.c
+++ b/plugins/random/random.c
@@ -1,5 +1,5 @@
 /* nbdkit
- * Copyright (C) 2017-2018 Red Hat Inc.
+ * Copyright (C) 2017-2019 Red Hat Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
@@ -146,6 +146,15 @@ random_pread (void *handle, void *buf, uint32_t count,
uint64_t offset,
   return 0;
 }

+/* Cache. */
+static int
+random_cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags)
+{
+  /* Everything is already in memory, falling back to .pread is
+     actually slower than treating this as a no-op. */
+  return 0;
+}
+
 static struct nbdkit_plugin plugin = {
   .name              = "random",
   .version           = PACKAGE_VERSION,
@@ -157,6 +166,7 @@ static struct nbdkit_plugin plugin = {
   .get_size          = random_get_size,
   .can_multi_conn    = random_can_multi_conn,
   .pread             = random_pread,
+  .cache             = random_cache,
   /* In this plugin, errno is preserved properly along error return
    * paths from failed system calls.
    */
diff --git a/plugins/streaming/streaming.c b/plugins/streaming/streaming.c
index 4ca3e76..505ed91 100644
--- a/plugins/streaming/streaming.c
+++ b/plugins/streaming/streaming.c
@@ -246,6 +246,16 @@ streaming_pread (void *handle, void *buf, uint32_t count,
uint64_t offset)
   return -1;
 }

+/* Cache. */
+static int
+streaming_cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags)
+{
+  /* nbdkit's default of falling back to .pread is pointless: it will
+   * fail for addresses already written, and waste a memset() for
+   * addresses not yet reached. Treat this as a no-op instead. */
+  return 0;
+}
+
 static struct nbdkit_plugin plugin = {
   .name              = "streaming",
   .longname          = "nbdkit streaming plugin",
@@ -259,6 +269,7 @@ static struct nbdkit_plugin plugin = {
   .get_size          = streaming_get_size,
   .pwrite            = streaming_pwrite,
   .pread             = streaming_pread,
+  .cache             = streaming_cache,
   .errno_is_preserved = 1,
 };

diff --git a/plugins/zero/zero.c b/plugins/zero/zero.c
index 49ce08e..be8fc2a 100644
--- a/plugins/zero/zero.c
+++ b/plugins/zero/zero.c
@@ -1,5 +1,5 @@
 /* nbdkit
- * Copyright (C) 2017-2018 Red Hat Inc.
+ * Copyright (C) 2017-2019 Red Hat Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
@@ -76,6 +76,15 @@ zero_pread (void *handle, void *buf, uint32_t count, uint64_t
offset,
   return -1;
 }

+/* Cache. */
+static int
+zero_cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags)
+{
+  /* Everything is already in memory, falling back to .pread is
+     actually slower than treating this as a no-op. */
+  return 0;
+}
+
 static struct nbdkit_plugin plugin = {
   .name              = "zero",
   .version           = PACKAGE_VERSION,
@@ -83,6 +92,7 @@ static struct nbdkit_plugin plugin = {
   .open              = zero_open,
   .get_size          = zero_get_size,
   .pread             = zero_pread,
+  .cache             = zero_cache,
   /* In this plugin, errno is preserved properly along error return
    * paths from failed system calls.
    */
-- 
2.20.1

It's easy to expose new callbacks to sh plugins. I have no idea how
any plugin would actually usefully cache anything in shell (you can't
really store it in a shell variable, since a new shell process is
started for each command); but if nothing else, this allows a user to
implement .can_cache returning true to bypass nbdkit's normal fallback
to .pread.

The shell plugin, coupled with Rich's work on libnbd as a client-side
library for actually exercising calls to NBD_CMD_CACHE, will be a
useful way to prove that cache commands even make it through the
stack. (Remember, qemu 3.0 was released with a fatally flawed
NBD_CMD_CACHE server implementation, because there were no open source
clients at the time that could actually send the command to test the
server with).

