Merge pull request #4526 from dnnr/detect-bitrot-in-diff

Add bitrot detection to "diff" command

changelog/unreleased/pull-4526

@@ -0,0 +1,11 @@
+Enhancement: Add bitrot detection to `diff` command
+
+The output of the `diff` command now includes the modifier `?` for files
+to indicate bitrot in backed up files. It will appear whenever there is a
+difference in content while the metadata is exactly the same. Since files with
+unchanged metadata are normally not read again when creating a backup, the
+detection is only effective if the right-hand side of the diff has been created
+with "backup --force".
+
+https://github.com/restic/restic/issues/805
+https://github.com/restic/restic/pull/4526

cmd/restic/cmd_diff.go

@@ -27,6 +27,10 @@ directory:
 * U  The metadata (access mode, timestamps, ...) for the item was updated
 * M  The file's content was modified
 * T  The type was changed, e.g. a file was made a symlink
+* ?  Bitrot detected: The file's content has changed but all metadata is the same
+
+Metadata comparison will likely not work if a backup was created using the
+'--ignore-inode' or '--ignore-ctime' option.
 
 To only compare files in specific subfolders, you can use the
 "<snapshotID>:<subfolder>" syntax, where "subfolder" is a path within the
@@ -272,6 +276,16 @@ func (c *Comparer) diffTree(ctx context.Context, stats *DiffStatsContainer, pref
 				!reflect.DeepEqual(node1.Content, node2.Content) {
 				mod += "M"
 				stats.ChangedFiles++
+
+				node1NilContent := *node1
+				node2NilContent := *node2
+				node1NilContent.Content = nil
+				node2NilContent.Content = nil
+				// the bitrot detection may not work if `backup --ignore-inode` or `--ignore-ctime` were used
+				if node1NilContent.Equals(node2NilContent) {
+					// probable bitrot detected
+					mod += "?"
+				}
 			} else if c.opts.ShowMetadata && !node1.Equals(*node2) {
 				mod += "U"
 			}

doc/075_scripting.rst

@@ -201,7 +201,8 @@ change
 +------------------+--------------------------------------------------------------+
 | ``modifier``     | Type of change, a concatenation of the following characters: |
 |                  | "+" = added, "-" = removed, "T" = entry type changed,        |
-|                  | "M" = file content changed, "U" = metadata changed           |
+|                  | "M" = file content changed, "U" = metadata changed,          |
+|                  | "?" = bitrot detected                                        |
 +------------------+--------------------------------------------------------------+
 
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