[PATCH v2 08/15] merge: handle NULL value for branch.<name>.mergeOptions

["Thomas Bachem via GitGitGadget" <gitgitgadget@gmail.com>]

From: Thomas Bachem <mail@thomasbachem.com>

A branch.<name>.mergeOptions without a value makes "git merge"
segfault on that branch: git_merge_config() passes the NULL value to
xstrdup().

Reject a missing value with config_error_nonbool(), as
git_config_string() already does for the other strings that
git_merge_config() reads.

Assisted-by: Claude Opus 5.5
Signed-off-by: Thomas Bachem <mail@thomasbachem.com>
---
 builtin/merge.c  | 2 ++
 t/t7600-merge.sh | 6 ++++++
 2 files changed, 8 insertions(+)

diff --git a/builtin/merge.c b/builtin/merge.c
index d7bf209114..c6f9356ccb 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -668,6 +668,8 @@ static int git_merge_config(const char *k, const char *v,
 	    skip_prefix(k, "branch.", &str) &&
 	    skip_prefix(str, branch, &str) &&
 	    !strcmp(str, ".mergeoptions")) {
+		if (!v)
+			return config_error_nonbool(k);
 		free(branch_mergeoptions);
 		branch_mergeoptions = xstrdup(v);
 		return 0;
diff --git a/t/t7600-merge.sh b/t/t7600-merge.sh
index 8f6109fb91..2da12d6662 100755
--- a/t/t7600-merge.sh
+++ b/t/t7600-merge.sh
@@ -552,6 +552,12 @@ test_expect_success 'merge c1 with c2 (squash in config)' '
 	test_cmp squash.1-5 .git/SQUASH_MSG
 '
 
+test_expect_success 'merge rejects branch.main.mergeOptions without a value' '
+	git reset --hard c1 &&
+	test_must_fail git -c branch.main.mergeOptions merge c2 2>err &&
+	test_grep "missing value for .branch.main.mergeoptions." err
+'
+
 test_debug 'git log --graph --decorate --oneline --all'
 
 test_expect_success 'override config option -n with --summary' '
-- 
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