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 plugins/sh/nbdkit-sh-plugin.pod | 12 +++++++++-
 plugins/sh/sh.c                 | 40 +++++++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+), 1 deletion(-)

diff --git a/plugins/sh/nbdkit-sh-plugin.pod b/plugins/sh/nbdkit-sh-plugin.pod
index 8af88b4..bfd58ab 100644
--- a/plugins/sh/nbdkit-sh-plugin.pod
+++ b/plugins/sh/nbdkit-sh-plugin.pod
@@ -218,9 +218,11 @@ This method is required.

 =item C<can_extents>

+=item C<can_cache>
+
 Unlike in other languages, you B<must> provide the C<can_*> methods
 otherwise they are assumed to all return false and your C<pwrite>,
-C<flush>, C<trim>, C<zero> and C<extents> methods will
never be
+C<flush>, C<trim>, C<zero>, C<extents>, and
C<cache> methods will never be
 called.  The reason for this is obscure: In other languages we can
 detect if (eg) a C<pwrite> method is defined and synthesize an
 appropriate response if no actual C<can_write> method is defined.
@@ -232,6 +234,7 @@ possible with this plugin.
  /path/to/script can_trim <handle>
  /path/to/script can_zero <handle>
  /path/to/script can_extents <handle>
+ /path/to/script can_cache <handle>

 The script should exit with code C<0> for true or code C<3> for
false.

@@ -334,6 +337,13 @@ Unlike in other languages, if you provide an
C<extents> method you
 B<must> also provide a C<can_extents> method which exits with code
 C<0> (true).

+=item C<cache>
+
+ /path/to/script cache <handle> <count> <offset>
+
+Unlike in other languages, if you provide a C<cache> method you
B<must>
+also provide a C<can_cache> method which exits with code C<0>
(true).
+
 =back

 =head2 Missing callbacks
diff --git a/plugins/sh/sh.c b/plugins/sh/sh.c
index a5beb57..7aded44 100644
--- a/plugins/sh/sh.c
+++ b/plugins/sh/sh.c
@@ -578,6 +578,12 @@ sh_can_multi_conn (void *handle)
   return boolean_method (handle, "can_multi_conn");
 }

+static int
+sh_can_cache (void *handle)
+{
+  return boolean_method (handle, "can_cache");
+}
+
 static int
 sh_flush (void *handle, uint32_t flags)
 {
@@ -782,6 +788,38 @@ sh_extents (void *handle, uint32_t count, uint64_t offset,
uint32_t flags,
   }
 }

+static int
+sh_cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags)
+{
+  char *h = handle;
+  char cbuf[32], obuf[32];
+  const char *args[] = { script, "cache", h, cbuf, obuf, NULL };
+
+  snprintf (cbuf, sizeof cbuf, "%" PRIu32, count);
+  snprintf (obuf, sizeof obuf, "%" PRIu64, offset);
+  assert (!flags);
+
+  switch (call (args)) {
+  case OK:
+    return 0;
+
+  case MISSING:
+    /* Ignore lack of cache callback. */
+    return 0;
+
+  case ERROR:
+    return -1;
+
+  case RET_FALSE:
+    nbdkit_error ("%s: %s method returned unexpected code (3/false)",
+                  script, "cache");
+    errno = EIO;
+    return -1;
+
+  default: abort ();
+  }
+}
+
 #define sh_config_help \
   "script=<FILENAME>     (required) The shell script to run.\n"
\
   "[other arguments may be used by the plugin that you load]"
@@ -812,6 +850,7 @@ static struct nbdkit_plugin plugin = {
   .can_extents       = sh_can_extents,
   .can_fua           = sh_can_fua,
   .can_multi_conn    = sh_can_multi_conn,
+  .can_cache         = sh_can_cache,

   .pread             = sh_pread,
   .pwrite            = sh_pwrite,
@@ -819,6 +858,7 @@ static struct nbdkit_plugin plugin = {
   .trim              = sh_trim,
   .zero              = sh_zero,
   .extents           = sh_extents,
+  .cache             = sh_cache,

   .errno_is_preserved = 1,
 };
-- 
2.20.1

Make it possible for filters to adjust the behavior for
NBD_CMD_CACHE. To avoid any 'git bisect' breakage, this patch defaults
.can_cache to false until all necessary filters have been patched
first.

