Re: [PATCH 1/2] replay: fail gracefully when a merge input is unreadable

From: Elijah Newren <redacted>

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 <redacted>
---
 replay.c                 |  7 +++++++
 t/t3650-replay-basics.sh | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+)

diff --git a/replay.c b/replay.c
index 463c900d6c..33e21b2032 100644
--- a/replay.c
+++ b/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;