Improve AARP

aarp.go

@@ -53,6 +53,7 @@ type AARPMachine struct {
 	mu         sync.RWMutex
 	myAddr     aarp.AddrPair
 	probes     int
+	assigned   bool
 	assignedCh chan struct{}
 }
 
@@ -74,7 +75,7 @@ func NewAARPMachine(cfg *config, pcapHandle *pcap.Handle, myHWAddr ethernet.Addr
 func (a *AARPMachine) Address() (aarp.AddrPair, bool) {
 	a.mu.RLock()
 	defer a.mu.RUnlock()
-	return a.myAddr, a.assigned()
+	return a.myAddr, a.assigned
 }
 
 // Assigned returns a channel that is closed when the local address is valid.
@@ -102,9 +103,11 @@ func (a *AARPMachine) Run(ctx context.Context, incomingCh <-chan *ethertalk.Pack
 			return ctx.Err()
 
 		case <-ticker.C:
-			if a.assigned() {
+			if a.probes >= 10 {
+				a.mu.Lock()
+				a.assigned = true
+				a.mu.Unlock()
 				close(a.assignedCh)
-				// No need to keep the ticker running if assigned
 				ticker.Stop()
 				continue
 			}
@@ -135,7 +138,7 @@ func (a *AARPMachine) Run(ctx context.Context, incomingCh <-chan *ethertalk.Pack
 				a.addressMappingTable.Learn(aapkt.Src.Proto, aapkt.Src.Hardware)
 				log.Printf("AARP: Gleaned that %v -> %v", aapkt.Src.Proto, aapkt.Src.Hardware)
 
-				if !(aapkt.Dst.Proto == a.myAddr.Proto && a.assigned()) {
+				if !(aapkt.Dst.Proto == a.myAddr.Proto && a.assigned) {
 					continue
 				}
 
@@ -152,7 +155,7 @@ func (a *AARPMachine) Run(ctx context.Context, incomingCh <-chan *ethertalk.Pack
 				if aapkt.Dst.Proto != a.myAddr.Proto {
 					continue
 				}
-				if !a.assigned() {
+				if !a.assigned {
 					a.reroll()
 				}
 
@@ -163,7 +166,7 @@ func (a *AARPMachine) Run(ctx context.Context, incomingCh <-chan *ethertalk.Pack
 				if aapkt.Dst.Proto != a.myAddr.Proto {
 					continue
 				}
-				if !a.assigned() {
+				if !a.assigned {
 					// Another node is probing for the same address! Unlucky
 					a.reroll()
 					continue
@@ -182,11 +185,30 @@ func (a *AARPMachine) Run(ctx context.Context, incomingCh <-chan *ethertalk.Pack
 // If the address is in the cache (AMT) and is still valid, that is used.
 // Otherwise, the address is resolved using AARP.
 func (a *AARPMachine) Resolve(ctx context.Context, ddpAddr ddp.Addr) (ethernet.Addr, error) {
-	result, waitCh := a.lookupOrWait(ddpAddr)
+	result, waitCh, winner := a.lookupOrWait(ddpAddr)
 	if waitCh == nil {
 		return result, nil
 	}
 
+	ctx, cancel := context.WithTimeout(ctx, aarpRequestTimeout)
+	defer cancel()
+
+	if !winner {
+		// some other goroutine is running the request
+		for {
+			select {
+			case <-ctx.Done():
+				return ethernet.Addr{}, ctx.Err()
+			case <-waitCh:
+				result, waitCh, _ = a.lookupOrWait(ddpAddr)
+				if waitCh == nil {
+					return result, nil
+				}
+			}
+		}
+	}
+
+	// I am the winner! I get to send the request packets and run the ticker
 	if err := a.request(ddpAddr); err != nil {
 		return ethernet.Addr{}, err
 	}
@@ -194,16 +216,13 @@ func (a *AARPMachine) Resolve(ctx context.Context, ddpAddr ddp.Addr) (ethernet.A
 	ticker := time.NewTicker(aarpRequestRetransmit)
 	defer ticker.Stop()
 