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 docs/nbdkit-filter.pod  | 27 ++++++++++++++++++++++++++-
 include/nbdkit-filter.h |  8 ++++++++
 server/filters.c        | 39 +++++++++++++++++++++++++++++++++------
 3 files changed, 67 insertions(+), 7 deletions(-)

diff --git a/docs/nbdkit-filter.pod b/docs/nbdkit-filter.pod
index 6aeaa7b..787333a 100644
--- a/docs/nbdkit-filter.pod
+++ b/docs/nbdkit-filter.pod
@@ -356,6 +356,8 @@ calls.

 =head2 C<.can_multi_conn>

+=head2 C<.can_cache>
+
  int (*can_write) (struct nbdkit_next_ops *next_ops, void *nxdata,
                    void *handle);
  int (*can_flush) (struct nbdkit_next_ops *next_ops, void *nxdata,
@@ -373,6 +375,8 @@ calls.
                  void *handle);
  int (*can_multi_conn) (struct nbdkit_next_ops *next_ops, void *nxdata,
                         void *handle);
+ int (*can_cache) (struct nbdkit_next_ops *next_ops, void *nxdata,
+                   void *handle);

 These intercept the corresponding plugin methods, and control feature
 bits advertised to the client.
@@ -597,6 +601,27 @@ Returns the number of extents in the list.

 Returns a copy of the C<i>'th extent.

+=head2 C<.cache>
+
+ int (*cache) (struct nbdkit_next_ops *next_ops, void *nxdata,
+               void *handle, uint32_t count, uint64_t offset,
+               uint32_t flags, int *err);
+
+This intercepts the plugin C<.cache> method and can be used to modify
+cache requests.
+
+This function will not be called if C<.can_cache> returned false; in
+turn, the filter should not call C<next_ops-E<gt>cache> if
+C<next_ops-E<gt>can_cache> did not return true.
+
+The parameter C<flags> exists in case of future NBD protocol
+extensions; at this time, it will be 0 on input, and the filter should
+not pass any flags to C<next_ops-E<gt>cache>.
+
+If there is an error, C<.cache> should call C<nbdkit_error> with an
+error message B<and> return -1 with C<err> set to the positive
errno
+value to return to the client.
+
 =head1 ERROR HANDLING

 If there is an error in the filter itself, the filter should call
@@ -708,4 +733,4 @@ Richard W.M. Jones

 =head1 COPYRIGHT

-Copyright (C) 2013-2018 Red Hat Inc.
+Copyright (C) 2013-2019 Red Hat Inc.
diff --git a/include/nbdkit-filter.h b/include/nbdkit-filter.h
index 9b6cd6e..5893dd8 100644
--- a/include/nbdkit-filter.h
+++ b/include/nbdkit-filter.h
@@ -74,6 +74,7 @@ struct nbdkit_next_ops {
   int (*can_extents) (void *nxdata);
   int (*can_fua) (void *nxdata);
   int (*can_multi_conn) (void *nxdata);
+  int (*can_cache) (void *nxdata);

   int (*pread) (void *nxdata, void *buf, uint32_t count, uint64_t offset,
                 uint32_t flags, int *err);
@@ -87,6 +88,8 @@ struct nbdkit_next_ops {
                int *err);
   int (*extents) (void *nxdata, uint32_t count, uint64_t offset, uint32_t
flags,
                   struct nbdkit_extents *extents, int *err);
+  int (*cache) (void *nxdata, uint32_t count, uint64_t offset, uint32_t flags,
+                int *err);
 };

 struct nbdkit_filter {
@@ -142,6 +145,8 @@ struct nbdkit_filter {
                   void *handle);
   int (*can_multi_conn) (struct nbdkit_next_ops *next_ops, void *nxdata,
                          void *handle);
+  int (*can_cache) (struct nbdkit_next_ops *next_ops, void *nxdata,
+                    void *handle);

   int (*pread) (struct nbdkit_next_ops *next_ops, void *nxdata,
                 void *handle, void *buf, uint32_t count, uint64_t offset,
@@ -161,6 +166,9 @@ struct nbdkit_filter {
   int (*extents) (struct nbdkit_next_ops *next_ops, void *nxdata,
                   void *handle, uint32_t count, uint64_t offset, uint32_t
flags,
                   struct nbdkit_extents *extents, int *err);
+  int (*cache) (struct nbdkit_next_ops *next_ops, void *nxdata,
+                void *handle, uint32_t count, uint64_t offset, uint32_t flags,
+                int *err);
 };

