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

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(a)redhat.com&gt; --- 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); }