Detect `git commit --amend` in pre-commit hooks so staged files include everything in commit

[sds]

Running pre-commit hooks when amending a commit currently behaves a little strangely in that only the changes being added to the commit (not all changes in the commit) are checked by Overcommit.

This sometimes results in weird behavior where depending on the order you add files and amend them to a commit you can get around certain pre-commit hooks. For example, with Rubocop's MethodLength cop set to 20 lines, if you add a method that is 19 lines long, commit it, then amend the commit to add another couple of lines to the method body, the Rubocop lint message will now become a warning because it is reported on a line that is not currently staged, since it's already in the commit.

Benefits

• Prevents potential bugs/problems being introduced due to order of changes being amended
• Assuming we can detect when a pre-commit hook is being executed as part of an "amend", this allows us to write a hook that prevents amending commits on certain branches (e.g. master) to enforce certain workflows

Costs

• Unclear if there is a definitive way to detect an amend on pre-commit (if it requires a non-reliable hack, this feature may be dead in the water)
• Significant behavior change, which may be surprising to existing users

Feel free to comment or ask questions in this issue.

[jawshooah]

Since no new commits have been created when the pre-commit hook runs, and the hook receives no helpful information via args or STDIN, really the only way is to check the command that initiated the commit via ps -ocommand= -p $PPID or similar.

[sds]

What happens if the user invoked git commit --amend via an alias in their .gitconfig? Running a quick test on my system and running ps f it looks like the command will have the alias' name (e.g. for amend = commit --amend):

/bin/zsh -l
\_ git amend
   \_ ruby .git/hooks/pre-commit

Luckily, we can easily see if an alias is defined using git config --get-regexp ^alias\.amend. Even better, git prevents you from having aliases point to other aliases or reference themselves, so we should be able to get away with just one check to see if commit.*--amend matches the alias.

For aliases that invoke shell scripts, those scripts will still end up triggering their own pre-commit hook, e.g. for:

[alias]
amend = "!f() { git commit --amend; }; f"

...running git amend would produce the following output in ps f:

/bin/zsh -l
\_ git amend
   \_ /bin/sh -c f() { git commit --amend; }; f f() { git commit --amend; }; f
      \_ git commit --amend
         \_ ruby .git/hooks/pre-commit

...and so our original code for detecting amends will work just fine.

[jawshooah]

Looks like shell aliases are expanded in the ps output as well, so this might actually work! 😄

[jawshooah]

Seems like git aliases are also expanded. Using a dummy repository with the following pre-commit hook:

#!/usr/bin/env bash
ps -ocommand= -p $PPID

I get the following:

$ git config alias.amd 'commit --amend'
$ git amd
/usr/local/bin/git commit --amend
[master 675d5e1] blargh
...

[sds]

Which OS are you running? On both CentOS 7 and Mac OS X 10.10.2 I get the following output using your script:

$ git amend
git amend

I've tested this with both bash 3.2.57(1) and zsh 5.0.5. It's possible there is some other setting that controls the output here?

[jawshooah]

Whoops, forgot that git was aliased to hub, which probably expands any aliases internally.

$ git amend
/usr/local/bin/git commit --amend

$ /usr/local/bin/git amend
/usr/local/bin/git amend

[sds]

Implemented in #167. Thanks!