[PATCH v4 4/5] builtin/hash-object: fix uninitialized hash function

[Junio C Hamano <gitster@pobox.com>]

From: Patrick Steinhardt <ps@pks.im>

The git-hash-object(1) command allows users to hash an object even
without a repository. Starting with c8aed5e8da (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 <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 builtin/hash-object.c   | 3 +++
 t/t1007-hash-object.sh  | 6 ++++++
 t/t1517-outside-repo.sh | 2 +-
 3 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/builtin/hash-object.c b/builtin/hash-object.c
index 82ca6d2bfd..c767414a0c 100644
--- a/builtin/hash-object.c
+++ b/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;
diff --git a/t/t1007-hash-object.sh b/t/t1007-hash-object.sh
index 64aea38486..4c138c6ca4 100755
--- a/t/t1007-hash-object.sh
+++ b/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' '
+	nongit git hash-object --stdin <hello >actual &&
+	echo "$(test_oid hello)" >expect &&
+	test_cmp expect actual
+'
+
 test_done
diff --git a/t/t1517-outside-repo.sh b/t/t1517-outside-repo.sh
index ac5f3191cc..854bb8f343 100755
--- a/t/t1517-outside-repo.sh
+++ b/t/t1517-outside-repo.sh
@@ -30,7 +30,7 @@ test_expect_success 'compute a patch-id outside repository' '
 	test_cmp patch-id.expect patch-id.actual
 '
 
-test_expect_failure 'hash-object outside repository' '
+test_expect_success 'hash-object outside repository' '
 	git hash-object --stdin <sample.patch >hash.expect &&
 	(
 		cd non-repo &&
-- 
2.45.0-145-g3e4a232f6e