Merge branch 'jk/no-optional-locks'

Some commands (most notably "git status") makes an opportunistic
update when performing a read-only operation to help optimize later
operations in the same repository.  The new "--no-optional-locks"
option can be passed to Git to disable them.

* jk/no-optional-locks:
  git: add --no-optional-locks option

Documentation/git.txt

@@ -159,6 +159,10 @@ foo.bar= ...`) sets `foo.bar` to the empty string which ` git config
 	Add "icase" magic to all pathspec. This is equivalent to setting
 	the `GIT_ICASE_PATHSPECS` environment variable to `1`.
 
+--no-optional-locks::
+	Do not perform optional operations that require locks. This is
+	equivalent to setting the `GIT_OPTIONAL_LOCKS` to `0`.
+
 GIT COMMANDS
 ------------
 
@@ -697,6 +701,14 @@ of clones and fetches.
 	which feed potentially-untrusted URLS to git commands.  See
 	linkgit:git-config[1] for more details.
 
+`GIT_OPTIONAL_LOCKS`::
+	If set to `0`, Git will complete any requested operation without
+	performing any optional sub-operations that require taking a lock.
+	For example, this will prevent `git status` from refreshing the
+	index as a side effect. This is useful for processes running in
+	the background which do not want to cause lock contention with
+	other operations on the repository.  Defaults to `1`.
+
 Discussion[[Discussion]]
 ------------------------
 

builtin/commit.c

@@ -1392,7 +1392,10 @@ int cmd_status(int argc, const char **argv, const char *prefix)
 	read_cache_preload(&s.pathspec);
 	refresh_index(&the_index, REFRESH_QUIET|REFRESH_UNMERGED, &s.pathspec, NULL, NULL);
 
-	fd = hold_locked_index(&index_lock, 0);
+	if (use_optional_locks())
+		fd = hold_locked_index(&index_lock, 0);
+	else
+		fd = -1;
 
 	s.is_initial = get_oid(s.reference, &oid) ? 1 : 0;
 	if (!s.is_initial)

cache.h

@@ -444,6 +444,7 @@ static inline enum object_type object_type(unsigned int mode)
 #define GIT_NOGLOB_PATHSPECS_ENVIRONMENT "GIT_NOGLOB_PATHSPECS"
 #define GIT_ICASE_PATHSPECS_ENVIRONMENT "GIT_ICASE_PATHSPECS"
 #define GIT_QUARANTINE_ENVIRONMENT "GIT_QUARANTINE_PATH"
+#define GIT_OPTIONAL_LOCKS_ENVIRONMENT "GIT_OPTIONAL_LOCKS"
 
 /*
  * This environment variable is expected to contain a boolean indicating
@@ -783,6 +784,11 @@ extern int protect_ntfs;
  */
 extern int ref_paranoia;
 
+/*
+ * Returns the boolean value of $GIT_OPTIONAL_LOCKS (or the default value).
+ */
+int use_optional_locks(void);
+
 /*
  * The character that begins a commented line in user-editable file
  * that is subject to stripspace.

environment.c

@@ -338,3 +338,8 @@ void reset_shared_repository(void)
 {
 	need_shared_repository_from_config = 1;
 }
+
+int use_optional_locks(void)
+{
+	return git_env_bool(GIT_OPTIONAL_LOCKS_ENVIRONMENT, 1);
+}

git.c

@@ -182,6 +182,10 @@ static int handle_options(const char ***argv, int *argc, int *envchanged)
 			setenv(GIT_ICASE_PATHSPECS_ENVIRONMENT, "1", 1);
 			if (envchanged)
 				*envchanged = 1;
+		} else if (!strcmp(cmd, "--no-optional-locks")) {
+			setenv(GIT_OPTIONAL_LOCKS_ENVIRONMENT, "0", 1);
+			if (envchanged)
+				*envchanged = 1;
 		} else if (!strcmp(cmd, "--shallow-file")) {
 			(*argv)++;
 			(*argc)--;

t/t7508-status.sh

@@ -1670,4 +1670,14 @@ test_expect_success '"Initial commit" should not be noted in commit template' '
 	test_i18ngrep ! "Initial commit" output
 '
 
+test_expect_success '--no-optional-locks prevents index update' '
+	test-chmtime =1234567890 .git/index &&
+	git --no-optional-locks status &&
+	test-chmtime -v +0 .git/index >out &&
+	grep ^1234567890 out &&
+	git status &&
+	test-chmtime -v +0 .git/index >out &&
+	! grep ^1234567890 out
+'
+
 test_done