 #define NBDKIT_REGISTER_FILTER(filter)                                  \
diff --git a/server/filters.c b/server/filters.c
index c619fd6..a48f67e 100644
--- a/server/filters.c
+++ b/server/filters.c
@@ -329,6 +329,13 @@ next_can_multi_conn (void *nxdata)
   return b_conn->b->can_multi_conn (b_conn->b, b_conn->conn);
 }

+static int
+next_can_cache (void *nxdata)
+{
+  struct b_conn *b_conn = nxdata;
+  return b_conn->b->can_cache (b_conn->b, b_conn->conn);
+}
+
 static int
 next_pread (void *nxdata, void *buf, uint32_t count, uint64_t offset,
             uint32_t flags, int *err)
@@ -379,6 +386,15 @@ next_extents (void *nxdata, uint32_t count, uint64_t
offset, uint32_t flags,
                              extents, err);
 }

+static int
+next_cache (void *nxdata, uint32_t count, uint64_t offset,
+            uint32_t flags, int *err)
+{
+  struct b_conn *b_conn = nxdata;
+  return b_conn->b->cache (b_conn->b, b_conn->conn, count, offset,
flags,
+                           err);
+}
+
 static struct nbdkit_next_ops next_ops = {
   .get_size = next_get_size,
   .can_write = next_can_write,
@@ -389,12 +405,14 @@ static struct nbdkit_next_ops next_ops = {
   .can_extents = next_can_extents,
   .can_fua = next_can_fua,
   .can_multi_conn = next_can_multi_conn,
+  .can_cache = next_can_cache,
   .pread = next_pread,
   .pwrite = next_pwrite,
   .flush = next_flush,
   .trim = next_trim,
   .zero = next_zero,
   .extents = next_extents,
+  .cache = next_cache,
 };

 static int
@@ -577,12 +595,16 @@ static int
 filter_can_cache (struct backend *b, struct connection *conn)
 {
   struct backend_filter *f = container_of (b, struct backend_filter, backend);
+  void *handle = connection_get_handle (conn, f->backend.i);
+  struct b_conn nxdata = { .b = f->backend.next, .conn = conn };

   debug ("%s: can_cache", f->name);

-  /* FIXME: Default to f->backend.next->can_cache, once all filters
-     have been audited */
-  return 0;
+  if (f->filter.can_cache)
+    return f->filter.can_cache (&next_ops, &nxdata, handle);
+  else
+    return 0; /* FIXME - allow passthrough once all filters are audited */
+  return f->backend.next->can_cache (f->backend.next, conn);
 }

 static int
@@ -720,15 +742,20 @@ filter_cache (struct backend *b, struct connection *conn,
               uint32_t flags, int *err)
 {
   struct backend_filter *f = container_of (b, struct backend_filter, backend);
+  void *handle = connection_get_handle (conn, f->backend.i);
+  struct b_conn nxdata = { .b = f->backend.next, .conn = conn };

   assert (flags == 0);

   debug ("%s: cache count=%" PRIu32 " offset=%" PRIu64
" flags=0x%" PRIx32,
          f->name, count, offset, flags);

-  /* FIXME: Allow filter to rewrite request */
-  return f->backend.next->cache (f->backend.next, conn,
-                                 count, offset, flags, err);
+  if (f->filter.cache)
+    return f->filter.cache (&next_ops, &nxdata, handle,
+                            count, offset, flags, err);
+  else
+    return f->backend.next->cache (f->backend.next, conn,
+                                   count, offset, flags, err);
 }

 static struct backend filter_functions = {
-- 
2.20.1

This also forms the first instance that I am aware of where an open
source client actually sends NBD_CMD_CACHE to the server - even if the
client is just a test app (the nbd plugin doesn't count - it won't
send to the server unless a client sends to the plugin first).

