[PATCH] completion: do not cache if --git-completion-helper fails

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

"git <cmd> --git-completion-helper" could fail if the command checks for
a repo before parse_options(). If the result is cached, later on when
the user moves to a worktree with repo, tab completion will still fail.

Avoid this by detecting errors and not cache the completion output. We
can try again and hopefully succeed next time (e.g. when a repo is
found).

Of course if --git-completion-helper fails permanently because of other
reasons (*), this will slow down completion. But I don't see any better
option to handle that case.

(*) one of those cases is if __gitcomp_builtin is called on a command
  that does not support --git-completion-helper. And we do have a
  generic call

    __git_complete_common "$command"

  but this case is protected with __git_support_parseopt_helper so we're
  good.

Reported-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 contrib/completion/git-completion.bash | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index 9f71bcde96..a515de8501 100644
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@ -398,13 +398,15 @@ __gitcomp_builtin ()
 	eval "options=\$$var"
 
 	if [ -z "$options" ]; then
+		local nocache=
 		# leading and trailing spaces are significant to make
 		# option removal work correctly.
-		options=" $incl $(__git ${cmd/_/ } --git-completion-helper) "
+		options=" $incl $(__git ${cmd/_/ } --git-completion-helper) " || nocache=t
+
 		for i in $excl; do
 			options="${options/ $i / }"
 		done
-		eval "$var=\"$options\""
+		test -n "$nocache" || eval "$var=\"$options\""
 	fi
 
 	__gitcomp "$options"
-- 
2.22.0.rc0.322.g2b0371e29a