[PATCH 1/2] Revert "check_repository_format_gently(): refuse extensions for old repositories"

From: Jonathan Nieder <jrnieder@gmail.com>
Date: Jul 16, 2020, 06:24 UTC
Message-ID: <20200716062429.GB3242764@google.com>
In-Reply-To: <20200716062054.GA3242764@google.com>

This reverts commit 14c7fa269e42df4133edd9ae7763b678ed6594cd.

The core.repositoryFormatVersion field was introduced in ab9cb76f661 (Repository format version check., 2005-11-25), providing a welcome bit of forward compatibility, thanks to some welcome analysis by Martin Atukunda. The semantics are simple: a repository with core.repositoryFormatVersion set to 0 should be comprehensible by all Git implementations in active use; and Git implementations should error out early instead of trying to act on Git repositories with higher core.repositoryFormatVersion values representing new formats that they do not understand.

A new repository format did not need to be defined until 00a09d57eb8 (introduce "extensions" form of core.repositoryformatversion, 2015-06-23). This provided a finer-grained extension mechanism for Git repositories. In a repository with core.repositoryFormatVersion set to 1, Git implementations can act on "extensions.*" settings that modify how a repository is interpreted. In repository format version 1, unrecognized extensions settings cause Git to error out.

What happens if a user sets an extension setting but forgets to increase the repository format version to 1? The extension settings were still recognized in that case; worse, unrecognized extensions settings do *not* cause Git to error out. So combining repository format version 0 with extensions settings produces in some sense the worst of both worlds.

To improve that situation, since 14c7fa269e4 (check_repository_format_gently(): refuse extensions for old repositories, 2020-06-05) Git instead ignores extensions in v0 mode. This way, v0 repositories get the historical (pre-2015) behavior and maintain compatibility with Git implementations that do not know about the v1 format. Unfortunately, users had been using this sort of configuration and this behavior change came to many as a surprise:

- users of "git config --worktree" that had followed its advice
  to enable extensions.worktreeConfig (without also increasing the
  repository format version) would find their worktree configuration
  no longer taking effect

- tools such as copybara[*] that had set extensions.partialClone in
  existing repositories (without also increasing the repository format
  version) would find that setting no longer taking effect

The behavior introduced in 14c7fa269e4 might be a good behavior if we were traveling back in time to 2015, but we're far too late. For some reason I thought that it was what had been originally implemented and that it had regressed. Apologies for not doing my research when 14c7fa269e4 was under development.

Let's return to the behavior we've had since 2015: always act on extensions.* settings, regardless of repository format version. While we're here, include some tests to describe the effect on the "upgrade repository version" code path.

[*] https://github.com/google/copybara/commit/ca76c0b1e13c4e36448d12c2aba4a5d9d98fb6e7

Reported-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
---
 setup.c                  | 12 +++---------
 t/t0410-partial-clone.sh | 15 +++++++++++++--
 2 files changed, 16 insertions(+), 11 deletions(-)

diff --git a/setup.c b/setup.c
index dbac2eabe8f..87bf0112cf3 100644
--- a/setup.c
+++ b/setup.c
@@ -507,15 +507,9 @@ static int check_repository_format_gently(const char *gitdir, struct repository_
 		die("%s", err.buf);
 	}
 
-	if (candidate->version >= 1) {
-		repository_format_precious_objects = candidate->precious_objects;
-		set_repository_format_partial_clone(candidate->partial_clone);
-		repository_format_worktree_config = candidate->worktree_config;
-	} else {
-		repository_format_precious_objects = 0;
-		set_repository_format_partial_clone(NULL);
-		repository_format_worktree_config = 0;
-	}
+	repository_format_precious_objects = candidate->precious_objects;
+	set_repository_format_partial_clone(candidate->partial_clone);
+	repository_format_worktree_config = candidate->worktree_config;
 	string_list_clear(&candidate->unknown_extensions, 0);
 
 	if (repository_format_worktree_config) {
diff --git a/t/t0410-partial-clone.sh b/t/t0410-partial-clone.sh
index 463dc3a8be0..51d1eba6050 100755
--- a/t/t0410-partial-clone.sh
+++ b/t/t0410-partial-clone.sh
@@ -42,14 +42,25 @@ test_expect_success 'convert shallow clone to partial clone' '
 	test_cmp_config -C client 1 core.repositoryformatversion
 '
 
-test_expect_success 'convert shallow clone to partial clone must fail with any extension' '
+test_expect_success 'converting to partial clone fails with noop extension' '
 	rm -fr server client &&
 	test_create_repo server &&
 	test_commit -C server my_commit 1 &&
 	test_commit -C server my_commit2 1 &&
 	git clone --depth=1 "file://$(pwd)/server" client &&
 	test_cmp_config -C client 0 core.repositoryformatversion &&
-	git -C client config extensions.partialclone origin &&
+	git -C client config extensions.noop true &&
+	test_must_fail git -C client fetch --unshallow --filter="blob:none"
+'
+
+test_expect_success 'converting to partial clone fails with unrecognized extension' '
+	rm -fr server client &&
+	test_create_repo server &&
+	test_commit -C server my_commit 1 &&
+	test_commit -C server my_commit2 1 &&
+	git clone --depth=1 "file://$(pwd)/server" client &&
+	test_cmp_config -C client 0 core.repositoryformatversion &&
+	git -C client config extensions.nonsense true &&
 	test_must_fail git -C client fetch --unshallow --filter="blob:none"
 '

-- 
2.28.0.rc0.105.gf9edc3c819