Bug: 'git am --abort' can silently reset the wrong branch

From: Eduardo Habkost <ehabkost@raisama.net>
Date: May 6, 2009, 19:19 UTC
Message-ID: <20090506191945.GG6325@blackpad>

Hi,

I've ben bitten by this multiple times, while maintaining a multi-branch repository. 'git am --abort' can drop the whole history of a branch, if you switch branches before running it. Below are the steps to reproduce the problem:

[foo/branch1]$ git --version git version 1.6.3.rc4.29.g8146

I have two branches, with distinct changes:

[foo/branch1]$ git diff branch1 branch2

diff --git a/file.txt b/file.txt
index e8a7b03..a11bf50 100644
--- a/file.txt
+++ b/file.txt
@@ -1,2 +1,2 @@
 initial content
-branch1 content, version 1
+branch2 content. lots of changes here. version 12345.
[foo/branch1]$ git status
# On branch branch1
nothing to commit (working directory clean)
[foo/branch1]$


I receive a patch and try to apply it to branch1:

[foo/branch1]$ git am /tmp/patch.patch
Applying: patch to be applied
fatal: corrupt patch at line 13
Patch failed at 0001 patch to be applied
When you have resolved this problem run "git am --resolved".
If you would prefer to skip this patch, instead run "git am --skip".
To restore the original branch and stop patching run "git am --abort".
[foo/branch1]$

(Seeing that solving this conflict will take some time. I go out to grab some
coffee).

(When back to work, somebody asks me to take a look on patch2.patch, that
applies to branch2).

[foo/branch1]$ git checkout branch2
Switched to branch 'branch2'
[foo/branch2]$ git am /tmp/patch2.patch
previous rebase directory /tmp/foo/.git/rebase-apply still exists but mbox given.
[foo/branch2]$


(Oh, I see. There was a pending 'git am' operation. Let's just cancel it, I can
start it again later anyway).

Here is where "git am" silently does the wrong thing:

[foo/branch2]$ git am --abort
[foo/branch2]$ cat file.txt
initial content
branch1 content, version 1
[foo/branch2]$

(Hey! where are the changes from branch2?)

[foo/branch2]$ git diff branch1 branch2
[foo/branch2]$

Ouch.   :(


When that happens, I need to resort to git reflog, and manually git-reset
branch2 to point to the original commit.

I don't know what would be the best approach to avoid this issue. I tested a
similar scenario using 'git rebase' and 'git rebase --abort' switched back to
branch1 before resetting the branch state, instead of resetting branch2. Maybe
'git am' could do the same.

-- 
Eduardo