[PATCH v2 1/1] setup: recognise extensions.objectFormat

[Patryk Obara <patryk.obara@gmail.com>]

This extension selects which hashing algorithm from vtable should be
used for reading and writing objects in the object store.  At the moment
supports only single value (sha-1).

In case value of objectFormat is an unknown hashing algorithm, Git
command will fail with following message:

  fatal: unknown repository extensions found:
	  objectformat = <value>

To indicate, that this specific objectFormat value is not recognised.

The objectFormat extension is not allowed in repository marked as
version 0 to prevent any possibility of accidentally writing a NewHash
object in the sha-1 object store. This extension behaviour is different
than preciousObjects extension (which is allowed in repo version 0).

Add tests and documentation note about new extension.

Signed-off-by: Patryk Obara <patryk.obara@gmail.com>
---
 Documentation/technical/repository-version.txt | 12 ++++++++++++
 setup.c                                        | 27 ++++++++++++++++++++++++++
 t/t1302-repo-version.sh                        | 15 ++++++++++++++
 3 files changed, 54 insertions(+)

diff --git a/Documentation/technical/repository-version.txt b/Documentation/technical/repository-version.txt
index 00ad37986e..7e2b832603 100644
--- a/Documentation/technical/repository-version.txt
+++ b/Documentation/technical/repository-version.txt
@@ -86,3 +86,15 @@ for testing format-1 compatibility.
 When the config key `extensions.preciousObjects` is set to `true`,
 objects in the repository MUST NOT be deleted (e.g., by `git-prune` or
 `git repack -d`).
+
+`objectFormat`
+~~~~~~~~~~~~~~
+
+This extension instructs Git to use a specific algorithm for addressing
+and interpreting objects in the object store. Currently, the only
+supported object format is `sha-1`. At the moment, the primary purpose
+of this option is to enable Git developers to experiment with different
+hashing algorithms without re-compilation of git client.
+
+See `hash-function-transition.txt` document for more detailed explanation.
+
diff --git a/setup.c b/setup.c
index 8cc34186ce..9b9993a14e 100644
--- a/setup.c
+++ b/setup.c
@@ -405,6 +405,31 @@ void setup_work_tree(void)
 	initialized = 1;
 }
 
+static int find_object_format(const char *value)
+{
+	int i;
+	for (i = GIT_HASH_SHA1; i < GIT_HASH_NALGOS; ++i) {
+		if (strcmp(value, hash_algos[i].name) == 0)
+			return i;
+	}
+	return GIT_HASH_UNKNOWN;
+}
+
+static void detect_object_format(struct repository_format *data,
+				 const char *value)
+{
+	if (data->version == 0)
+		die(_("invalid repository format version '%d'"), data->version);
+
+	data->hash_algo = find_object_format(value);
+	if (data->hash_algo == GIT_HASH_UNKNOWN) {
+		struct strbuf object_format = STRBUF_INIT;
+		strbuf_addf(&object_format, "objectformat = %s", value);
+		string_list_append(&data->unknown_extensions, object_format.buf);
+		strbuf_release(&object_format);
+	}
+}
+
 static int check_repo_format(const char *var, const char *value, void *vdata)
 {
 	struct repository_format *data = vdata;
@@ -422,6 +447,8 @@ static int check_repo_format(const char *var, const char *value, void *vdata)
 			;
 		else if (!strcmp(ext, "preciousobjects"))
 			data->precious_objects = git_config_bool(var, value);
+		else if (!strcmp(ext, "objectformat"))
+			detect_object_format(data, value);
 		else
 			string_list_append(&data->unknown_extensions, ext);
 	} else if (strcmp(var, "core.bare") == 0) {
diff --git a/t/t1302-repo-version.sh b/t/t1302-repo-version.sh
index ce4cff13bb..227b397ff2 100755
--- a/t/t1302-repo-version.sh
+++ b/t/t1302-repo-version.sh
@@ -107,4 +107,19 @@ test_expect_success 'gc runs without complaint' '
 	git gc
 '
 
+test_expect_success 'object-format not allowed in repo version=0' '
+	mkconfig 0 "objectFormat = sha-1" >.git/config &&
+	check_abort
+'
+
+test_expect_success 'object-format=sha-1 allowed' '
+	mkconfig 1 "objectFormat = sha-1" >.git/config &&
+	check_allow
+'
+
+test_expect_success 'object-format=foo unsupported' '
+	mkconfig 1 "objectFormat = foo" >.git/config &&
+	check_abort
+'
+
 test_done
-- 
2.14.3