The dumb HTTP transport tries to read the remote HEAD reference by
downloading the "HEAD" file and then parsing it via `http_fetch_ref()`.
This function will either parse the file as an object ID in case it is
exactly `the_hash_algo->hexsz` long, or otherwise it will check whether
the reference starts with "ref :" and parse it as a symbolic ref.

This is broken when parsing detached HEADs of a remote SHA256 repository
because we never update `the_hash_algo` to the discovered remote object
hash. Consequently, `the_hash_algo` will always be the fallback SHA1
hash algorithm, which will cause us to fail parsing HEAD altogteher when
it contains a SHA256 object ID.

Fix this issue by setting up `the_hash_algo` via `repo_set_hash_algo()`.
While at it, let's make the expected SHA1 fallback explicit in our code,
which also addresses an upcoming issue where we are going to remove the
SHA1 fallback for `the_hash_algo`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 remote-curl.c              | 19 ++++++++++++++++++-
 t/t5550-http-fetch-dumb.sh | 15 +++++++++++++++
 2 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/remote-curl.c b/remote-curl.c
index 0b6d7815fd..004b707fdf 100644
--- a/remote-curl.c
+++ b/remote-curl.c
@@ -266,12 +266,23 @@ static struct ref *parse_git_refs(struct discovery *heads, int for_push)
 	return list;
 }
 
+/*
+ * Try to detect the hash algorithm used by the remote repository when using
+ * the dumb HTTP transport. As dumb transports cannot tell us the object hash
+ * directly have to derive it from the advertised ref lengths.
+ */
 static const struct git_hash_algo *detect_hash_algo(struct discovery *heads)
 {
 	const char *p = memchr(heads->buf, '\t', heads->len);
 	int algo;
+
+	/*
+	 * In case the remote has no refs we have no way to reliably determine
+	 * the object hash used by that repository. In that case we simply fall
+	 * back to SHA1, which may or may not be correct.
+	 */
 	if (!p)
-		return the_hash_algo;
+		return &hash_algos[GIT_HASH_SHA1];
 
 	algo = hash_algo_by_length((p - heads->buf) / 2);
 	if (algo == GIT_HASH_UNKNOWN)
@@ -295,6 +306,12 @@ static struct ref *parse_info_refs(struct discovery *heads)
 		    "is this a git repository?",
 		    transport_anonymize_url(url.buf));
 
+	/*
+	 * Set the repository's hash algo to whatever we have just detected.
+	 * This ensures that we can correctly parse the remote references.
+	 */
+	repo_set_hash_algo(the_repository, hash_algo_by_ptr(options.hash_algo));
+
 	data = heads->buf;
 	start = NULL;
 	mid = data;
diff --git a/t/t5550-http-fetch-dumb.sh b/t/t5550-http-fetch-dumb.sh
index 4c3b32785d..5f16cbc58d 100755
--- a/t/t5550-http-fetch-dumb.sh
+++ b/t/t5550-http-fetch-dumb.sh
@@ -55,6 +55,21 @@ test_expect_success 'list refs from outside any repository' '
 	test_cmp expect actual
 '
 
+
+test_expect_success 'list detached HEAD from outside any repository' '
+	git clone --mirror "$HTTPD_DOCUMENT_ROOT_PATH/repo.git" \
+		"$HTTPD_DOCUMENT_ROOT_PATH/repo-detached.git" &&
+	git -C "$HTTPD_DOCUMENT_ROOT_PATH/repo-detached.git" \
+		update-ref --no-deref HEAD refs/heads/main &&
+	git -C "$HTTPD_DOCUMENT_ROOT_PATH/repo-detached.git" update-server-info &&
+	cat >expect <<-EOF &&
+	$(git rev-parse main)	HEAD
+	$(git rev-parse main)	refs/heads/main
+	EOF
+	nongit git ls-remote "$HTTPD_URL/dumb/repo-detached.git" >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'create password-protected repository' '
 	mkdir -p "$HTTPD_DOCUMENT_ROOT_PATH/auth/dumb/" &&
 	cp -Rf "$HTTPD_DOCUMENT_ROOT_PATH/repo.git" \
-- 
2.45.0-rc0