Re: git-am failed, what's next ?

From: Junio C Hamano <junkio@cox.net>
Date: Feb 22, 2007, 20:02 UTC
Message-ID: <7v3b4yezao.fsf@assigned-by-dhcp.cox.net>
In-Reply-To: <200702221113.44338.andyparkins@gmail.com>

Andy Parkins <andyparkins@gmail.com> writes:

Show 14 quoted lines

> On Thursday 2007 February 22 09:47, Junio C Hamano wrote:
>
>> If you have the pre-image blobs the patch was created against,
>> and the patch was created with git and records "index" lines
>> like these correctly:
>>
>>         diff --git a/builtin-config.c b/builtin-config.c
>>         index 0f9051d..f1433a4 100644
>>
>> then you can tell git to fall back on 3-way merge to apply the
>> patch.  After seeing "git am ./mbox" fail, you could try
>
> Magic.  I had no idea about this.  Could it be made even more
> magical by doing this for you?

You can start with --3way from the beginning, not just after seeing it fail.

In the earlier description, I made it sound as if "-3" is an option for recovery, but that is not the case.

Running "git am" without mbox parameter and an existing .dotest/ is the instruction for "git am" to continue, and the first thing it does when told to continue is different depending on if either --skip or --resolved is given. --skip makes it skip the patch. --resolved takes the index, uses the metainfo to make a commit. Lack of these options makes it re-try the one recorded in .dotest/next file. After that, if it fails (and --skip would not fail), it stops. Otherwise it goes on to the next patch.

The example you referred to as "magic" is just a normal "retry it" codepath but running with the "--3way" option.

The 3-way fallback is not enabled by default. A minor reason for this is that it did not exist in the original and is an optional feature that was added later on.

But the real reason is that as a principle, git tools err on the safe side by default, just like our use of "git-apply" in "git-am" does not allow fuzz in the patch nor reducing context by default for strictness. That way, the patch application process is stopped and gives the non-interactive user a chance to inspect _why_ the patch does not apply, before deciding to continue, either with fuzz'ed patch application, fixing the patch text, or falling back on 3-way.

But these days, I almost always run:

	$ git am -3 -s ./mbox

and inspect the ones that actually used 3-way fallback after the fact. It _might_ make sense to add a configuration option to default to -3 (or -s for that matter), like:

	[am]
		signoff
                threeway

and do something like this:

---
 git-am.sh |   14 ++++++++++++++
 1 files changed, 14 insertions(+), 0 deletions(-)

diff --git a/git-am.sh b/git-am.sh
index 6db9cb5..c0395f5 100755
--- a/git-am.sh
+++ b/git-am.sh
@@ -109,6 +109,7 @@ prec=4
 dotest=.dotest sign= utf8=t keep= skip= interactive= resolved= binary= resolvemsg=
 git_apply_opt=
 
+has_opt=
 while case "$#" in 0) break;; esac
 do
 	case "$1" in
@@ -155,8 +156,21 @@ do
 	*)
 	break ;;
 	esac
+	has_opt=t
 done
 
+if test -z "$has_opt"
+then
+	if test true = "$(git config --bool am.signoff)"
+	then
+		sign=t
+	fi
+	if test true = "$(git config --bool am.threeway)"
+	then
+		threeway=t
+	fi
+fi
+
 # If the dotest directory exists, but we have finished applying all the
 # patches in them, clear it out.
 if test -d "$dotest" &&