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 tests/test-layers-filter.c | 22 +++++++++++++++++++++-
 tests/test-layers-plugin.c | 17 +++++++++++++++++
 tests/test-layers.c        | 36 ++++++++++++++++++++++++++++++++++++
 3 files changed, 74 insertions(+), 1 deletion(-)

diff --git a/tests/test-layers-filter.c b/tests/test-layers-filter.c
index a8f4723..bd063bd 100644
--- a/tests/test-layers-filter.c
+++ b/tests/test-layers-filter.c
@@ -1,5 +1,5 @@
 /* nbdkit
- * Copyright (C) 2018 Red Hat Inc.
+ * Copyright (C) 2018-2019 Red Hat Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
@@ -184,6 +184,15 @@ test_layers_filter_can_extents (struct nbdkit_next_ops
*next_ops,
   return next_ops->can_extents (nxdata);
 }

+static int
+test_layers_filter_can_cache (struct nbdkit_next_ops *next_ops,
+                              void *nxdata,
+                              void *handle)
+{
+  DEBUG_FUNCTION;
+  return next_ops->can_cache (nxdata);
+}
+
 static int
 test_layers_filter_pread (struct nbdkit_next_ops *next_ops, void *nxdata,
                            void *handle, void *buf,
@@ -241,6 +250,15 @@ test_layers_filter_extents (struct nbdkit_next_ops
*next_ops, void *nxdata,
   return next_ops->extents (nxdata, count, offset, flags, extents, err);
 }

+static int
+test_layers_filter_cache (struct nbdkit_next_ops *next_ops, void *nxdata,
+                          void *handle, uint32_t count, uint64_t offset,
+                          uint32_t flags, int *err)
+{
+  DEBUG_FUNCTION;
+  return next_ops->cache (nxdata, count, offset, flags, err);
+}
+
 static struct nbdkit_filter filter = {
   .name              = "testlayers" layer,
   .version           = PACKAGE_VERSION,
@@ -262,12 +280,14 @@ static struct nbdkit_filter filter = {
   .can_fua           = test_layers_filter_can_fua,
   .can_multi_conn    = test_layers_filter_can_multi_conn,
   .can_extents       = test_layers_filter_can_extents,
+  .can_cache         = test_layers_filter_can_cache,
   .pread             = test_layers_filter_pread,
   .pwrite            = test_layers_filter_pwrite,
   .flush             = test_layers_filter_flush,
   .trim              = test_layers_filter_trim,
   .zero              = test_layers_filter_zero,
   .extents           = test_layers_filter_extents,
+  .cache             = test_layers_filter_cache,
 };

 NBDKIT_REGISTER_FILTER(filter)
diff --git a/tests/test-layers-plugin.c b/tests/test-layers-plugin.c
index f9b2014..981641e 100644
--- a/tests/test-layers-plugin.c
+++ b/tests/test-layers-plugin.c
@@ -143,6 +143,13 @@ test_layers_plugin_can_multi_conn (void *handle)
   return 1;
 }

+static int
+test_layers_plugin_can_cache (void *handle)
+{
+  DEBUG_FUNCTION;
+  return 1;
+}
+
 static int
 test_layers_plugin_can_extents (void *handle)
 {
@@ -201,6 +208,14 @@ test_layers_plugin_extents (void *handle,
   return nbdkit_add_extent (extents, offset, count, 0);
 }

