Re: [PATCH v2 3/3] builtin/unpack-objects.c: change xwrite to write_in_full avoid truncation.

From: Jeff King <peff@peff.net>
Date: Feb 27, 2024, 19:25 UTC
Message-ID: <20240227192530.GD3784114@coredump.intra.peff.net>
In-Reply-To: <03be01da69af$d8366e10$88a34a30$@nexbridge.com>

On Tue, Feb 27, 2024 at 02:04:46PM -0500, rsbecker@nexbridge.com wrote:

Show 22 quoted lines

> >> diff --git a/builtin/unpack-objects.c b/builtin/unpack-objects.c index
> >> e0a701f2b3..6935c4574e 100644
> >> --- a/builtin/unpack-objects.c
> >> +++ b/builtin/unpack-objects.c
> >> @@ -680,7 +680,7 @@ int cmd_unpack_objects(int argc, const char
> >> **argv, const char *prefix UNUSED)
> >>
> >>  	/* Write the last part of the buffer to stdout */
> >>  	while (len) {
> >> -		int ret = xwrite(1, buffer + offset, len);
> >> +		int ret = write_in_full(1, buffer + offset, len);
> >>  		if (ret <= 0)
> >>  			break;
> >>  		len -= ret;
> [...]
> I experimented with using write_in_full vs. keeping xwrite. With xwrite in
> this loop, t7704.9 consistently fails as described in the other thread. With
> write_in_full, the code works correctly. I assume there are side-effects
> that are present. This change is critical to having the code work on
> NonStop. Otherwise git seems to be at risk of actually being seriously
> broken if unpack does not work correctly. I am happy to have my series
> ignored as long as the problem is otherwise corrected.

I'm somewhat skeptical that this code is to blame, as it should be run very rarely at all; it is just dumping any content in the pack stream after the end of the checksum to stdout. But in normal use by Git, there is no such content in the first place.

If I do this:

diff --git a/builtin/unpack-objects.c b/builtin/unpack-objects.c
index e0a701f2b3..affe55035d 100644
--- a/builtin/unpack-objects.c
+++ b/builtin/unpack-objects.c
@@ -680,11 +680,7 @@ int cmd_unpack_objects(int argc, const char **argv, const char *prefix UNUSED)
 
 	/* Write the last part of the buffer to stdout */
 	while (len) {
-		int ret = xwrite(1, buffer + offset, len);
-		if (ret <= 0)
-			break;
-		len -= ret;
-		offset += ret;
+		BUG("cruft at the end of the pack!");
 	}
 
 	/* All done */

then t7704 still passes, as it does not run this code at all. In fact,
nothing in the test suite fails. Which is not to say we should get rid
of those code. If we were writing today we might flag it as an error,
but we should keep it for historical compatibility.

But I do not see any bug in the code, and nor do I think it could
contribute to a test failure.

-Peff