[PATCH v2 1/4] setup: add an escape hatch for "no more default hash algorithm" change

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

Partially revert c8aed5e8 (repository: stop setting SHA1 as the
default object hash, 2024-05-07), to keep end-user systems still
broken when we have gap in our test coverage but yet give them an
escape hatch to set the GIT_TEST_DEFAULT_HASH_IS_SHA1=1 environment
variable to revert to the previous behaviour.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 repository.c  | 24 ++++++++++++++++++++++++
 t/test-lib.sh |  4 ++++
 2 files changed, 28 insertions(+)

diff --git a/repository.c b/repository.c
index 15c10015b0..6f67966e35 100644
--- a/repository.c
+++ b/repository.c
@@ -26,6 +26,30 @@ void initialize_repository(struct repository *repo)
 	repo->parsed_objects = parsed_object_pool_new();
 	ALLOC_ARRAY(repo->index, 1);
 	index_state_init(repo->index, repo);
+
+	/*
+	 * Unfortunately, we need to keep this hack around for the time being:
+	 *
+	 *   - Not setting up the hash algorithm for `the_repository` leads to
+	 *     crashes because `the_hash_algo` is a macro that expands to
+	 *     `the_repository->hash_algo`. So if Git commands try to access
+	 *     `the_hash_algo` without a Git directory we crash.
+	 *
+	 *   - Setting up the hash algorithm to be SHA1 by default breaks other
+	 *     commands when running with SHA256.
+	 *
+	 * This is another point in case why having global state is a bad idea.
+	 * Eventually, we should remove this hack and stop setting the hash
+	 * algorithm in this function altogether. Instead, it should only ever
+	 * be set via our repository setup procedures. But that requires more
+	 * work.
+	 *
+	 * As we discover the code paths that need fixing, we may remove this
+	 * code completely, but we are not there yet.
+	 */
+	if (repo == the_repository &&
+	    git_env_bool("GIT_TEST_DEFAULT_HASH_IS_SHA1", 0))
+		repo_set_hash_algo(repo, GIT_HASH_SHA1);
 }
 
 static void expand_base_dir(char **out, const char *in,
diff --git a/t/test-lib.sh b/t/test-lib.sh
index 79d3e0e7d9..36d311fb4a 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -542,6 +542,10 @@ export EDITOR
 
 GIT_DEFAULT_HASH="${GIT_TEST_DEFAULT_HASH:-sha1}"
 export GIT_DEFAULT_HASH
+
+GIT_TEST_DEFAULT_HASH_IS_SHA1=0
+export GIT_TEST_DEFAULT_HASH_IS_SHA1
+
 GIT_DEFAULT_REF_FORMAT="${GIT_TEST_DEFAULT_REF_FORMAT:-files}"
 export GIT_DEFAULT_REF_FORMAT
 GIT_TEST_MERGE_ALGORITHM="${GIT_TEST_MERGE_ALGORITHM:-ort}"
-- 
2.45.0-145-g3e4a232f6e