[PATCH 1/4] merge-tree: fail with a non-zero exit code on missing tree objects

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

When `git merge-tree` encounters a missing tree object, it should error
out and not continue quietly as if nothing had happened.

However, as of time of writing, `git merge-tree` _does_ continue, and
then offers the empty tree as result.

Let's fix this.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 merge-ort.c                      |  7 ++++---
 t/t4301-merge-tree-write-tree.sh | 10 ++++++++++
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/merge-ort.c b/merge-ort.c
index 6491070d965..c37fc035f13 100644
--- a/merge-ort.c
+++ b/merge-ort.c
@@ -1659,9 +1659,10 @@ static int collect_merge_info(struct merge_options *opt,
 	info.data = opt;
 	info.show_all_errors = 1;
 
-	parse_tree(merge_base);
-	parse_tree(side1);
-	parse_tree(side2);
+	if (parse_tree(merge_base) < 0 ||
+	    parse_tree(side1) < 0 ||
+	    parse_tree(side2) < 0)
+		return -1;
 	init_tree_desc(t + 0, merge_base->buffer, merge_base->size);
 	init_tree_desc(t + 1, side1->buffer, side1->size);
 	init_tree_desc(t + 2, side2->buffer, side2->size);
diff --git a/t/t4301-merge-tree-write-tree.sh b/t/t4301-merge-tree-write-tree.sh
index 7d0fa74da74..4ea1b74445d 100755
--- a/t/t4301-merge-tree-write-tree.sh
+++ b/t/t4301-merge-tree-write-tree.sh
@@ -950,5 +950,15 @@ test_expect_success '--merge-base with tree OIDs' '
 	git merge-tree --merge-base=side1^^{tree} side1^{tree} side3^{tree} >with-trees &&
 	test_cmp with-commits with-trees
 '
+test_expect_success 'error out on missing tree objects' '
+	git init --bare missing-tree.git &&
+	(
+		git rev-list side3 &&
+		git rev-parse side3^:
+	) | git pack-objects missing-tree.git/objects/pack/side3-tree-is-missing &&
+	side3=$(git rev-parse side3) &&
+	test_must_fail git --git-dir=missing-tree.git merge-tree $side3^ $side3 >actual &&
+	test_must_be_empty actual
+'
 
 test_done
-- 
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