packed-backend: mmap large "packed-refs" file during fsck

During fsck, we use "strbuf_read" to read the content of "packed-refs"
without using mmap mechanism. This is a bad practice which would consume
more memory than using mmap mechanism. Besides, as all code paths in
"packed-backend.c" use this way, we should make "fsck" align with the
current codebase.

As we have introduced the helper function "allocate_snapshot_buffer", we
can simply use this function to use mmap mechanism.

Suggested-by: Jeff King <[email protected]>
Suggested-by: Patrick Steinhardt <[email protected]>
Signed-off-by: shejialuo <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: shejialuo

refs/packed-backend.c

@@ -2068,7 +2068,7 @@ static int packed_fsck(struct ref_store *ref_store,
 {
 	struct packed_ref_store *refs = packed_downcast(ref_store,
 							REF_STORE_READ, "fsck");
-	struct strbuf packed_ref_content = STRBUF_INIT;
+	struct snapshot snapshot = { 0 };
 	unsigned int sorted = 0;
 	struct stat st;
 	int ret = 0;
@@ -2112,7 +2112,7 @@ static int packed_fsck(struct ref_store *ref_store,
 		goto cleanup;
 	}
 
-	if (!st.st_size) {
+	if (!allocate_snapshot_buffer(&snapshot, fd, &st)) {
 		struct fsck_ref_report report = { 0 };
 		report.path = "packed-refs";
 		ret = fsck_report_ref(o, &report,
@@ -2121,21 +2121,16 @@ static int packed_fsck(struct ref_store *ref_store,
 		goto cleanup;
 	}
 
-	if (strbuf_read(&packed_ref_content, fd, 0) < 0) {
-		ret = error_errno(_("unable to read '%s'"), refs->path);
-		goto cleanup;
-	}
-
-	ret = packed_fsck_ref_content(o, ref_store, &sorted, packed_ref_content.buf,
-				      packed_ref_content.buf + packed_ref_content.len);
+	ret = packed_fsck_ref_content(o, ref_store, &sorted, snapshot.start,
+				      snapshot.eof);
 	if (!ret && sorted)
-		ret = packed_fsck_ref_sorted(o, ref_store, packed_ref_content.buf,
-					     packed_ref_content.buf + packed_ref_content.len);
+		ret = packed_fsck_ref_sorted(o, ref_store, snapshot.start,
+					     snapshot.eof);
 
 cleanup:
 	if (fd >= 0)
 		close(fd);
-	strbuf_release(&packed_ref_content);
+	clear_snapshot_buffer(&snapshot);
 	return ret;
 }