[Outreachy PATCH v3 1/2] parse-options: answer --git-completion-helper before command runs

fast-import parses its options only after reading the "feature"
lines from its input, and it creates a temporary pack file before
that. So "git fast-import --git-completion-helper" waits for input,
and each run leaves a tmp_pack_* file behind.

bash completion runs the helper with the terminal as input, so
"git fast-import --<TAB>" would freeze once fast-import is no
longer marked with the NO_PARSEOPT flag.

Since show_usage_with_options_if_asked() already answers -h and
--help-all before a command does any work, it now also answers
the completion helper, so fast-import and other commands that
call it reply without reading input.

Reported-by: René Scharfe <redacted>
Helped-by: Junio C Hamano [off-list ref]
Signed-off-by: Tanga Joris <redacted>
---
Changes since v2:
 - Moved the check from fast-import into
   show_usage_with_options_if_asked() in parse-options,
   as Junio suggested.
 - 2/2 is unchanged

 parse-options.c        | 6 ++++++
 t/t9300-fast-import.sh | 8 ++++++++
 2 files changed, 14 insertions(+)

diff --git a/parse-options.c b/parse-options.c
index 283a0f1dc2..f2a23b79a4 100644
--- a/parse-options.c
+++ b/parse-options.c

@@ -1536,6 +1536,12 @@ void show_usage_with_options_if_asked(int ac, const char **av,
 			usage_with_options_internal(NULL, usagestr, opts,
 						    USAGE_FULL, USAGE_TO_STDOUT);
 			exit(0);
+		} else if (!strcmp(av[1], "--git-completion-helper")) {
+			show_gitcomp(opts, 0);
+			exit(0);
+		} else if (!strcmp(av[1], "--git-completion-helper-all")) {
+			show_gitcomp(opts, 1);
+			exit(0);
 		}
 	}
 }

diff --git a/t/t9300-fast-import.sh b/t/t9300-fast-import.sh
index 73c6e3d448..99d7843217 100755
--- a/t/t9300-fast-import.sh
+++ b/t/t9300-fast-import.sh

@@ -2914,6 +2914,14 @@ test_expect_success 'R: blob appears only once' '
 	test 1 = $n
 '
 
+test_expect_success 'R: --git-completion-helper does not read from stdin' '
+	find .git/objects/pack -name "tmp_pack_*" >before &&
+	echo garbage | git fast-import --git-completion-helper >out &&
+	test_grep -e "--date-format=" out &&
+	find .git/objects/pack -name "tmp_pack_*" >after &&
+	test_cmp before after
+'
+
 ###
 ### series S (mark and path parsing)
 ###

-- 
2.43.0