Libguestfs - Feb 2021 - [nbdkit PATCH] checkwrite: Implement .can_fast_zero, .can_multi_conn

Blindly passing through the plugin's .can_fast_zero when our own .zero
never calls the plugin is fishy; rather, we can use the heuristic that
if .extents shows all zeroes, we were fast, otherwise we fail fast
rather than calling the plugin's .pread.  And since we never modify
any of the plugin's data, we can advertise multi-conn (note that it is
already unwise to use this filter with a plugin that reports different
data on read without any intervening writes, such as the info plugin).
---
 filters/checkwrite/checkwrite.c | 29 ++++++++++++++++++++++++++++-
 1 file changed, 28 insertions(+), 1 deletion(-)

diff --git a/filters/checkwrite/checkwrite.c b/filters/checkwrite/checkwrite.c
index 2792d4d8..bbd7f1cd 100644
--- a/filters/checkwrite/checkwrite.c
+++ b/filters/checkwrite/checkwrite.c
@@ -1,5 +1,5 @@
 /* nbdkit
- * Copyright (C) 2019-2020 Red Hat Inc.
+ * Copyright (C) 2019-2021 Red Hat Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
@@ -91,6 +91,23 @@ checkwrite_can_zero (struct nbdkit_next_ops *next_ops, void
*nxdata,
   return NBDKIT_ZERO_NATIVE;
 }

+static int
+checkwrite_can_fast_zero (struct nbdkit_next_ops *next_ops, void *nxdata,
+                          void *handle)
+{
+  /* It is better to advertise support, even if we always reject fast
+   * zero attempts when the plugin lacks .can_extents.
+   */
+  return 1;
+}
+
+static int
+checkwrite_can_multi_conn (struct nbdkit_next_ops *next_ops, void *nxdata,
+                           void *handle)
+{
+  return 1;
+}
+
 static inline int
 data_does_not_match (int *err)
 {
@@ -171,6 +188,10 @@ checkwrite_trim_zero (struct nbdkit_next_ops *next_ops,
void *nxdata,
       }

       /* Otherwise we have to read the underlying data and check. */
+      if (flags & NBDKIT_FLAG_FAST_ZERO) {
+        *err = ENOTSUP;
+        return -1;
+      }
       while (count > 0) {
         size_t buflen = MIN (MAX_REQUEST_SIZE, count);
         buflen = MIN (buflen, next_extent_offset - offset);
@@ -199,6 +220,10 @@ checkwrite_trim_zero (struct nbdkit_next_ops *next_ops,
void *nxdata,
   else {
     CLEANUP_FREE char *buf;

+    if (flags & NBDKIT_FLAG_FAST_ZERO) {
+      *err = ENOTSUP;
+      return -1;
+    }
     buf = malloc (MIN (MAX_REQUEST_SIZE, count));
     if (buf == NULL) {
       *err = errno;
@@ -231,6 +256,8 @@ static struct nbdkit_filter filter = {
   .can_fua           = checkwrite_can_fua,
   .can_trim          = checkwrite_can_trim,
   .can_zero          = checkwrite_can_zero,
+  .can_fast_zero     = checkwrite_can_fast_zero,
+  .can_multi_conn    = checkwrite_can_multi_conn,

   .pwrite            = checkwrite_pwrite,
   .flush             = checkwrite_flush,
-- 
2.30.1