[PATCH v2 04/12] reftable/stack: guard against NULL list_file in stack_destroy

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

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

When reftable_new_stack() fails partway through initialization
(e.g., reftable_buf_addstr returns an OOM error before
reftable_buf_detach assigns p->list_file), it jumps to the error
path which calls reftable_stack_destroy(p). At that point,
p->list_file is still NULL because the detach never happened.

reftable_stack_destroy() passes st->list_file unconditionally to
read_lines(), which calls open(filename, O_RDONLY). Passing NULL
to open() is undefined behavior and will typically crash.

Guard the read_lines() call with a NULL check on st->list_file.
When list_file is NULL, there are no table files to clean up
anyway, so skipping read_lines is the correct behavior.

Pointed out by Coverity.

Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 reftable/stack.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/reftable/stack.c b/reftable/stack.c
index 1fba96ddb3..3fc3c0b2d1 100644
--- a/reftable/stack.c
+++ b/reftable/stack.c
@@ -171,7 +171,8 @@ void reftable_stack_destroy(struct reftable_stack *st)
 		st->merged = NULL;
 	}
 
-	err = read_lines(st->list_file, &names);
+	if (st->list_file)
+		err = read_lines(st->list_file, &names);
 	if (err < 0) {
 		REFTABLE_FREE_AND_NULL(names);
 	}
-- 
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