[PATCH] merge: avoid write merge state when unable to write index

["Kyle Zhao via GitGitGadget" <gitgitgadget@gmail.com>]

From: Kyle Zhao <kylezhao@tencent.com>

Currently, when index.lock exists, if a merge occurs, the merge state
will be written and the index will be unchanged.

If the user exec "git commit" instead of "git merge --abort" after this,
the commit will be successful and all modifications from the source
branch will be lost.

Signed-off-by: Kyle Zhao <kylezhao@tencent.com>
---
    merge: avoid write merge state when unable to write index
    
    In some of our monorepos, code is sometimes lost after merging.
    
    After investigation, we discovered the problem.
    
    This happens if we perform "git pull" or "git merge" when another git
    process is writing to the index, especially in a monorepo (because its
    index will be larger).
    
    How to reproduce:
    
    git init demo
    cd demo
    touch 1.txt && git add . && git commit -m "1"
    git checkout -b source-branch
    touch 2.txt && git add . && git commit -m "2"
    echo "1" >> 1.txt && git add . && git commit -m "3"
    touch .git/index.lock
    git merge source-branch
    rm .git/index.lock
    git commit -m "4"
    
    
    Then the modifications from the source branch are lost.
    
    Regards, Kyle
    
    cc: Elijah Newren newren@gmail.com cc: Ævar Arnfjörð Bjarmason
    avarab@gmail.com cc: Junio C Hamano gitster@pobox.com

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1731%2Fkeyu98%2Fkz%2Ffix-merge-when-index-lock-exists-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1731/keyu98/kz/fix-merge-when-index-lock-exists-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1731

 builtin/merge.c  |  6 ++++--
 t/t7600-merge.sh | 10 ++++++++++
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/builtin/merge.c b/builtin/merge.c
index 6a6d3798858..80e3438be25 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -698,8 +698,10 @@ static int try_merge_strategy(const char *strategy, struct commit_list *common,
 
 	if (repo_refresh_and_write_index(the_repository, REFRESH_QUIET,
 					 SKIP_IF_UNCHANGED, 0, NULL, NULL,
-					 NULL) < 0)
-		return error(_("Unable to write index."));
+					 NULL) < 0) {
+		error(_("Unable to write index."));
+		return 2;
+	}
 
 	if (!strcmp(strategy, "recursive") || !strcmp(strategy, "subtree") ||
 	    !strcmp(strategy, "ort")) {
diff --git a/t/t7600-merge.sh b/t/t7600-merge.sh
index e5ff073099a..f03709ea4be 100755
--- a/t/t7600-merge.sh
+++ b/t/t7600-merge.sh
@@ -236,6 +236,16 @@ test_expect_success 'merge c1 with c2' '
 	verify_parents $c1 $c2
 '
 
+test_expect_success 'merge c1 with c2 when index.lock exists' '
+	test_when_finished rm .git/index.lock &&
+	git reset --hard c1 &&
+	touch .git/index.lock &&
+	test_must_fail git merge c2 &&
+	test_path_is_missing .git/MERGE_HEAD &&
+	test_path_is_missing .git/MERGE_MODE &&
+	test_path_is_missing .git/MERGE_MSG
+'
+
 test_expect_success 'merge --squash c3 with c7' '
 	git reset --hard c3 &&
 	test_must_fail git merge --squash c7 &&

base-commit: 19fe900cfce8096b7645ec9611a0b981f6bbd154
-- 
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