+static int
+test_layers_plugin_cache (void *handle,
+                         uint32_t count, uint64_t offset, uint32_t flags)
+{
+  DEBUG_FUNCTION;
+  return 0;
+}
+
 static struct nbdkit_plugin plugin = {
   .name              = "testlayersplugin",
   .version           = PACKAGE_VERSION,
@@ -220,12 +235,14 @@ static struct nbdkit_plugin plugin = {
   .can_fua           = test_layers_plugin_can_fua,
   .can_multi_conn    = test_layers_plugin_can_multi_conn,
   .can_extents       = test_layers_plugin_can_extents,
+  .can_cache         = test_layers_plugin_can_cache,
   .pread             = test_layers_plugin_pread,
   .pwrite            = test_layers_plugin_pwrite,
   .flush             = test_layers_plugin_flush,
   .trim              = test_layers_plugin_trim,
   .zero              = test_layers_plugin_zero,
   .extents           = test_layers_plugin_extents,
+  .cache             = test_layers_plugin_cache,
   /* In this plugin, errno is preserved properly along error return
    * paths from failed system calls.
    */
diff --git a/tests/test-layers.c b/tests/test-layers.c
index 627e4ec..a820ba5 100644
--- a/tests/test-layers.c
+++ b/tests/test-layers.c
@@ -361,6 +361,12 @@ main (int argc, char *argv[])
      "filter1: test_layers_filter_can_extents",
      "test_layers_plugin_can_extents",
      NULL);
+  log_verify_seen_in_order
+    ("filter3: test_layers_filter_can_cache",
+     "filter2: test_layers_filter_can_cache",
+     "filter1: test_layers_filter_can_cache",
+     "test_layers_plugin_can_cache",
+     NULL);

   fprintf (stderr, "%s: protocol connected\n", program_name);

@@ -526,6 +532,36 @@ main (int argc, char *argv[])
      "test_layers_plugin_zero",
      NULL);

+  request.type = htobe16 (NBD_CMD_CACHE);
+  request.offset = htobe64 (0);
+  request.count = htobe32 (512);
+  request.flags = htobe16 (0);
+  if (send (sock, &request, sizeof request, 0) != sizeof request) {
+    perror ("send: NBD_CMD_CACHE");
+    exit (EXIT_FAILURE);
+  }
+  if (recv (sock, &reply, sizeof reply, MSG_WAITALL) != sizeof reply) {
+    perror ("recv: NBD_CMD_CACHE");
+    exit (EXIT_FAILURE);
+  }
+  if (reply.error != NBD_SUCCESS) {
+    fprintf (stderr, "%s: NBD_CMD_CACHE failed with %d\n",
+             program_name, reply.error);
+    exit (EXIT_FAILURE);
+  }
+
+  sleep (1);
+  log_verify_seen_in_order
+    ("testlayersfilter3: cache count=512 offset=0 flags=0x0",
+     "filter3: test_layers_filter_cache",
+     "testlayersfilter2: cache count=512 offset=0 flags=0x0",
+     "filter2: test_layers_filter_cache",
+     "testlayersfilter1: cache count=512 offset=0 flags=0x0",
+     "filter1: test_layers_filter_cache",
+     "testlayersplugin: debug: cache count=512 offset=0",
+     "test_layers_plugin_cache",
+     NULL);
+
   /* XXX We should test NBD_CMD_BLOCK_STATUS here.  However it
    * requires that we negotiate structured replies and base:allocation
    * in the handshake, and the format of the reply is more complex
-- 
2.20.1

Rely on .can_cache passthrough to imply that our .cache won't be
called unless the plugin also has .cache. [Technically, that won't
happen until a later patch flips the default in filters.c].  Round the
cache request out, to cache the same range as would otherwise be
passed to the plugin's .pread if we had set .can_cache to
false. Fortunately, when maxlen is larger than maxblock, and the
plugin supports .cache, this results in a lot fewer calls into the
plugin than the .pread fallback.

Oddly enough, a client can submit an unaligned request for just under
4G of caching where our rounding would overflow a 32-bit integer, so
our rounding has to use a 64-bit temporary.

