builtin/hash-object: fix uninitialized hash function

The git-hash-object(1) command allows users to hash an object even
without a repository. Starting with c8aed5e (repository: stop setting
SHA1 as the default object hash, 2024-05-07), this will make us hit an
uninitialized hash function, which subsequently leads to a segfault.

Fix this by falling back to SHA-1 explicitly when running outside of
a Git repository. Users can use GIT_DEFAULT_HASH environment to
specify what hash algorithm they want, so arguably this code should
not be needed.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

builtin/hash-object.c

@@ -123,6 +123,9 @@ int cmd_hash_object(int argc, const char **argv, const char *prefix)
 	else
 		prefix = setup_git_directory_gently(&nongit);
 
+	if (nongit && !the_hash_algo)
+		repo_set_hash_algo(the_repository, GIT_HASH_SHA1);
+
 	if (vpath && prefix) {
 		vpath_free = prefix_filename(prefix, vpath);
 		vpath = vpath_free;

t/t1007-hash-object.sh

@@ -260,4 +260,10 @@ test_expect_success '--literally with extra-long type' '
 	echo example | git hash-object -t $t --literally --stdin
 '
 
+test_expect_success '--stdin outside of repository (uses SHA-1)' '
+	nongit git hash-object --stdin <hello >actual &&
+	echo "$(test_oid --hash=sha1 hello)" >expect &&
+	test_cmp expect actual
+'
+
 test_done

t/t1517-outside-repo.sh

@@ -29,7 +29,7 @@ test_expect_success 'compute a patch-id outside repository (uses SHA-1)' '
 	test_cmp patch-id.expect patch-id.actual
 '
 
-test_expect_failure 'hash-object outside repository (uses SHA-1)' '
+test_expect_success 'hash-object outside repository (uses SHA-1)' '
 	nongit env GIT_DEFAULT_HASH=sha1 \
 		git hash-object --stdin <sample.patch >hash.expect &&
 	nongit \