device: allow reducing queue constants on iOS

Heavier network extensions might require the wireguard-go component to
use less ram, so let users of this reduce these as needed.

At some point we'll put this behind a configuration method of sorts, but
for now, just expose the consts as vars.

Requested-by: Josh Bleecher Snyder <josh@tailscale.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

device/constants.go

@@ -35,7 +35,6 @@ const (
 /* Implementation constants */
 
 const (
-	UnderLoadQueueSize = QueueHandshakeSize / 8
 	UnderLoadAfterTime = time.Second // how long does the device remain under load after detected
 	MaxPeers           = 1 << 16     // maximum number of configured peers
 )

device/device.go

@@ -210,7 +210,7 @@ func (device *Device) Down() error {
 func (device *Device) IsUnderLoad() bool {
 	// check if currently under load
 	now := time.Now()
-	underLoad := len(device.queue.handshake.c) >= UnderLoadQueueSize
+	underLoad := len(device.queue.handshake.c) >= QueueHandshakeSize/8
 	if underLoad {
 		atomic.StoreInt64(&device.rate.underLoadUntil, now.Add(UnderLoadAfterTime).UnixNano())
 		return true

device/queueconstants_ios.go

@@ -7,13 +7,15 @@
 
 package device
 
-/* Fit within memory limits for iOS's Network Extension API, which has stricter requirements */
-
-const (
+// Fit within memory limits for iOS's Network Extension API, which has stricter requirements.
+// These are vars instead of consts, because heavier network extensions might want to reduce
+// them further.
+var (
 	QueueStagedSize                   = 128
 	QueueOutboundSize                 = 1024
 	QueueInboundSize                  = 1024
 	QueueHandshakeSize                = 1024
-	MaxSegmentSize             = 1700
-	PreallocatedBuffersPerPool = 1024
+	PreallocatedBuffersPerPool uint32 = 1024
 )
+
+const MaxSegmentSize = 1700