Re: git gui replaces amend message when prepare-commit-msg hook is used

> On Thu, Jul 04, 2013 at 01:59:10PM +0300, Orgad Shaneh wrote:
>> On Thu, Jul 4, 2013 at 1:34 PM, John Keeping <john@keeping.me.uk> wrote:
>> > On Thu, Jul 04, 2013 at 12:47:28PM +0300, Orgad Shaneh wrote:
>> >> Hi,
>> >>
>> >> If a prepare-commit-msg hook is used, git gui executes it for "New Commit".
>> >>
>> >> If the "New Commit" is selected, and then immediately "Amend" (before
>> >> the hook returns), when the hook returns the message is replaced with
>> >> the one produced by the hook.
>> >
>> > I think this is a problem with the hook you are running.  The hook is
>> > given arguments specifying the message file and optionally the source of
>> > whatever is already in the file (see githooks(5) for details).
>> >
>> > It sounds like your hook is blindly overwriting the file, rather than
>> > preserving its contents in the cases where you wish to do so.
>>
>> Let me try to explain.
>>
>> When git gui is executed, it calls the prepare-commit-msg script with
>> .git/PREPARE_COMMIT_MSG as an argument.
>>
>> When amend is selected, the hook is *not* called at all (what would it
>> prepare? The message is already committed)
>>
>> Use the following hook to reproduce:
>> --- snip ---
>> #!/bin/sh
>>
>> sleep 5
>> echo "$@" >> /tmp/hook.log
>> echo 'Hello hook' > "$1"
>> --- snip ---
>>
>> Now run git gui (or press F5 if it is already running), and before 5
>> seconds pass, click Amend last commit. You'll see the commit's
>> message, but when the 5 seconds pass it is replaced with "Hello hook".
>> That's the bug.
>
> Yes, and that's a bug in the hook.  The hook is called with a second
> argument "commit" but it is ignoring this and blindly overwriting the
> message.  githooks(5) says:
>
>     prepare-commit-msg
>         This hook is invoked by git commit right after preparing the default
>         log message, and before the editor is started.
>
>         It takes one to three parameters. The first is the name of the
>         file that contains the commit log message. The second is the
>         source of the commit message, and can be: message (if a -m or -F
>         option was given); template (if a -t option was given or the
>         configuration option commit.template is set); merge (if the
>         commit is a merge or a .git/MERGE_MSG file exists); squash (if a
>         .git/SQUASH_MSG file exists); or commit, followed by a commit
>         SHA1 (if a -c, -C or --amend option was given).
>
>         If the exit status is non-zero, git commit will abort.
>
>         The purpose of the hook is to edit the message file in place,
>         and it is not suppressed by the --no-verify option. A non-zero
>         exit means a failure of the hook and aborts the commit. It
>         should not be used as replacement for pre-commit hook.
>
> Your problem is that your hook script is not checking $2 so it is
> overwriting the message even when you do not want to do so.