fix multiple TTL bugs

The first fix independently extends the address expiration time and the address
TTL:

By example:

* We have an address with a TTL of 4s that will expire in 1s.
* We update it with a TTL of 3s.

Before this change:

* We end up with an address with a TTL of 3s that will expire in 3s.

After this change:

* We end up with an address with a TTL of 4s that will expire in 3s.

---

The second fix prevents the in-memory addressbook from announcing existing
addresses every time their TTLs get updated.

---

The third fix correctly updates TTLs for existing addresses in the on-disk
addressbook.

This fixes libp2p/go-libp2p-identify#2

pstoreds/addr_book.go

@@ -347,11 +347,21 @@ Outer:
 		for _, have := range pr.Addrs {
 			if incoming.Equal(have.Addr) {
 				existed[i] = true
-				if mode == ttlExtend && have.Expiry > newExp {
-					// if we're only extending TTLs but the addr already has a longer one, we skip it.
-					continue Outer
+				switch mode {
+				case ttlOverride:
+					have.Ttl = int64(ttl)
+				case ttlExtend:
+					if int64(ttl) > have.Ttl {
+						have.Ttl = int64(ttl)
+					}
+				default:
+					panic("BUG: unimplemented ttl mode")
 				}
-				have.Expiry = newExp
+
+				if newExp > have.Expiry {
+					have.Expiry = newExp
+				}
+
 				// we found the address, and addresses cannot be duplicate,
 				// so let's move on to the next.
 				continue Outer

pstoremem/addr_book.go

@@ -132,7 +132,7 @@ func (mab *memoryAddrBook) AddAddr(p peer.ID, addr ma.Multiaddr, ttl time.Durati
 
 // AddAddrs gives memoryAddrBook addresses to use, with a given ttl
 // (time-to-live), after which the address is no longer valid.
-// If the manager has a longer TTL, the operation is a no-op for that address
+// This function never reduces the TTL or expiration of an address.
 func (mab *memoryAddrBook) AddAddrs(p peer.ID, addrs []ma.Multiaddr, ttl time.Duration) {
 	// if ttl is zero, exit. nothing to do.
 	if ttl <= 0 {
@@ -156,10 +156,19 @@ func (mab *memoryAddrBook) AddAddrs(p peer.ID, addrs []ma.Multiaddr, ttl time.Du
 		}
 		asBytes := addr.Bytes()
 		a, found := amap[string(asBytes)] // won't allocate.
-		if !found || exp.After(a.Expires) {
+		if !found {
+			// not found, save and announce it.
 			amap[string(asBytes)] = &expiringAddr{Addr: addr, Expires: exp, TTL: ttl}
-
 			mab.subManager.BroadcastAddr(p, addr)
+		} else {
+			// Update expiration/TTL independently.
+			// We never want to reduce either.
+			if ttl > a.TTL {
+				a.TTL = ttl
+			}
+			if exp.After(a.Expires) {
+				a.Expires = exp
+			}
 		}
 	}
 }