[PATCH v2 8/9] t: allow specifying compatibility hash

From: brian m. carlson <sandals@crustytoothpaste.net>
Date: Oct 2, 2025, 22:38 UTC
Message-ID: <20251002223855.1022847-9-sandals@crustytoothpaste.net>
In-Reply-To: <20251002223855.1022847-1-sandals@crustytoothpaste.net>

We want to specify a compatibility hash for testing interactions for SHA-256 repositories where we have SHA-1 compatibility enabled. Allow the user to specify this scenario in the test suite by setting GIT_TEST_DEFAULT_HASH to "sha256:sha1".

Note that this will get passed into GIT_DEFAULT_HASH, which Git itself does not presently support. However, we will support this in a future commit.

Since we'll now want to know the value for a specific version, let's add the ability to specify either the storage hash (in this case, SHA-256) or the compatibility hash (SHA-1). We use a different value for the compatibility hash that will be enabled for all repositories (test_repo_compat_hash_algo) versus the one that is used individually in some tests (test_compat_hash_algo), since we want to still run those individual tests without requiring that the testsuite be run fully in a compatibility mode.

In some cases, we'll need to adjust our test suite to work in a proper way with a compatibility hash. For example, in such a case, we'll only use pack index v3, since v1 and v2 lack support for multiple algorithms. Since we won't want to write those older formats, we'll need to skip tests that do so. Let's add a COMPAT_HASH prerequisite for this purpose.

Finally, in this scenario, we can no longer rely on having broken objects work since we lack compatibility mappings to rewrite objects in the repository. Add a prerequisite, BROKEN_OBJECTS, that we define in terms of COMPAT_HASH and checks to see if creating deliberately broken objects is possible, so that we can disable these tests if not.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
---
 t/test-lib-functions.sh |  9 +++++++--
 t/test-lib.sh           | 13 +++++++++++++
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index a28de7b19b..52d7759bf5 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -1708,11 +1708,16 @@ test_set_hash () {
 # Detect the hash algorithm in use.
 test_detect_hash () {
 	case "${GIT_TEST_DEFAULT_HASH:-$GIT_TEST_BUILTIN_HASH}" in
-	"sha256")
+	*:*)
+	    test_hash_algo="${GIT_TEST_DEFAULT_HASH%%:*}"
+	    test_compat_hash_algo="${GIT_TEST_DEFAULT_HASH##*:}"
+	    test_repo_compat_hash_algo="$test_compat_hash_algo"
+	    ;;
+	sha256)
 	    test_hash_algo=sha256
 	    test_compat_hash_algo=sha1
 	    ;;
-	*)
+	sha1)
 	    test_hash_algo=sha1
 	    test_compat_hash_algo=sha256
 	    ;;
diff --git a/t/test-lib.sh b/t/test-lib.sh
index 621cd31ae1..9eb79324ee 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -1917,6 +1917,19 @@ test_lazy_prereq DEFAULT_HASH_ALGORITHM '
 test_lazy_prereq DEFAULT_REPO_FORMAT '
 	test_have_prereq SHA1,REFFILES
 '
+# BROKEN_OBJECTS is a test whether we can write deliberately broken objects and
+# expect them to work.  When running using SHA-256 mode with SHA-1
+# compatibility, we cannot write such objects because there's no SHA-1
+# compatibility value for a nonexistent object.
+test_lazy_prereq BROKEN_OBJECTS '
+	! test_have_prereq COMPAT_HASH
+'
+
+# COMPAT_HASH is a test if we're operating in a repository with SHA-256 with
+# SHA-1 compatibility.
+test_lazy_prereq COMPAT_HASH '
+	test -n "$test_repo_compat_hash_algo"
+'
 
 # Ensure that no test accidentally triggers a Git command
 # that runs the actual maintenance scheduler, affecting a user's