replay: fail gracefully when a merge input is unreadable

When objects involved in the merge cannot be read, the merge machinery
will return early with result.clean = -1, and result.tree left as NULL.
pick_regular_commit() tested only "if (!result->clean)", ignoring the
case where "clean < 0".  That causes the code to try to use
result->tree, resulting in a SIGSEGV.

Handle clean < 0 explicitly; the merge machinery will already have printed
messages such as "Could not read <object>" and "collecting merge info
failed for trees...", so we don't need to add much detail beyond the
fact that the merge failed.

Signed-off-by: Elijah Newren <newren@gmail.com>

Author: newren

replay.c

@@ -327,6 +327,13 @@ static struct commit *pick_regular_commit(struct repository *repo,
 	merge_opt->ancestor = NULL;
 	merge_opt->branch2 = NULL;
 
+	if (result->clean < 0) {
+		error(_("merge of %s onto %s failed"),
+		      oid_to_hex(&pickme->object.oid),
+		      oid_to_hex(&replayed_base->object.oid));
+		return NULL;
+	}
+
 	if (!result->clean)
 		return NULL;
 

t/t3650-replay-basics.sh

@@ -565,4 +565,38 @@ test_expect_success '--onto with --ref rejects multiple revision ranges' '
 	test_grep "cannot be used with multiple revision ranges" err
 '
 
+test_expect_success 'replay fails without segfault when objects are missing' '
+	test_when_finished "rm -fr unreadable" &&
+	git init unreadable &&
+	(
+		cd unreadable &&
+
+		test_write_lines l1 l2 l3 l4 l5 l6 l7 l8 >f &&
+		git add f &&
+		git commit -m base &&
+		git branch base &&
+
+		test_write_lines l1 l2 l3 l4 l5 l6 l7 CHANGED >f &&
+		git commit -am side &&
+		git branch side &&
+
+		git switch -c onto base &&
+		test_write_lines CHANGED l2 l3 l4 l5 l6 l7 l8 >f &&
+		git commit -am onto &&
+
+		# The replay works while every object is readable.
+		git replay --onto onto base..side &&
+
+		# Removing the onto tree makes parse_tree() fail during the
+		# incore merge, driving clean < 0 with a NULL result tree.
+		onto_tree=$(git rev-parse onto^{tree}) &&
+		obj=$(test_oid_to_path "$onto_tree") &&
+		mv .git/objects/${obj} saved-tree &&
+
+		# Ensure replay gracefully handles the missing object
+		test_must_fail git replay --onto onto base..side 2>err &&
+		test_grep "Could not read\|collecting merge info failed" err
+	)
+'
+
 test_done