taildrop: implement asynchronous file deletion (#9844)

File resumption requires keeping partial files around for some time,
but we must still eventually delete them if never resumed.
Thus, we implement asynchronous file deletion, which could
spawn a background goroutine to delete the files.

We also use the same mechanism for deleting files on Windows,
where a file can't be deleted if there is still an open file handle.
We can enqueue those with the asynchronous file deleter as well.

Updates tailscale/corp#14772

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

ipn/ipnlocal/local.go

@@ -646,6 +646,9 @@ func (b *LocalBackend) Shutdown() {
 	if b.sockstatLogger != nil {
 		b.sockstatLogger.Shutdown()
 	}
+	if b.peerAPIServer != nil {
+		b.peerAPIServer.taildrop.Shutdown()
+	}
 
 	b.unregisterNetMon()
 	b.unregisterHealthWatch()
@@ -3614,14 +3617,14 @@ func (b *LocalBackend) initPeerAPIListener() {
 
 	ps := &peerAPIServer{
 		b: b,
-		taildrop: &taildrop.Manager{
+		taildrop: taildrop.ManagerOptions{
 			Logf:             b.logf,
-			Clock:            tstime.DefaultClock{b.clock},
+			Clock:            tstime.DefaultClock{Clock: b.clock},
 			Dir:              fileRoot,
 			DirectFileMode:   b.directFileRoot != "",
 			AvoidFinalRename: !b.directFileDoFinalRename,
 			SendFileNotify:   b.sendFileNotify,
-		},
+		}.New(),
 	}
 	if dm, ok := b.sys.DNSManager.GetOK(); ok {
 		ps.resolver = dm.Resolver()