split-index: accept that a base index can be empty

We are about to fix an ancient bug where `do_read_index()` pretended
that the index was not initialized when there are no index entries.

Before the `index_state` structure gained the `initialized` flag in
913e0e99b6a (unpack_trees(): protect the handcrafted in-core index from
read_cache(), 2008-08-23), that was the best we could do (even if it was
incorrect: it is totally possible to read a Git index file that contains
no index entries).

This pattern was repeated also in 998330ac2e7 (read-cache: look for
shared index files next to the index, too, 2021-08-26), which we fix
here by _not_ mistaking an empty base index for a missing
`sharedindex.*` file.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/read-cache.c b/read-cache.c
index 1ac4defff3303b64bb4fb8aa21110c7ca15ca182..d0a3c9082b18bae56041f39b155f08991a440da2 100644 (file)
--- a/read-cache.c
+++ b/read-cache.c
@@ -2500,12 +2500,14 @@ int read_index_from(struct index_state *istate, const char *path,
 
        base_oid_hex = oid_to_hex(&split_index->base_oid);
        base_path = xstrfmt("%s/sharedindex.%s", gitdir, base_oid_hex);
-       trace2_region_enter_printf("index", "shared/do_read_index",
-                                  the_repository, "%s", base_path);
-       ret = do_read_index(split_index->base, base_path, 0);
-       trace2_region_leave_printf("index", "shared/do_read_index",
-                                  the_repository, "%s", base_path);
-       if (!ret) {
+       if (file_exists(base_path)) {
+               trace2_region_enter_printf("index", "shared/do_read_index",
+                                       the_repository, "%s", base_path);
+
+               ret = do_read_index(split_index->base, base_path, 0);
+               trace2_region_leave_printf("index", "shared/do_read_index",
+                                       the_repository, "%s", base_path);
+       } else {
                char *path_copy = xstrdup(path);
                char *base_path2 = xstrfmt("%s/sharedindex.%s",
                                           dirname(path_copy), base_oid_hex);