[Libguestfs] [nbdkit PATCH] sh: Allow pwrite to not consume all data

[Eric Blake]

I hit another transient failure in libnbd CI when a poorly-written
eval script did not consume all of stdin during .pwrite.  As behaving
as a data sink can be a somewhat reasonable feature of a
quickly-written sh or eval plugin, we should not be so insistent as
treating an EPIPE failure as an immediate return of EIO to the client.

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

I probably need to add unit test coverage of this before committing
(although proving that I win the data race on a client process exiting
faster than the parent can write enough data to hit EPIPE is hard).

 plugins/sh/nbdkit-sh-plugin.pod |  8 +++++++
 plugins/sh/call.c               | 38 ++++++++++++++++++++++-----------
 2 files changed, 34 insertions(+), 12 deletions(-)

diff --git a/plugins/sh/nbdkit-sh-plugin.pod b/plugins/sh/nbdkit-sh-plugin.pod
index b2c946a0..8b83a5b3 100644
--- a/plugins/sh/nbdkit-sh-plugin.pod
+++ b/plugins/sh/nbdkit-sh-plugin.pod
@@ -505,6 +505,14 @@ Unlike in other languages, if you provide a C<pwrite> method you
 B<must> also provide a C<can_write> method which exits with code C<0>
 (true).

+With nbdkit E<ge> 1.36, this method may return C<0> without consuming
+any data from stdin, and without producing any output, in order to
+behave as an intentional data sink.  But in older versions, nbdkit
+would treat any C<EPIPE> failure in writing to your script as an error
+condition even if your script returns success; to avoid unintended
+failures, you may want to include C<"cat >/dev/null"> in a script
+intending to ignore the client's write requests.
+
 =item C<flush>

  /path/to/script flush <handle>
diff --git a/plugins/sh/call.c b/plugins/sh/call.c
index 888c6459..79c67a04 100644
--- a/plugins/sh/call.c
+++ b/plugins/sh/call.c
@@ -34,6 +34,7 @@

 #include <assert.h>
 #include <fcntl.h>
+#include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <inttypes.h>
@@ -130,6 +131,7 @@ debug_call (const char **argv)
  */
 static int
 call3 (const char *wbuf, size_t wbuflen, /* sent to stdin (can be NULL) */
+       bool *pipe_full,                  /* set if wbuf not fully written */
        string *rbuf,                     /* read from stdout */
        string *ebuf,                     /* read from stderr */
        const char **argv)                /* script + parameters */
@@ -275,15 +277,8 @@ call3 (const char *wbuf, size_t wbuflen, /* sent to stdin (can be NULL) */
       r = write (pfds[0].fd, wbuf, wbuflen);
       if (r == -1) {
         if (errno == EPIPE) {
-          /* We tried to write to the script but it didn't consume
-           * the data.  Probably the script exited without reading
-           * from stdin.  This is an error in the script.
-           */
-          nbdkit_error ("%s: write to script failed because of a broken pipe: "
-                        "this can happen if the script exits without "
-                        "consuming stdin, which usually indicates a bug "
-                        "in the script",
-                        argv0);
+          *pipe_full = true;
+          r = wbuflen;
         }
         else
           nbdkit_error ("%s: write: %m", argv0);
@@ -555,7 +550,7 @@ call (const char **argv)
   CLEANUP_FREE_STRING string rbuf = empty_vector;
   CLEANUP_FREE_STRING string ebuf = empty_vector;

-  r = call3 (NULL, 0, &rbuf, &ebuf, argv);
+  r = call3 (NULL, 0, NULL, &rbuf, &ebuf, argv);
   return handle_script_error (argv[0], &ebuf, r);
 }

@@ -568,7 +563,7 @@ call_read (string *rbuf, const char **argv)
   int r;
   CLEANUP_FREE_STRING string ebuf = empty_vector;

-  r = call3 (NULL, 0, rbuf, &ebuf, argv);
+  r = call3 (NULL, 0, NULL, rbuf, &ebuf, argv);
   r = handle_script_error (argv[0], &ebuf, r);
   if (r == ERROR)
     string_reset (rbuf);
@@ -584,7 +579,26 @@ call_write (const char *wbuf, size_t wbuflen, const char **argv)
   int r;
   CLEANUP_FREE_STRING string rbuf = empty_vector;
   CLEANUP_FREE_STRING string ebuf = empty_vector;
+  bool pipe_full = false;

-  r = call3 (wbuf, wbuflen, &rbuf, &ebuf, argv);
+  r = call3 (wbuf, wbuflen, &pipe_full, &rbuf, &ebuf, argv);
+  if (pipe_full && r == OK) {
+    /* We allow scripts to intentionally ignore data, but they must
+     * have no output when doing so.
+     */
+    if (rbuf.len > 0 || ebuf.len > 0) {
+      nbdkit_error ("%s: write to script failed because of a broken pipe: "
+                    "this can happen if the script exits without "
+                    "consuming stdin, which usually indicates a bug "
+                    "in the script",
+                    argv[0]);
+      r = ERROR;
+    }
+    else
+      nbdkit_debug ("%s: write to script failed because of a broken pipe; "
+                    "assuming this was an intentional data sink, although it "
+                    "may indicate a bug in the script",
+                    argv[0]);
+  }
   return handle_script_error (argv[0], &ebuf, r);
 }
-- 
2.41.0