grep: move sha1-reading mutex into low-level code

The multi-threaded git-grep code needs to serialize access
to the thread-unsafe read_sha1_file call. It does this with
a mutex that is local to builtin/grep.c.

Let's instead push this down into grep.c, where it can be
used by both builtin/grep.c and grep.c. This will let us
safely teach the low-level grep.c code tricks that involve
reading from the object db.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/grep.c

@@ -85,21 +85,6 @@ static inline void grep_unlock(void)
 		pthread_mutex_unlock(&grep_mutex);
 }
 
-/* Used to serialize calls to read_sha1_file. */
-static pthread_mutex_t read_sha1_mutex;
-
-static inline void read_sha1_lock(void)
-{
-	if (use_threads)
-		pthread_mutex_lock(&read_sha1_mutex);
-}
-
-static inline void read_sha1_unlock(void)
-{
-	if (use_threads)
-		pthread_mutex_unlock(&read_sha1_mutex);
-}
-
 /* Signalled when a new work_item is added to todo. */
 static pthread_cond_t cond_add;
 
@@ -254,7 +239,7 @@ static void start_threads(struct grep_opt *opt)
 	int i;
 
 	pthread_mutex_init(&grep_mutex, NULL);
-	pthread_mutex_init(&read_sha1_mutex, NULL);
+	pthread_mutex_init(&grep_read_mutex, NULL);
 	pthread_mutex_init(&grep_attr_mutex, NULL);
 	pthread_cond_init(&cond_add, NULL);
 	pthread_cond_init(&cond_write, NULL);
@@ -303,7 +288,7 @@ static int wait_all(void)
 	}
 
 	pthread_mutex_destroy(&grep_mutex);
-	pthread_mutex_destroy(&read_sha1_mutex);
+	pthread_mutex_destroy(&grep_read_mutex);
 	pthread_mutex_destroy(&grep_attr_mutex);
 	pthread_cond_destroy(&cond_add);
 	pthread_cond_destroy(&cond_write);
@@ -313,8 +298,6 @@ static int wait_all(void)
 	return hit;
 }
 #else /* !NO_PTHREADS */
-#define read_sha1_lock()
-#define read_sha1_unlock()
 
 static int wait_all(void)
 {
@@ -376,9 +359,9 @@ static void *lock_and_read_sha1_file(const unsigned char *sha1, enum object_type
 {
 	void *data;
 
-	read_sha1_lock();
+	grep_read_lock();
 	data = read_sha1_file(sha1, type, size);
-	read_sha1_unlock();
+	grep_read_unlock();
 	return data;
 }
 
@@ -617,10 +600,10 @@ static int grep_object(struct grep_opt *opt, const struct pathspec *pathspec,
 		struct strbuf base;
 		int hit, len;
 
-		read_sha1_lock();
+		grep_read_lock();
 		data = read_object_with_reference(obj->sha1, tree_type,
 						  &size, NULL);
-		read_sha1_unlock();
+		grep_read_unlock();
 
 		if (!data)
 			die(_("unable to read tree (%s)"), sha1_to_hex(obj->sha1));

grep.c

@@ -826,6 +826,12 @@ static inline void grep_attr_unlock(void)
 	if (grep_use_locks)
 		pthread_mutex_unlock(&grep_attr_mutex);
 }
+
+/*
+ * Same as git_attr_mutex, but protecting the thread-unsafe object db access.
+ */
+pthread_mutex_t grep_read_mutex;
+
 #else
 #define grep_attr_lock()
 #define grep_attr_unlock()

grep.h

@@ -139,6 +139,23 @@ extern int grep_threads_ok(const struct grep_opt *opt);
  */
 extern int grep_use_locks;
 extern pthread_mutex_t grep_attr_mutex;
+extern pthread_mutex_t grep_read_mutex;
+
+static inline void grep_read_lock(void)
+{
+	if (grep_use_locks)
+		pthread_mutex_lock(&grep_read_mutex);
+}
+
+static inline void grep_read_unlock(void)
+{
+	if (grep_use_locks)
+		pthread_mutex_unlock(&grep_read_mutex);
+}
+
+#else
+#define grep_read_lock()
+#define grep_read_unlock()
 #endif
 
 #endif