[PATCH] am -3: recover the diagnostic messages for corrupt patches

Junio C Hamano· Apr 14, 2010, 20:33 UTC · lore

"git am -3" first tries to apply the patch without any extra trick, and applies it to a synthesized tree for 3-way merge after the first attempt fails. "git apply" exits with status 1 for a patch that is well-formed but is not applicable (and it dies on other errors with non-zereo, non-1 status) and has an optimization to fall back to the 3-way merge only in the case.

An earlier patch 3ddd170 (am: suppress apply errors when using 3-way, 2009-06-16) squelched diagnostic messages from the first attempt, not to be shown to the end user. This worked reasonably well if the reason the first application failed was because the patch was made against a wrong version.

When the patch is corrupt (e.g. line-wrapped or leading whitespaces got dropped), however, because the second patch application is not even attempted, the error message from the first application is never shown and is forever lost. This message is necessary to locate where the patch is corrupt and fix it up.

We could fix this issue by reverting 3dd170, or keeping the error message to somewhere and showing it, but because this is an error codepath, the easiest is to disable the optimization. The second patch application is attempted even when the input is corrupt, and it will notice, diagnose, and stop with an error message.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 git-am.sh |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

Show changes to git-am.sh +1 −1

diff --git a/git-am.sh b/git-am.sh
index e26c54a..4e0e406 100755
--- a/git-am.sh
+++ b/git-am.sh
@@ -534,7 +534,7 @@ do
 		;;
 	esac
 
-	if test $apply_status = 1 && test "$threeway" = t
+	if test $apply_status != 0 && test "$threeway" = t
 	then
 		if (fall_back_3way)
 		then

-- 
1.7.1.rc1.252.gce30c

Stephen Boyd· Apr 15, 2010, 03:28 UTC · re: Junio C Hamano · lore

Re: [PATCH] am -3: recover the diagnostic messages for corrupt patches

On 04/14/2010 01:33 PM, Junio C Hamano wrote: [...]

Show 7 quoted lines

> We could fix this issue by reverting 3dd170, or keeping the error message
> to somewhere and showing it, but because this is an error codepath, the
> easiest is to disable the optimization.  The second patch application is
> attempted even when the input is corrupt, and it will notice, diagnose,
> and stop with an error message.
>
> Signed-off-by: Junio C Hamano <gitster@pobox.com>

Looks fine to me. Two invocations on an error path where user intervention is most likely required shouldn't be too bad like you say.

This reminds me, a while ago I looked into pushing 3way into git-apply (that was a GSoC project suggestion). We could introduce an internal "I'm attempting a 3way if this fails" option for git-apply and then it could be smart and output the errors if the patch is malformed. Otherwise it would be silent like it should be on 3way apply for the first time failures. Obviously this will have to happen for 3way to be pushed into git-apply at all.

Sounds a little too smart to me though (this is the stupid content tracker after all) so let's just do what you have ;-)

patcham -3: recover the diagnostic messages for corrupt patches