htlcswitch: fix alignment of the packetQueue's fields for 32-bit systems (#507)

In this commit, we fix an existing issue that would cause lnd to panic
on 32-bit systems. Within the packetQueue we utilize atomics heavily.
However, it's the caller's job to ensure 64-bit alignment of 64-bit words
accessed atomically. This is documented within the sync/atomic package
as a set of known bugs.

The old alignment of this struct was:

⛰  structlayout github.com/lightningnetwork/lnd/htlcswitch packetQueue
packetQueue.queueLen int32: 0-4 (size 4, align 4)
padding: 4-8 (size 4, align 0)
packetQueue.totalHtlcAmt int64: 8-16 (size 8, align 8)
packetQueue.queueCond *sync.Cond: 16-24 (size 8, align 8)
packetQueue.queueMtx.state int32: 24-28 (size 4, align 4)
packetQueue.queueMtx.sema uint32: 28-32 (size 4, align 4)
packetQueue.queue []*github.com/lightningnetwork/lnd/htlcswitch.htlcPacket: 32-56 (size 24, align 8)
packetQueue.outgoingPkts chan *github.com/lightningnetwork/lnd/htlcswitch.htlcPacket: 56-64 (size 8, align 8)
packetQueue.freeSlots chan struct{}: 64-72 (size 8, align 8)
packetQueue.wg.noCopy sync.noCopy: 72-72 (size 0, align 1)
packetQueue.wg.state1 [12]byte: 72-84 (size 12, align 1)
packetQueue.wg.sema uint32: 84-88 (size 4, align 4)
packetQueue.quit chan struct{}: 88-96 (size 8, align 8)

After this commit, the new alignment of this sturct is:

⛰  structlayout  -json github.com/lightningnetwork/lnd/htlcswitch packetQueue | structlayout-optimize
packetQueue.queue []*github.com/lightningnetwork/lnd/htlcswitch.htlcPacket: 0-24 (size 24, align 8)
packetQueue.wg struct: 24-40 (size 16, align 8)
packetQueue.freeSlots chan struct{}: 40-48 (size 8, align 8)
packetQueue.queueCond *sync.Cond: 48-56 (size 8, align 8)
packetQueue.queueMtx struct: 56-64 (size 8, align 8)
packetQueue.outgoingPkts chan *github.com/lightningnetwork/lnd/htlcswitch.htlcPacket: 64-72 (size 8, align 8)
packetQueue.totalHtlcAmt int64: 72-80 (size 8, align 8)
packetQueue.quit chan struct{}: 80-88 (size 8, align 8)
packetQueue.queueLen int32: 88-92 (size 4, align 8)
padding: 92-96 (size 4, align 0)

Fixes #505, and #463.

htlcswitch/queue.go

@@ -17,6 +17,29 @@ import (
 // to signal the number of slots available, and a condition variable to allow
 // the packetQueue to know when new items have been added to the queue.
 type packetQueue struct {
+	queue []*htlcPacket
+
+	wg sync.WaitGroup
+
+	// freeSlots serves as a semaphore who's current value signals the
+	// number of available slots on the commitment transaction.
+	freeSlots chan struct{}
+
+	queueCond *sync.Cond
+	queueMtx  sync.Mutex
+
+	// outgoingPkts is a channel that the channelLink will receive on in
+	// order to drain the packetQueue as new slots become available on the
+	// commitment transaction.
+	outgoingPkts chan *htlcPacket
+
+	// totalHtlcAmt is the sum of the value of all pending HTLC's currently
+	// residing within the overflow queue. This value should only read or
+	// modified *atomically*.
+	totalHtlcAmt int64
+
+	quit chan struct{}
+
 	// queueLen is an internal counter that reflects the size of the queue
 	// at any given instance. This value is intended to be use atomically
 	// as this value is used by internal methods to obtain the length of
@@ -24,27 +47,6 @@ type packetQueue struct {
 	// deadlock situation where the main goroutine is attempting a send
 	// with the lock held.
 	queueLen int32
-
-	// totalHtlcAmt is the sum of the value of all pending HTLC's currently
-	// residing within the overflow queue. This value should only read or
-	// modified *atomically*.
-	totalHtlcAmt int64
-
-	queueCond *sync.Cond
-	queueMtx  sync.Mutex
-	queue     []*htlcPacket
-
-	// outgoingPkts is a channel that the channelLink will receive on in
-	// order to drain the packetQueue as new slots become available on the
-	// commitment transaction.
-	outgoingPkts chan *htlcPacket
-
-	// freeSlots serves as a semaphore who's current value signals the
-	// number of available slots on the commitment transaction.
-	freeSlots chan struct{}
-
-	wg   sync.WaitGroup
-	quit chan struct{}
 }
 
 // newPacketQueue returns a new instance of the packetQueue. The maxFreeSlots