local: make --local-no-updated provide a consistent view of the objects

Before this change the --local-no-updated flag would not error if the
files changed in size during the transfer. The file could still be
read beyond the size advertised though which caused problems with
certain backends.

After this change we attempt to provide a consistent view of the file
once it has been opened.

Once the file has had stat() called on it for the first time we

- Only transfer the size that stat gave
- Only checksum the size that stat gave
- Don't update the stat info for the file

This means that files that are extending can be transferred - rclone
will transfer the length it saw the first time it listed the file.

See: https://forum.rclone.org/t/transport-connection-broken/16494/21

backend/local/local.go

@@ -90,7 +90,24 @@ are being uploaded and aborts with a message which starts "can't copy
 
 However on some file systems this modification time check may fail (eg
 [Glusterfs #2206](https://github.com/rclone/rclone/issues/2206)) so this
-check can be disabled with this flag.`,
+check can be disabled with this flag.
+
+If this flag is set, rclone will use its best efforts to transfer a
+file which is being updated. If the file is only having things
+appended to it (eg a log) then rclone will transfer the log file with
+the size it had the first time rclone saw it.
+
+If the file is being modified throughout (not just appended to) then
+the transfer may fail with a hash check failure.
+
+In detail, once the file has had stat() called on it for the first
+time we:
+
+- Only transfer the size that stat gave
+- Only checksum the size that stat gave
+- Don't update the stat info for the file
+
+`,
 			Default:  false,
 			Advanced: true,
 		}, {
@@ -809,6 +826,10 @@ func (o *Object) Hash(ctx context.Context, r hash.Type) (string, error) {
 		} else {
 			in, err = o.openTranslatedLink(0, -1)
 		}
+		// If not checking for updates, only read size given
+		if o.fs.opt.NoCheckUpdated {
+			in = readers.NewLimitedReadCloser(in, o.size)
+		}
 		if err != nil {
 			return "", errors.Wrap(err, "hash: failed to open")
 		}
@@ -955,6 +976,13 @@ func (o *Object) Open(ctx context.Context, options ...fs.OpenOption) (in io.Read
 		}
 	}
 
+	// If not checking updated then limit to current size.  This means if
+	// file is being extended, readers will read a o.Size() bytes rather
+	// than the new size making for a consistent upload.
+	if limit < 0 && o.fs.opt.NoCheckUpdated {
+		limit = o.size
+	}
+
 	// Handle a translated link
 	if o.translatedLink {
 		return o.openTranslatedLink(offset, limit)
@@ -1151,6 +1179,10 @@ func (f *Fs) OpenWriterAt(ctx context.Context, remote string, size int64) (fs.Wr
 
 // setMetadata sets the file info from the os.FileInfo passed in
 func (o *Object) setMetadata(info os.FileInfo) {
+	// if not checking updated then don't update the stat
+	if o.fs.opt.NoCheckUpdated && !o.modTime.IsZero() {
+		return
+	}
 	// Don't overwrite the info if we don't need to
 	// this avoids upsetting the race detector
 	if o.size != info.Size() {