Examine file ctime when checking if files have changed.

changelog/unreleased/issue-2179

@@ -0,0 +1,15 @@
+Enhancement: Use ctime when checking for file changes
+
+Previously, restic only checked a file's mtime (along with other non-timestamp
+metadata) to decide if a file has changed. This could cause restic to not notice
+that a file has changed (and therefore continue to store the old version, as
+opposed to the modified version) if something edits the file and then resets the
+timestamp. Restic now also checks the ctime of files, so any modifications to a
+file should be noticed, and the modified file will be backed up. The ctime check
+will be disabled if the --ignore-inode flag was given.
+
+If this change causes problems for you, please open an issue, and we can look in
+to adding a seperate flag to disable just the ctime check.
+
+https://github.com/restic/restic/issues/2179
+https://github.com/restic/restic/pull/2212

internal/archiver/archiver.go

@@ -453,8 +453,13 @@ func fileChanged(fi os.FileInfo, node *restic.Node, ignoreInode bool) bool {
 		return true
 	}
 
-	// check size
+	// check status change timestamp
 	extFI := fs.ExtendedStat(fi)
+	if !ignoreInode && !extFI.ChangeTime.Equal(node.ChangeTime) {
+		return true
+	}
+
+	// check size
 	if uint64(fi.Size()) != node.Size || uint64(extFI.Size) != node.Size {
 		return true
 	}

internal/archiver/archiver_test.go

@@ -1,6 +1,7 @@
 package archiver
 
 import (
+	"bytes"
 	"context"
 	"io/ioutil"
 	"os"
@@ -576,6 +577,19 @@ func TestFileChanged(t *testing.T) {
 				save(t, filename, defaultContent)
 			},
 		},
+		{
+			Name: "new-content-same-timestamp",
+			Modify: func(t testing.TB, filename string) {
+				fi, err := os.Stat(filename)
+				if err != nil {
+					t.Fatal(err)
+				}
+				extFI := fs.ExtendedStat(fi)
+				save(t, filename, bytes.ToUpper(defaultContent))
+				sleep()
+				setTimestamp(t, filename, extFI.AccessTime, extFI.ModTime)
+			},
+		},
 		{
 			Name: "other-content",
 			Modify: func(t testing.TB, filename string) {

internal/fs/stat.go

@@ -22,6 +22,7 @@ type ExtendedFileInfo struct {
 
 	AccessTime time.Time // last access time stamp
 	ModTime    time.Time // last (content) modification time stamp
+	ChangeTime time.Time // last status change time stamp
 }
 
 // ExtendedStat returns an ExtendedFileInfo constructed from the os.FileInfo.

internal/fs/stat_bsd.go

@@ -30,6 +30,7 @@ func extendedStat(fi os.FileInfo) ExtendedFileInfo {
 
 		AccessTime: time.Unix(s.Atimespec.Unix()),
 		ModTime:    time.Unix(s.Mtimespec.Unix()),
+		ChangeTime: time.Unix(s.Ctimespec.Unix()),
 	}
 
 	return extFI

internal/fs/stat_unix.go

@@ -30,6 +30,7 @@ func extendedStat(fi os.FileInfo) ExtendedFileInfo {
 
 		AccessTime: time.Unix(s.Atim.Unix()),
 		ModTime:    time.Unix(s.Mtim.Unix()),
+		ChangeTime: time.Unix(s.Ctim.Unix()),
 	}
 
 	return extFI

internal/fs/stat_windows.go

@@ -27,5 +27,7 @@ func extendedStat(fi os.FileInfo) ExtendedFileInfo {
 	mtime := syscall.NsecToTimespec(s.LastWriteTime.Nanoseconds())
 	extFI.ModTime = time.Unix(mtime.Unix())
 
+	extFI.ChangeTime = extFI.ModTime
+
 	return extFI
 }