-	ctx, cancel := context.WithTimeout(ctx, aarpRequestTimeout)
-	defer cancel()
-
 	for {
 		select {
 		case <-ctx.Done():
 			return ethernet.Addr{}, ctx.Err()
 
 		case <-waitCh:
-			result, waitCh = a.lookupOrWait(ddpAddr)
+			result, waitCh, _ = a.lookupOrWait(ddpAddr)
 			if waitCh == nil {
 				return result, nil
 			}
@@ -216,8 +235,6 @@ func (a *AARPMachine) Resolve(ctx context.Context, ddpAddr ddp.Addr) (ethernet.A
 	}
 }
 
-func (a *AARPMachine) assigned() bool { return a.probes >= 10 }
-
 // Re-roll a local address
 func (a *AARPMachine) reroll() {
 	a.mu.Lock()
@@ -229,7 +246,7 @@ func (a *AARPMachine) reroll() {
 		) + a.cfg.EtherTalk.NetStart
 	}
 
-	// Can't use: 0x00, 0xff, 0xfe, or the existing node number
+	// Can't use: 0x00, 0xff, 0xfe, and should avoid the existing node number
 	newNode := rand.N[ddp.Node](0xfd) + 1
 	for newNode != a.myAddr.Proto.Node {
 		newNode = rand.N[ddp.Node](0xfd) + 1
@@ -280,9 +297,10 @@ func (a *AARPMachine) request(ddpAddr ddp.Addr) error {
 }
 
 type amtEntry struct {
-	hwAddr  ethernet.Addr
+	hwAddr     ethernet.Addr
-	last    time.Time
+	last       time.Time
-	updated chan struct{}
+	updated    chan struct{}
+	requesting bool
 }
 
 // addressMappingTable implements a concurrent-safe Address Mapping Table for
@@ -302,38 +320,45 @@ func (t *addressMappingTable) Learn(ddpAddr ddp.Addr, hwAddr ethernet.Addr) {
 	oldEnt := t.table[ddpAddr]
 	if oldEnt == nil {
 		t.table[ddpAddr] = &amtEntry{
-			hwAddr:  hwAddr,
+			hwAddr:     hwAddr,
-			last:    time.Now(),
+			last:       time.Now(),
-			updated: make(chan struct{}),
+			updated:    make(chan struct{}),
+			requesting: false,
 		}
 		return
 	}
 
-	if oldEnt.hwAddr == hwAddr && time.Since(oldEnt.last) < maxAMTEntryAge {
-		oldEnt.last = time.Now()
-		return
-	}
 	oldEnt.hwAddr = hwAddr
 	oldEnt.last = time.Now()
+	oldEnt.requesting = false
 	close(oldEnt.updated)
 	oldEnt.updated = make(chan struct{})
 }
 
 // lookupOrWait returns either the valid cached Ethernet address for the given
-// DDP address, or a channel that is closed when the entry is updated.
+// DDP address, or a non-nil channel that is closed when the entry is updated.
-func (t *addressMappingTable) lookupOrWait(ddpAddr ddp.Addr) (ethernet.Addr, <-chan struct{}) {
+// It also reports if this is the first call since the entry became invalid.
+func (t *addressMappingTable) lookupOrWait(ddpAddr ddp.Addr) (ethernet.Addr, <-chan struct{}, bool) {
 	t.mu.Lock()
 	defer t.mu.Unlock()
 	if t.table == nil {
 		t.table = make(map[ddp.Addr]*amtEntry)
 	}
-	ent, ok := t.table[ddpAddr]
+	ent := t.table[ddpAddr]
-	if ok && time.Since(ent.last) < maxAMTEntryAge {
+	if ent == nil {
-		return ent.hwAddr, nil
+		ch := make(chan struct{})
+		t.table[ddpAddr] = &amtEntry{
+			updated:    ch,
+			requesting: true,
+		}
+		return ethernet.Addr{}, ch, true
 	}
-	ch := make(chan struct{})
+	if time.Since(ent.last) >= maxAMTEntryAge {
-	t.table[ddpAddr] = &amtEntry{
+		if ent.requesting {
-		updated: ch,
+			return ent.hwAddr, ent.updated, false
+		}
+		ent.requesting = true
+		return ent.hwAddr, ent.updated, true
 	}
-	return ethernet.Addr{}, ch
+	return ent.hwAddr, nil, false
 }