Warn “file changed as we read it” less often

* src/create.c (dump_file0): Remove an fstatat call that is
unnecessary because the file wasn’t read so we can treat the first
fstatat as atomic.  Warn “file changed” when the file’s size,
mtime, user ID, group ID, or mode changes, instead of when the
file’s size or ctime changes.  Also, when such a change happens,
do not change exit status if --ignore-failed-read.  Finally, don’t
attempt to change atime back if it didn’t change.

1 files changed, 38 insertions, 16 deletions

diff --git a/src/create.c b/src/create.c
index 30db2b5c..ebca3a93 100644
--- a/src/create.c
+++ b/src/create.c
@@ -1639,8 +1639,6 @@ dump_file0 (struct tar_stat_info *st, char const *name, char const *p)
 {
   union block *header;
   char type;
-  off_t original_size;
-  struct timespec original_ctime;
   off_t block_ordinal = -1;
   int fd = 0;
   bool is_dir;
@@ -1683,10 +1681,11 @@ dump_file0 (struct tar_stat_info *st, char const *name, char const *p)
       return;
     }
 
-  st->archive_file_size = original_size = st->stat.st_size;
+  struct stat st1 = st->stat;
+  st->archive_file_size = st->stat.st_size;
   st->atime = get_stat_atime (&st->stat);
   st->mtime = get_stat_mtime (&st->stat);
-  st->ctime = original_ctime = get_stat_ctime (&st->stat);
+  st->ctime = get_stat_ctime (&st->stat);
 
 #ifdef S_ISHIDDEN
   if (S_ISHIDDEN (st->stat.st_mode))
@@ -1736,7 +1735,7 @@ dump_file0 (struct tar_stat_info *st, char const *name, char const *p)
   if (is_dir || S_ISREG (st->stat.st_mode) || S_ISCTG (st->stat.st_mode))
     {
       bool ok;
-      struct stat final_stat;
+      struct stat st2;
 
       xattrs_acls_get (parentfd, name, st, 0, !is_dir);
       xattrs_selinux_get (parentfd, name, st, fd);
@@ -1804,31 +1803,54 @@ dump_file0 (struct tar_stat_info *st, char const *name, char const *p)
 		  errno = - parentfd;
 		  ok = false;
 		}
-	      else
-		ok = fstatat (parentfd, name, &final_stat, fstatat_flags) == 0;
 	    }
 	  else
-	    ok = fstat (fd, &final_stat) == 0;
+	    ok = fstat (fd, &st2) == 0;
 
 	  if (! ok)
 	    file_removed_diag (p, top_level, stat_diag);
 	}
 
-      if (ok)
+      if (ok && fd)
 	{
-	  if ((timespec_cmp (get_stat_ctime (&final_stat), original_ctime) != 0
-	       /* Original ctime will change if the file is a directory and
-		  --remove-files is given */
-	       && !(remove_files_option && is_dir))
-	      || original_size < final_stat.st_size)
+	  /* Heuristically check whether the file is the same in all
+	     attributes that tar cares about and can easily check.
+	     Although the check is not perfect since it does not
+	     consult file contents, it is typically good enough.
+	     Do not check atime which is saved only to replace it later.
+	     Do not check ctime where changes might be benign (e.g.,
+	     another process creates a hard link to the file).  */
+
+	  /* If the file's user ID, group ID or mode changed, tar may
+	     have output the wrong info for the file.  */
+	  ok &= st1.st_uid == st2.st_uid;
+	  ok &= st1.st_gid == st2.st_gid;
+	  ok &= st1.st_mode == st2.st_mode;
+
+	  /* Likewise for the file's mtime, but skip this check if it
+	     is a directory possibly updated by --remove-files.  */
+	  if (! (is_dir && remove_files_option))
+	    ok &= ! timespec_cmp (get_stat_mtime (&st1),
+				  get_stat_mtime (&st2));
+
+	  /* Likewise for the file's size, but skip this check if it
+	     is a directory as tar does not output directory sizes.
+	     Although dump_regular_file caught regular file shrinkage,
+	     it shouldn't hurt to check for shrinkage again now;
+	     plus, the file may have grown.  */
+	  if (!is_dir)
+	    ok &= st1.st_size == st2.st_size;
+
+	  if (!ok)
 	    {
 	      WARNOPT (WARN_FILE_CHANGED,
 		       (0, 0, _("%s: file changed as we read it"),
 			quotearg_colon (p)));
-	      set_exit_status (TAREXIT_DIFFERS);
+	      if (! ignore_failed_read_option)
+		set_exit_status (TAREXIT_DIFFERS);
 	    }
 	  else if (atime_preserve_option == replace_atime_preserve
-		   && fd && (is_dir || original_size != 0)
+		   && timespec_cmp (st->atime, get_stat_atime (&st2)) != 0
 		   && set_file_atime (fd, parentfd, name, st->atime) != 0)
 	    utime_error (p);
 	}