Subject: Re: [PATCH] git-difftool--helper: honor `--trust-exit-code` with `--dir-diff`

From: Junio C Hamano <[email-masked]>
Date: Fri, 16 Feb 2024 10:12:32 -0800

Patrick Steinhardt <[email-masked]> writes:

> The `--trust-exit-code` option for git-diff-tool(1) was introduced via
> 2b52123fcf (difftool: add support for --trust-exit-code, 2014-10-26).
> When set, it makes us return the exit code of the invoked diff tool when
> diffing multiple files. This patch didn't change the code path where
> `--dir-diff` was passed because we already returned the exit code of the
> diff tool unconditionally in that case.
>
> This was changed a month later via c41d3fedd8 (difftool--helper: add
> explicit exit statement, 2014-11-20), where an explicit `exit 0` was
> added to the end of git-difftool--helper.sh. While the stated intent of
> that commit was merely a cleanup, it had the consequence that we now
> to ignore the exit code of the diff tool when `--dir-diff` was set. This
> change in behaviour is thus very likely an unintended side effect of
> this patch.
>
> Now there are two ways to fix this:
>
>   - We can either restore the original behaviour, which unconditionally
>     returned the exit code of the diffing tool when `--dir-diff` is
>     passed.
>
>   - Or we can make the `--dir-diff` case respect the `--trust-exit-code`
>     flag.
>
> The fact that we have been ignoring exit codes for 7 years by now makes
> me rather lean towards the latter option. Furthermore, respecting the
> flag in one case but not the other would needlessly make the user
> interface more complex.
>
> Fix the bug so that we also honor `--trust-exit-code` for dir diffs and
> adjust the documentation accordingly.
>
> Reported-by: Jean-Rémy Falleri <[email-masked]>
> Signed-off-by: Patrick Steinhardt <[email-masked]>
> ---

Sounds sensible.

The last time David was on list seems to be in April 2023; just in 
case let's CC him for an Ack (or something else).

>  Documentation/git-difftool.txt |  1 -
>  git-difftool--helper.sh        | 14 +++++
>  t/t7800-difftool.sh            | 99 ++++++++++++++++++----------------
>  3 files changed, 68 insertions(+), 46 deletions(-)
>
> diff --git a/Documentation/git-difftool.txt b/Documentation/git-difftool.txt
> index c05f97aca9..a616f8b2e6 100644
> --- a/Documentation/git-difftool.txt
> +++ b/Documentation/git-difftool.txt
> @@ -105,7 +105,6 @@ instead.  `--no-symlinks` is the default on Windows.
>  	`merge.tool` until a tool is found.
>  
>  --[no-]trust-exit-code::
> -	'git-difftool' invokes a diff tool individually on each file.
>  	Errors reported by the diff tool are ignored by default.
>  	Use `--trust-exit-code` to make 'git-difftool' exit when an
>  	invoked diff tool returns a non-zero exit code.
> diff --git a/git-difftool--helper.sh b/git-difftool--helper.sh
> index e4e820e680..09d8542917 100755
> --- a/git-difftool--helper.sh
> +++ b/git-difftool--helper.sh
> @@ -91,6 +91,20 @@ then
>  	# ignore the error from the above --- run_merge_tool
>  	# will diagnose unusable tool by itself
>  	run_merge_tool "$merge_tool" false
> +
> +	status=$?
> +	if test $status -ge 126
> +	then
> +		# Command not found (127), not executable (126) or
> +		# exited via a signal (>= 128).
> +		exit $status
> +	fi

So these errors spawning the tool backend are always reported,
regardless of the trust-exit-code settings.  OK.

> +	if test "$status" != 0 &&
> +		test "$GIT_DIFFTOOL_TRUST_EXIT_CODE" = true
> +	then
> +		exit $status
> +	fi

I found this somehow harder to reason about than necessary.  Just

	if test "$GIT_DIFFTOOL_TRUST_EXIT_CODE" = true
	then
		exit $status
	fi

would have been a more straight-forward expression of what we want
to happen here, i.e. "if we are told to report the tool's exit
status, we do so, regardless of what the exit status is".

Not that the construct in your patch is wrong---we will exit with 0
at the end even when "trust-exit-code" thing is true and the tool
returned success.