vfs: fix modtimes not updating when writing via cache - fixes #4763

This reads modtime from a dirty cache item if it exists. This mirrors
the way reading the size works.

This fixes the mod time not updating when the file is written, only
when the writeback completes.

See: https://forum.rclone.org/t/rclone-mount-and-changing-timestamps-after-writes/22629

vfs/file.go

@@ -300,6 +300,17 @@ func (f *File) ModTime() (modTime time.Time) {
 	if d.vfs.Opt.NoModTime {
 		return d.ModTime()
 	}
+	// Read the modtime from a dirty item if it exists
+	if f.d.vfs.Opt.CacheMode >= vfscommon.CacheModeMinimal {
+		if item := f.d.vfs.cache.DirtyItem(f._path()); item != nil {
+			modTime, err := item.GetModTime()
+			if err != nil {
+				fs.Errorf(f._path(), "ModTime: Item GetModTime failed: %v", err)
+			} else {
+				return modTime
+			}
+		}
+	}
 	if !pendingModTime.IsZero() {
 		return pendingModTime
 	}

vfs/vfscache/cache.go

@@ -237,7 +237,8 @@ func (c *Cache) InUse(name string) bool {
 	return item.inUse()
 }
 
-// DirtyItem the Item if it exists in the cache and is Dirty
+// DirtyItem returns the Item if it exists in the cache **and** is
+// dirty otherwise it returns nil.
 //
 // name should be a remote path not an osPath
 func (c *Cache) DirtyItem(name string) (item *Item) {

vfs/vfscache/item.go

@@ -359,17 +359,22 @@ func (item *Item) Truncate(size int64) (err error) {
 	return nil
 }
 
+// _stat gets the current stat of the backing file
+//
+// Call with mutex held
+func (item *Item) _stat() (fi os.FileInfo, err error) {
+	if item.fd != nil {
+		return item.fd.Stat()
+	}
+	osPath := item.c.toOSPath(item.name) // No locking in Cache
+	return os.Stat(osPath)
+}
+
 // _getSize gets the current size of the item and updates item.info.Size
 //
 // Call with mutex held
 func (item *Item) _getSize() (size int64, err error) {
-	var fi os.FileInfo
-	if item.fd != nil {
-		fi, err = item.fd.Stat()
-	} else {
-		osPath := item.c.toOSPath(item.name) // No locking in Cache
-		fi, err = os.Stat(osPath)
-	}
+	fi, err := item._stat()
 	if err != nil {
 		if os.IsNotExist(err) && item.o != nil {
 			size = item.o.Size()
@@ -1187,6 +1192,18 @@ func (item *Item) setModTime(modTime time.Time) {
 	item.mu.Unlock()
 }
 
+// GetModTime of the cache file
+func (item *Item) GetModTime() (modTime time.Time, err error) {
+	// defer log.Trace(item.name, "modTime=%v", modTime)("")
+	item.mu.Lock()
+	defer item.mu.Unlock()
+	fi, err := item._stat()
+	if err == nil {
+		modTime = fi.ModTime()
+	}
+	return modTime, nil
+}
+
 // ReadAt bytes from the file at off
 func (item *Item) ReadAt(b []byte, off int64) (n int, err error) {
 	n = 0