ipn/{ipnserver,ipnlocal}: support incoming Taildrop on Synology

If the user has a "Taildrop" shared folder on startup and
the "tailscale" system user has read/write access to it,
then the user can "tailscale file cp" to their NAS.

Updates #2179 (would be fixes, but not super ideal/easy yet)

Change-Id: I68e59a99064b302abeb6d8cc84f7d2a09f764990
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -142,7 +142,11 @@ type LocalBackend struct {
 	// same as the Network Extension lifetime and we can thus avoid
 	// double-copying files by writing them to the right location
 	// immediately.
-	directFileRoot string
+	// It's also used on Synology, but in that case DoFinalRename is
+	// also set true, which moves the *.partial file to its final
+	// name on completion.
+	directFileRoot          string
+	directFileDoFinalRename bool // false on macOS, true on Synology
 
 	// statusLock must be held before calling statusChanged.Wait() or
 	// statusChanged.Broadcast().
@@ -234,6 +238,17 @@ func (b *LocalBackend) SetDirectFileRoot(dir string) {
 	b.directFileRoot = dir
 }
 
+// SetDirectFileDoFinalRename sets whether the peerapi file server should rename
+// a received "name.partial" file to "name" when the download is complete.
+//
+// This only applies when SetDirectFileRoot is non-empty.
+// The default is false.
+func (b *LocalBackend) SetDirectFileDoFinalRename(v bool) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	b.directFileDoFinalRename = v
+}
+
 // b.mu must be held.
 func (b *LocalBackend) maybePauseControlClientLocked() {
 	if b.cc == nil {
@@ -2199,10 +2214,11 @@ func (b *LocalBackend) initPeerAPIListener() {
 	}
 
 	ps := &peerAPIServer{
-		b:              b,
-		rootDir:        fileRoot,
-		selfNode:       selfNode,
-		directFileMode: b.directFileRoot != "",
+		b:                       b,
+		rootDir:                 fileRoot,
+		selfNode:                selfNode,
+		directFileMode:          b.directFileRoot != "",
+		directFileDoFinalRename: b.directFileDoFinalRename,
 	}
 	if re, ok := b.e.(wgengine.ResolvingEngine); ok {
 		if r, ok := re.GetResolver(); ok {

ipn/ipnlocal/peerapi.go

@@ -56,10 +56,17 @@ type peerAPIServer struct {
 	// directFileMode is whether we're writing files directly to a
 	// download directory (as *.partial files), rather than making
 	// the frontend retrieve it over localapi HTTP and write it
-	// somewhere itself. This is used on GUI macOS version.
+	// somewhere itself. This is used on the GUI macOS versions
+	// and on Synology.
 	// In directFileMode, the peerapi doesn't do the final rename
-	// from "foo.jpg.partial" to "foo.jpg".
+	// from "foo.jpg.partial" to "foo.jpg" unless
+	// directFileDoFinalRename is set.
 	directFileMode bool
+
+	// directFileDoFinalRename is whether in directFileMode we
+	// additionally move the *.direct file to its final name after
+	// it's received.
+	directFileDoFinalRename bool
 }
 
 const (
@@ -697,7 +704,7 @@ func (h *peerAPIHandler) handlePeerPut(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return
 	}
-	if h.ps.directFileMode {
+	if h.ps.directFileMode && !h.ps.directFileDoFinalRename {
 		if inFile != nil { // non-zero length; TODO: notify even for zero length
 			inFile.markAndNotifyDone()
 		}