Libguestfs - Aug 2023 - [nbdkit PATCH] sh: Allow pwrite to not consume all data

On Wed, Aug 30, 2023 at 05:21:19PM -0500, 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.
I was thinking about this over night, and came to the conclusion that
it's always fine to ignore EPIPE errors.  For example a script might
be processing the input data gradually and then encounter an error and
want to exit immediately.  We also have plenty of plugins that discard
some or all of the written data.

So my counter-proposal (coming soon) is going to simply turn the EPIPE
error into a debug message and discard the rest of the write buffer.

Rich.

> 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
> 
> _______________________________________________
> Libguestfs mailing list
> Libguestfs at redhat.com
> https://listman.redhat.com/mailman/listinfo/libguestfs
-- 
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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On 8/31/23 10:02, Richard W.M. Jones wrote:
> 
> On Wed, Aug 30, 2023 at 05:21:19PM -0500, 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.
> 
> I was thinking about this over night, and came to the conclusion that
> it's always fine to ignore EPIPE errors.
Interesting; I formed the opposite impression!
> For example a script might
> be processing the input data gradually and then encounter an error and
> want to exit immediately.  We also have plenty of plugins that discard
> some or all of the written data.
But that would be associated with a nonzero exit status, right?

And that way the nbd client would see the pwrite operation as failed.

Laszlo
> 
> So my counter-proposal (coming soon) is going to simply turn the EPIPE
> error into a debug message and discard the rest of the write buffer.
> 
> Rich.
> 
> 
>> 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
>>
>> _______________________________________________
>> Libguestfs mailing list
>> Libguestfs at redhat.com
>> https://listman.redhat.com/mailman/listinfo/libguestfs
>