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

[Laszlo Ersek]

On 8/31/23 00:21, Eric Blake wrote:
> 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);
>  }

The patch appears to do what it says on the tin, but I'm unconvinced we
should relax the requirement. The "may indicate a bug" debug message is
easy to miss. A hard failure upon EPIPE is noticeable, and as you write
the script can just drain unwanted input with "cat >/dev/null".

I think I may not be appreciating this patch due to not understanding
how difficult it could be to update such a script. Can you elaborate on
that? Can you provide an example where adding "cat >/dev/null" to the
pwrite script is a disproportionate chore?

Thanks,
Laszlo