sftp: fix SetModTime stat failed: object not found with --sftp-set-modtime=false

Some sftp servers don't allow the user to access the file after upload.

In this case the error message indicates that using
--sftp-set-modtime=false would fix the problem. However it doesn't
because SetModTime does a stat call which can't be disabled.

    Update SetModTime failed: SetModTime stat failed: object not found

After upload this patch checks for an `object not found` error if
set_modtime == false and ignores it, returning the expected size of
the object instead.

It also makes SetModTime do nothing if set_modtime = false

https://forum.rclone.org/t/sftp-update-setmodtime-failed/22873

backend/sftp/sftp.go

@@ -1354,7 +1354,9 @@ func (o *Object) stat(ctx context.Context) error {
 //
 // it also updates the info field
 func (o *Object) SetModTime(ctx context.Context, modTime time.Time) error {
-	if o.fs.opt.SetModTime {
+	if !o.fs.opt.SetModTime {
+		return nil
+	}
 	c, err := o.fs.getSftpConnection(ctx)
 	if err != nil {
 		return errors.Wrap(err, "SetModTime")
@@ -1364,8 +1366,7 @@ func (o *Object) SetModTime(ctx context.Context, modTime time.Time) error {
 	if err != nil {
 		return errors.Wrap(err, "SetModTime failed")
 	}
-	}
-	err := o.stat(ctx)
+	err = o.stat(ctx)
 	if err != nil {
 		return errors.Wrap(err, "SetModTime stat failed")
 	}
@@ -1496,10 +1497,28 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 		remove()
 		return errors.Wrap(err, "Update Close failed")
 	}
+
+	// Set the mod time - this stats the object if o.fs.opt.SetModTime == true
 	err = o.SetModTime(ctx, src.ModTime(ctx))
 	if err != nil {
 		return errors.Wrap(err, "Update SetModTime failed")
 	}
+
+	// Stat the file after the upload to read its stats back if o.fs.opt.SetModTime == false
+	if !o.fs.opt.SetModTime {
+		err = o.stat(ctx)
+		if err == fs.ErrorObjectNotFound {
+			// In the specific case of o.fs.opt.SetModTime == false
+			// if the object wasn't found then don't return an error
+			fs.Debugf(o, "Not found after upload with set_modtime=false so returning best guess")
+			o.modTime = src.ModTime(ctx)
+			o.size = src.Size()
+			o.mode = os.FileMode(0666) // regular file
+		} else if err != nil {
+			return errors.Wrap(err, "Update stat failed")
+		}
+	}
+
 	return nil
 }