introduce "preciousObjects" repository extension

If this extension is used in a repository, then no
operations should run which may drop objects from the object
storage. This can be useful if you are sharing that storage
with other repositories whose refs you cannot see.

For instance, if you do:

  $ git clone -s parent child
  $ git -C parent config extensions.preciousObjects true
  $ git -C parent config core.repositoryformatversion 1

you now have additional safety when running git in the
parent repository. Prunes and repacks will bail with an
error, and `git gc` will skip those operations (it will
continue to pack refs and do other non-object operations).
Older versions of git, when run in the repository, will
fail on every operation.

Note that we do not set the preciousObjects extension by
default when doing a "clone -s", as doing so breaks
backwards compatibility. It is a decision the user should
make explicitly.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/repository-version.txt

@@ -79,3 +79,10 @@ The defined extensions are:
 
 This extension does not change git's behavior at all. It is useful only
 for testing format-1 compatibility.
+
+`preciousObjects`
+~~~~~~~~~~~~~~~~~
+
+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`).

builtin/gc.c

@@ -352,15 +352,17 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 	if (gc_before_repack())
 		return -1;
 
-	if (run_command_v_opt(repack.argv, RUN_GIT_CMD))
-		return error(FAILED_RUN, repack.argv[0]);
+	if (!repository_format_precious_objects) {
+		if (run_command_v_opt(repack.argv, RUN_GIT_CMD))
+			return error(FAILED_RUN, repack.argv[0]);
 
-	if (prune_expire) {
-		argv_array_push(&prune, prune_expire);
-		if (quiet)
-			argv_array_push(&prune, "--no-progress");
-		if (run_command_v_opt(prune.argv, RUN_GIT_CMD))
-			return error(FAILED_RUN, prune.argv[0]);
+		if (prune_expire) {
+			argv_array_push(&prune, prune_expire);
+			if (quiet)
+				argv_array_push(&prune, "--no-progress");
+			if (run_command_v_opt(prune.argv, RUN_GIT_CMD))
+				return error(FAILED_RUN, prune.argv[0]);
+		}
 	}
 
 	if (prune_worktrees_expire) {

builtin/prune.c

@@ -218,6 +218,9 @@ int cmd_prune(int argc, const char **argv, const char *prefix)
 		return 0;
 	}
 
+	if (repository_format_precious_objects)
+		die(_("cannot prune in a precious-objects repo"));
+
 	while (argc--) {
 		unsigned char sha1[20];
 		const char *name = *argv++;

builtin/repack.c

@@ -193,6 +193,9 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 	argc = parse_options(argc, argv, prefix, builtin_repack_options,
 				git_repack_usage, 0);
 
+	if (delete_redundant && repository_format_precious_objects)
+		die(_("cannot delete packs in a precious-objects repo"));
+
 	if (pack_kept_objects < 0)
 		pack_kept_objects = write_bitmaps;
 

cache.h

@@ -694,6 +694,7 @@ extern int grafts_replace_parents;
 #define GIT_REPO_VERSION 0
 #define GIT_REPO_VERSION_READ 1
 extern int repository_format_version;
+extern int repository_format_precious_objects;
 extern int check_repository_format(void);
 
 #define MTIME_CHANGED	0x0001

environment.c

@@ -26,6 +26,7 @@ int warn_ambiguous_refs = 1;
 int warn_on_object_refname_ambiguity = 1;
 int ref_paranoia = -1;
 int repository_format_version;
+int repository_format_precious_objects;
 const char *git_commit_encoding;
 const char *git_log_output_encoding;
 int shared_repository = PERM_UMASK;

setup.c

@@ -367,6 +367,8 @@ static int check_repo_format(const char *var, const char *value, void *cb)
 		 */
 		if (!strcmp(ext, "noop"))
 			;
+		else if (!strcmp(ext, "preciousobjects"))
+			repository_format_precious_objects = git_config_bool(var, value);
 		else
 			string_list_append(&unknown_extensions, ext);
 	}

t/t1302-repo-version.sh

@@ -105,4 +105,26 @@ abort 1 no-such-extension
 allow 0 no-such-extension
 EOF
 
+test_expect_success 'precious-objects allowed' '
+	mkconfig 1 preciousObjects >.git/config &&
+	check_allow
+'
+
+test_expect_success 'precious-objects blocks destructive repack' '
+	test_must_fail git repack -ad
+'
+
+test_expect_success 'other repacks are OK' '
+	test_commit foo &&
+	git repack
+'
+
+test_expect_success 'precious-objects blocks prune' '
+	test_must_fail git prune
+'
+
+test_expect_success 'gc runs without complaint' '
+	git gc
+'
+
 test_done