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>

diff --git a/editors/diff.c b/editors/diff.c
index ca4a4eae77bf4faf32214c2310be8086931247ae..3719bb49f9f1182f1fb16017141588a61bce3039 100644 (file)
--- 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);