Re: [PATCH] mergetools/meld: do not rely on the output of `meld --help`

[Junio C Hamano <gitster@pobox.com>]

Junio C Hamano <gitster@pobox.com> writes:

> This obviously breaks those who have happily been using their
> installed version of meld that understands and shows --output in the
> help text.  Is that a minority that is rapidly diminishing?
>
> I would understand it if the change were
>
>  - a configuration tells us to use or not use --output; when it is
>    set, then we do not try auto-detect by reading --help output
>
>  - when that new configuration is not set, we keep the current code
>    to read --help output, which may fail for recent meld but that is
>    not a regression.
>
> When versions of meld that support --output but do not mention it in
> their --help text are overwhelming majority, we would want to flip
> the fallback codepath from "read --help and decide" to "assume that
> --output can be used", but I do not know if now is the time to do
> so.

In other words, I am wondering if a milder fix would be along this
line of change instead.  Older versions seem to list --output
explicitly, and we assume newer ones including the one reported by
Andrey begin their output like so:

    $ meld --help
    Usage:
      meld [OPTION...]

hence we catch either of these patterns.

 mergetools/meld | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mergetools/meld b/mergetools/meld
index cb672a5..b6169c9 100644
--- a/mergetools/meld
+++ b/mergetools/meld
@@ -23,8 +23,12 @@ check_meld_for_output_version () {
 	meld_path="$(git config mergetool.meld.path)"
 	meld_path="${meld_path:-meld}"
 
-	if "$meld_path" --help 2>&1 | grep -e --output >/dev/null
+	if meld_has_output_option="$(git config --bool mergetool.meld.hasOutput)"
 	then
+		: use whatever is configured
+	elif "$meld_path" --help 2>&1 | grep -e '--output=' -e '\[OPTION\.\.\.\]' /dev/null
+	then
+		: old ones explicitly listed --output and new ones just say OPTION...
 		meld_has_output_option=true
 	else
 		meld_has_output_option=false