read-cache: make do_read_index() always set up istate->repo

If there is no index file, e.g., because the repository has just been
created, we return zero early (unless `must_exist` makes us die
instead.)

This early return means we do not set up `istate->repo`. With
`core.untrackedCache=true`, the recent e6a6535 ("untracked-cache:
support '--untracked-files=all' if configured", 2022-03-31) will
eventually pass down `istate->repo` as a null pointer to
`repo_config_get_string()`, causing a segmentation fault.

If we do hit this early return, set up `istate->repo` similar to when we
actually read the index.

Reported-by: Joey Hess <[email protected]>
Signed-off-by: Martin Ågren <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Martin Ågren

read-cache.c

@@ -2268,8 +2268,11 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
 	istate->timestamp.nsec = 0;
 	fd = open(path, O_RDONLY);
 	if (fd < 0) {
-		if (!must_exist && errno == ENOENT)
+		if (!must_exist && errno == ENOENT) {
+			if (!istate->repo)
+				istate->repo = the_repository;
 			return 0;
+		}
 		die_errno(_("%s: index file open failed"), path);
 	}
 

t/t7063-status-untracked-cache.sh

@@ -985,4 +985,9 @@ test_expect_success '"status" after file replacement should be clean with UC=fal
 	status_is_clean
 '
 
+test_expect_success 'empty repo (no index) and core.untrackedCache' '
+	git init emptyrepo &&
+	git -C emptyrepo -c core.untrackedCache=true write-tree
+'
+
 test_done