ipn/ipnlocal: add LocalBackend.SetDirectFileRoot

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/backend.go

@@ -94,6 +94,13 @@ type PartialFile struct {
 	Started      time.Time // time transfer started
 	DeclaredSize int64     // or -1 if unknown
 	Received     int64     // bytes copied thus far
+
+	// FinalPath is non-empty when the final has been completely
+	// written and renamed into place. This is then the complete
+	// path to the file post-rename. This is only set in
+	// "direct" file mode where the peerapi isn't being used; see
+	// LocalBackend.SetDirectFileRoot.
+	FinalPath string `json:",omitempty"`
 }
 
 // StateKey is an opaque identifier for a set of LocalBackend state