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.

NEWS

@@ -11,6 +11,23 @@ version 1.34.90 (git)
 
 * Bug fixes
 
+** Warn "file changed as we read it" less often.
+   Formerly, tar warned if the file's size or ctime changed.
+   However, this generated a false positive if tar read a file
+   while another process hard-linked to it, changing its ctime.
+   Now, tar warns if the file's size, mtime, user ID, group ID,
+   or mode changes.  Although neither heuristic is perfect,
+   the new one should work better in practice.
+
+** Fix --ignore-failed-read to ignore file-changed read errors
+   as far as exit status is concerned.  You can now suppress file-changed
+   issues entirely with --ignore-failed-read --warning=no-file-changed.
+
+** Fix --remove-files to not remove a file that changed while we read it.
+
+** Fix --atime-preserve=replace to not fail if there was no need to replace,
+   either because we did not read the file, or the atime did not change.
+
 ** Fix handling of prefix keywords not followed by "." in pax headers.
 
 ** Fix handling of out-of-range sparse entries in pax headers.

doc/tar.texi

@@ -2859,7 +2859,7 @@ Ignore exit codes of subprocesses. @xref{Writing to an External Program}.
 @opsummary{ignore-failed-read}
 @item --ignore-failed-read
 
-Do not exit unsuccessfully merely because an unreadable file was encountered.
+Do not exit unsuccessfully merely because reading failed.
 @xref{Ignore Failed Read}.
 
 @opsummary{ignore-zeros}
@@ -4641,7 +4641,8 @@ Disable all warning messages.
 @item file-changed
 @samp{%s: file changed as we read it}
 @item failed-read
-Suppresses warnings about unreadable files or directories. This
+Suppresses warnings about read failures, which can occur if files
+or directories are unreadable, or if they change while being read.  This
 keyword applies only if used together with the @option{--ignore-failed-read}
 option. @xref{Ignore Failed Read}.
 @end table
@@ -5764,11 +5765,12 @@ Disable SELinux context support.
 @table @option
 @item --ignore-failed-read
 @opindex ignore-failed-read
-Do not exit with nonzero on unreadable files or directories.
+Do not exit with nonzero if there are mild problems while reading.
 @end table
 
 This option has effect only during creation.  It instructs tar to
-treat as mild conditions any missing or unreadable files (directories).
+treat as mild conditions any missing or unreadable files (directories),
+or files that change while reading.
 Such failures don't affect the program exit code, and the
 corresponding diagnostic messages are marked as warnings, not errors.
 These warnings can be suppressed using the

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);
 	}