From a86492a5673983fe7755631996a23c2dc510808f Mon Sep 17 00:00:00 2001
From: Josh Bleecher Snyder <josh@tailscale.com>
Date: Sun, 17 Jan 2021 09:49:39 -0800
Subject: device: remove QueueInboundElement.dropped

Now that we block when enqueueing to the decryption queue,
there is only one case in which we "drop" a inbound element,
when decryption fails.

We can use a simple, obvious, sync-free sentinel for that, elem.packet == nil.
Also, we can return the message buffer to the pool slightly later,
which further simplifies the code.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>
---
 device/receive.go | 35 +++++------------------------------
 1 file changed, 5 insertions(+), 30 deletions(-)

diff --git a/device/receive.go b/device/receive.go
index 972b342..701e308 100644
--- a/device/receive.go
+++ b/device/receive.go
@@ -30,7 +30,6 @@ type QueueHandshakeElement struct {
 }
 
 type QueueInboundElement struct {
-	dropped int32
 	sync.Mutex
 	buffer   *[MaxMessageSize]byte
 	packet   []byte
@@ -50,14 +49,6 @@ func (elem *QueueInboundElement) clearPointers() {
 	elem.endpoint = nil
 }
 
-func (elem *QueueInboundElement) Drop() {
-	atomic.StoreInt32(&elem.dropped, AtomicTrue)
-}
-
-func (elem *QueueInboundElement) IsDropped() bool {
-	return atomic.LoadInt32(&elem.dropped) == AtomicTrue
-}
-
 func (device *Device) addToHandshakeQueue(queue chan QueueHandshakeElement, elem QueueHandshakeElement) bool {
 	select {
 	case queue <- elem:
@@ -180,7 +171,6 @@ func (device *Device) RoutineReceiveIncoming(IP int, bind conn.Bind) {
 			elem.packet = packet
 			elem.buffer = buffer
 			elem.keypair = keypair
-			elem.dropped = AtomicFalse
 			elem.endpoint = endpoint
 			elem.counter = 0
 			elem.Mutex = sync.Mutex{}
@@ -243,19 +233,11 @@ func (device *Device) RoutineDecryption() {
 	logDebug.Println("Routine: decryption worker - started")
 
 	for elem := range device.queue.decryption.c {
-		// check if dropped
-
-		if elem.IsDropped() {
-			continue
-		}
-
 		// split message into fields
-
 		counter := elem.packet[MessageTransportOffsetCounter:MessageTransportOffsetContent]
 		content := elem.packet[MessageTransportOffsetContent:]
 
 		// decrypt and release to consumer
-
 		var err error
 		elem.counter = binary.LittleEndian.Uint64(counter)
 		// copy counter to nonce
@@ -267,8 +249,7 @@ func (device *Device) RoutineDecryption() {
 			nil,
 		)
 		if err != nil {
-			elem.Drop()
-			device.PutMessageBuffer(elem.buffer)
+			elem.packet = nil
 		}
 		elem.Unlock()
 	}
@@ -484,9 +465,7 @@ func (peer *Peer) RoutineSequentialReceiver() {
 		logDebug.Println(peer, "- Routine: sequential receiver - stopped")
 		peer.routines.stopping.Done()
 		if elem != nil {
-			if !elem.IsDropped() {
-				device.PutMessageBuffer(elem.buffer)
-			}
+			device.PutMessageBuffer(elem.buffer)
 			device.PutInboundElement(elem)
 		}
 	}()
@@ -495,9 +474,7 @@ func (peer *Peer) RoutineSequentialReceiver() {
 
 	for {
 		if elem != nil {
-			if !elem.IsDropped() {
-				device.PutMessageBuffer(elem.buffer)
-			}
+			device.PutMessageBuffer(elem.buffer)
 			device.PutInboundElement(elem)
 			elem = nil
 		}
@@ -513,15 +490,13 @@ func (peer *Peer) RoutineSequentialReceiver() {
 		}
 
 		// wait for decryption
-
 		elem.Lock()
-
-		if elem.IsDropped() {
+		if elem.packet == nil {
+			// decryption failed
 			continue
 		}
 
 		// check for replay
-
 		if !elem.keypair.replayFilter.ValidateCounter(elem.counter, RejectAfterMessages) {
 			continue
 		}
-- 
cgit v1.2.3-59-g8ed1b