remote-curl: fall back to default hash outside repo

When a remote helper like git-remote-http is invoked outside of a
repository (for example, by running git ls-remote in a non-git
directory), setup_git_directory_gently() leaves the_hash_algo
uninitialized as NULL.

If the user has a globally configured fetch refspec, remote-curl
attempts to parse it during initialization. Inside parse_refspec(),
it checks whether the LHS of the refspec is an exact OID by evaluating
llen == the_hash_algo->hexsz. Because the_hash_algo is NULL, this
results in a segmentation fault.

In 9e89dcb66a (builtin/ls-remote: fall back to SHA1 outside of a repo,
2024-08-02), we added a workaround to ls-remote to fall back to the
default hash algorithm to prevent exactly this type of crash when
parsing refspec capabilities. However, because remote-curl runs as a
separate process, it does not inherit that fallback and crashes anyway.

Instead of pushing a NULL-guard workaround down into parse_refspec(),
fix this by mirroring the ls-remote workaround directly in
remote-curl.c. If we are operating outside a repository, initialize
the_hash_algo to GIT_HASH_DEFAULT. This keeps the HTTP transport
consistent with non-HTTP transports that execute in-process, preventing
crashes without altering the generic refspec parsing logic.

Reported-by: Jo Liss <joliss@gmail.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: K Jayatheerth <jayatheerthkulkarni2005@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

remote-curl.c

diff --git a/remote-curl.c b/remote-curl.c
index 69f9194..a489cbe 100644
--- a/remote-curl.c
+++ b/remote-curl.c
@@ -1551,6 +1551,13 @@ int cmd_main(int argc, const char **argv)
 		goto cleanup;
 	}
 
+	/*
+	 * yuck, see 9e89dcb66a (builtin/ls-remote: fall back to SHA1 outside
+	 * of a repo, 2024-08-02)
+	 */
+	if (nongit)
+		repo_set_hash_algo(the_repository, GIT_HASH_DEFAULT);
+
 	options.verbosity = 1;
 	options.progress = !!isatty(2);
 	options.thin = 1;

t/t5551-http-fetch-smart.sh

diff --git a/t/t5551-http-fetch-smart.sh b/t/t5551-http-fetch-smart.sh
index 73cf531..a26b6c2 100755
--- a/t/t5551-http-fetch-smart.sh
+++ b/t/t5551-http-fetch-smart.sh
@@ -782,4 +782,11 @@
 	test_cmp expect actual
 '
 
+test_expect_success 'ls-remote outside repo does not segfault with fetch refspec' '
+	nongit git \
+		-c remote.origin.url="$HTTPD_URL/smart/repo.git" \
+		-c remote.origin.fetch=anything \
+		ls-remote origin
+'
+
 test_done