passive MAC learning and ARP cache revamp (#96)

* begin working on arp cache

* fix little things

* rework arp cache priority

* fix test

* keep working on ARP

* push changes before thinking about ip prefix issue

* add passive peer MAC setting

* patch egress mac with correct ethernet CRCs

* consolidate subnet learning in subnetTable type

* add tests and fix subnet table bug

arp/cache.go

@@ -0,0 +1,167 @@
+package arp
+
+import (
+	"github.com/soypat/lneto"
+	"github.com/soypat/lneto/ethernet"
+	"github.com/soypat/lneto/internal"
+)
+
+type cache struct {
+	entries []entry
+}
+
+// entry is designed for compactness. size=class=24 bytes, same as a slice header on x86.
+type entry struct {
+	addr  [16]byte
+	mac   [6]byte
+	age   uint8
+	flags eflags
+}
+
+func (e *entry) use(mac [6]byte, proto []byte, flags eflags) {
+	e.flags = eflagInUse | flags
+	if copy(e.addr[:], proto) == 16 {
+		e.flags |= eflagIPv6
+	}
+	e.age = 0
+	e.mac = mac
+}
+
+func (e *entry) destroy() { *e = entry{} }
+
+func (e *entry) put(frame, ourAddr []byte, ourMAC [6]byte, op Operation) (int, error) {
+	if len(ourMAC) != 6 {
+		return 0, lneto.ErrInvalidAddr
+	}
+	f, err := NewFrame(frame)
+	if err != nil {
+		return 0, err
+	}
+	f.SetHardware(1, 6)
+	f.SetOperation(op)
+	var n int
+	if e.flags&eflagIPv6 != 0 {
+		if len(ourAddr) != 16 {
+			return 0, lneto.ErrInvalidAddr
+		} else if len(frame) < sizeHeaderv6 {
+			return 0, lneto.ErrShortBuffer
+		}
+		f.SetProtocol(ethernet.TypeIPv6, 16)
+		hw, addr := f.Sender16()
+		*hw = ourMAC
+		copy(addr[:], ourAddr)
+		hw, addr = f.Target16()
+		copy(hw[:], e.mac[:])
+		copy(addr[:], e.addr[:])
+		n = sizeHeaderv6
+	} else {
+		if len(ourAddr) != 4 {
+			return 0, lneto.ErrInvalidAddr
+		}
+		f.SetProtocol(ethernet.TypeIPv4, 4)
+		hw, addr := f.Sender4()
+		*hw = ourMAC
+		copy(addr[:], ourAddr)
+		hw, addr = f.Target4()
+		copy(hw[:], e.mac[:])
+		copy(addr[:], e.addr[:])
+		n = sizeHeaderv4
+	}
+	return n, nil
+}
+
+func (flags eflags) hasAny(bits eflags) bool { return flags&bits != 0 }
+
+type eflags uint8
+
+// unset eflagInUse to signal the entry can be acquired for a new query.
+const (
+	// eflagInUse set when in use. Discarded/unused entries have this bit unset.
+	eflagInUse eflags = 1 << iota
+	// set for IPv6 addressed entries.
+	eflagIPv6
+	// network device queried our address and we must respond to it.
+	// Both MAC and IP are valid in this case.
+	eflagPendingResponse
+	// user asked to query this address and query has yet to be answered. May or may not be sent.
+	eflagIncomplete
+	// user asked to query this address and the query has not been sent out yet.
+	// The MAC address is invalid in this case.
+	eflagIncompletePendingQuery
+	// eflagPriority set for prioritized cache entries. These entries are discarded last.
+	// i.e: set for user created queries, unset for external incoming network queries.
+	eflagPriority
+	// trigger callback, you know the drill.
+	eflagResolveTriggersCallback
+)
+
+func (c *cache) age() {
+	for i := range c.entries {
+		if c.entries[i].flags&eflagInUse != 0 && c.entries[i].age < 255 {
+			c.entries[i].age++
+		}
+	}
+}
+
+func (c *cache) reset(size int) {
+	internal.SliceReuse(&c.entries, size)
+	c.entries = c.entries[:cap(c.entries)] // maximize queries given allocation.
+}
+
+func (c *cache) getNextFlagged(entryHasFlags eflags) *entry {
+	for i := range c.entries {
+		flags := c.entries[i].flags
+		if flags&eflagInUse != 0 && flags.hasAny(entryHasFlags) {
+			return &c.entries[i]
+		}
+	}
+	return nil
+}
+
+func (c *cache) clearFlags(entryHasFlags, clrTheseFlagsIfMatch eflags) {
+	for i := range c.entries {
+		// Can clear flags on unused too, simpler.
+		if c.entries[i].flags&entryHasFlags != 0 {
+			c.entries[i].flags &^= clrTheseFlagsIfMatch
+		}
+	}
+}
+
+func (c *cache) Lookup(addr []byte) *entry {
+	n := len(addr)
+	for i := range c.entries {
+		if c.entries[i].flags&eflagInUse != 0 && internal.BytesEqual(c.entries[i].addr[:n], addr) {
+			return &c.entries[i]
+		}
+	}
+	return nil
+}
+
+// acquireNext gets next available entry for use. If all are in use evicts
+// the oldest passive entry (learned from incoming requests) before touching
+// active user queries or pending responses.
+func (c *cache) acquireNext() *entry {
+	const priorityFlags = eflagPendingResponse | eflagIncomplete | eflagPriority
+	oldest, oldestPassive := 0, -1
+	for i := range c.entries {
+		if c.entries[i].flags&eflagInUse == 0 {
+			oldest = i
+			break
+		}
+		if !c.entries[i].flags.hasAny(priorityFlags) {
+			if oldestPassive < 0 || c.entries[i].age > c.entries[oldestPassive].age {
+				oldestPassive = i
+			}
+		}
+		if c.entries[i].age > c.entries[oldest].age {
+			oldest = i
+		}
+	}
+	if oldestPassive >= 0 && c.entries[oldest].flags&eflagInUse != 0 {
+		oldest = oldestPassive
+	}
+	c.age()
+	e := &c.entries[oldest]
+	e.destroy()
+	return e
+}