submodule: add extension to encode gitdir paths

Add a submoduleEncoding extension which fixes filesystem collisions by
encoding gitdir paths. At a high level, this implements a mechanism to
encode -> validate -> retry until a working gitdir path is found.

Credit goes to Junio for coming up with this design: encoding is only
applied when necessary, e.g. uppercase characters are encoded only on
case-folding filesystems and only if a real conflict is detected.

To make this work, we rely on the submodule.<name>.gitdir config as the
single source of truth for gitidir paths: the config is always set when
the extension is enabled. Users who care about gitdir paths are expected
to get/set the config and not the underlying encoding implementation.

This commit adds the basic encoding logic which addresses nested gitdirs.
The next commit fixes case-folding, the next commit fixes names longer
than NAME_MAX. The idea is the encoding can be improved over time in a
way which is transparent to users.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Suggested-by: Phillip Wood <phillip.wood123@gmail.com>
Suggested-by: Patrick Steinhardt <ps@pks.im>
Based-on-patch-by: Brandon Williams <bwilliams.eng@gmail.com>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 24 insertions, 0 deletions

diff --git a/t/lib-verify-submodule-gitdir-path.sh b/t/lib-verify-submodule-gitdir-path.sh
new file mode 100644
index 0000000000..62794df976
--- /dev/null
+++ b/t/lib-verify-submodule-gitdir-path.sh
@@ -0,0 +1,24 @@
+# Helper to verify if repo $1 contains a submodule named $2 with gitdir path $3
+
+# This does not check filesystem existence. That is done in submodule.c via the
+# submodule_name_to_gitdir() API which this helper ends up calling. The gitdirs
+# might or might not exist (e.g. when adding a new submodule), so this only
+# checks the expected configuration path, which might be overridden by the user.
+
+verify_submodule_gitdir_path() {
+	repo="$1" &&
+	name="$2" &&
+	path="$3" &&
+	(
+		cd "$repo" &&
+		# Compute expected absolute path
+		expected="$(git rev-parse --git-common-dir)/$path" &&
+		expected="$(test-tool path-utils real_path "$expected")" &&
+		# Compute actual absolute path
+		actual="$(git submodule--helper gitdir "$name")" &&
+		actual="$(test-tool path-utils real_path "$actual")" &&
+		echo "$expected" >expect &&
+		echo "$actual" >actual &&
+		test_cmp expect actual
+	)
+}