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 filters/blocksize/blocksize.c | 31 ++++++++++++++++++++++++++++++-
 1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/filters/blocksize/blocksize.c b/filters/blocksize/blocksize.c
index 4f3e9a3..2d28222 100644
--- a/filters/blocksize/blocksize.c
+++ b/filters/blocksize/blocksize.c
@@ -1,5 +1,5 @@
 /* nbdkit
- * Copyright (C) 2018 Red Hat Inc.
+ * Copyright (C) 2018-2019 Red Hat Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
@@ -389,6 +389,34 @@ blocksize_extents (struct nbdkit_next_ops *next_ops, void
*nxdata,
                             flags, extents, err);
 }

+static int
+blocksize_cache (struct nbdkit_next_ops *next_ops, void *nxdata,
+                 void *handle, uint32_t count, uint64_t offs, uint32_t flags,
+                 int *err)
+{
+  uint32_t limit;
+  uint64_t remaining = count; /* Rounding out could exceed 32 bits */
+
+  /* Unaligned head */
+  limit = offs & (minblock - 1);
+  remaining += limit;
+  offs -= limit;
+
+  /* Unaligned tail */
+  remaining = ROUND_UP (remaining, minblock);
+
+  /* Aligned body */
+  while (remaining) {
+    limit = MIN (maxdata, remaining);
+    if (next_ops->cache (nxdata, limit, offs, flags, err) == -1)
+      return -1;
+    offs += limit;
+    remaining -= limit;
+  }
+
+  return 0;
+}
+
 static struct nbdkit_filter filter = {
   .name              = "blocksize",
   .longname          = "nbdkit blocksize filter",
@@ -404,6 +432,7 @@ static struct nbdkit_filter filter = {
   .trim              = blocksize_trim,
   .zero              = blocksize_zero,
   .extents           = blocksize_extents,
+  .cache             = blocksize_cache,
 };

 NBDKIT_REGISTER_FILTER(filter)
-- 
2.20.1

On Thu, May 09, 2019 at 10:03:34PM -0500, Eric Blake
wrote:
> Until the next few patches expose and implement new callbacks for
> filters and plugins, our initial implementation for NBD_CMD_CACHE is
> to just blindly advertise the feature, and call into .pread with an
> ignored buffer.
> 
> Note that for bisection reasons, this patch treats any use of a filter
> as a forced .can_cache of false to bypass the filter's caching; once
> all affected filters are patched to handle cache requests correctly, a
> later patch will then switch the filter default to passthrough for the
> sake of remaining filters.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>
> 
> ---
> 
> Note: we could also choose to always advertise caching, but make the
> nbdkit default version be a no-op rather than call out to .pread. For
> that matter, maybe a plugin's .can_cache should allow the plugin to
> choose between a tri-state of no-op, fallback to .pread, or call
> .cache; I'm not sure which is the saner default for a random plugin
> compiled against older nbdkit. At least it's fairly easy to write a
> filter that changes the default from no-op to .pread or from .pread to
> no-op. Be thinking about that as you review the rest of the series.
I'm worried that we end up in the same situation that we do with
.zero: because we emulate it, we end up with a slow zero support,
whereas clients [well, Nir S. mainly] actually want
NBD_FLAG_SEND_WRITE_ZEROES == *fast* zeroing is available.

In particular in the NBD_CMD_CACHE case: for some plugins emulating
this behind the scenes with pread makes some sense.  eg. It works for
nbdkit-file-plugin because it will prefetch the data into Linux's page
cache.  However if a plugin makes a network access,
eg. nbdkit-vddk-plugin, that can be both very expensive and has no
prefetching benefit.

Therefore I do think we should default to ignoring NBD_CMD_CACHE
commands and *not* advertising the feature, except when the plugin
says that it is able to handle it.

For tri-stating .can_cache: yes that makes sense, because there is
moderate difficulty for a plugin to emulate .cache using .pread.  The
plugin has to at least allocate a dummy buffer and throw it away, and
we could probably do something more efficient than that if we
implement an emulate mode in the server.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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For this plugin, if a tri-state can_cache is implemented, the code
from sh_can_fua could be reused:

https://github.com/libguestfs/nbdkit/blob/7a4f38f98920a1192e5c50ea9bd212e759e3f4e7/plugins/sh/sh.c#L530

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-df lists disk usage of guests without needing to install any
software inside the virtual machine.  Supports Linux and Windows.
http://people.redhat.com/~rjones/virt-df/