diff: optimize diffing of files with the same metadata

Signed-off-by: Roman Borisov <ext-roman.borisov@nokia.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 25 insertions, 0 deletions

diff --git a/editors/diff.c b/editors/diff.c
index ca4a4eae7..3719bb49f 100644
--- a/editors/diff.c
+++ b/editors/diff.c
@@ -952,6 +952,31 @@ int diff_main(int argc UNUSED_PARAM, char **argv)
         if (gotstdin && (S_ISDIR(stb[0].st_mode) || S_ISDIR(stb[1].st_mode)))
                 bb_error_msg_and_die("can't compare stdin to a directory");
 
+        /* Compare metadata to check if the files are the same physical file.
+         *
+         * Comment from diffutils source says:
+         * POSIX says that two files are identical if st_ino and st_dev are
+         * the same, but many file systems incorrectly assign the same (device,
+         * inode) pair to two distinct files, including:
+         * GNU/Linux NFS servers that export all local file systems as a
+         * single NFS file system, if a local device number (st_dev) exceeds
+         * 255, or if a local inode number (st_ino) exceeds 16777215.
+         */
+        if (ENABLE_DESKTOP
+         && stb[0].st_ino == stb[1].st_ino
+         && stb[0].st_dev == stb[1].st_dev
+         && stb[0].st_size == stb[1].st_size
+         && stb[0].st_mtime == stb[1].st_mtime
+         && stb[0].st_ctime == stb[1].st_ctime
+         && stb[0].st_mode == stb[1].st_mode
+         && stb[0].st_nlink == stb[1].st_nlink
+         && stb[0].st_uid == stb[1].st_uid
+         && stb[0].st_gid == stb[1].st_gid
+        ) {
+                /* files are physically the same; no need to compare them */
+                return STATUS_SAME;
+        }
+
         if (S_ISDIR(stb[0].st_mode) && S_ISDIR(stb[1].st_mode)) {
 #if ENABLE_FEATURE_DIFF_DIR
                 diffdir(file, s_start);