peerstore: make it possible to use an empty peer ID (#2006)

p2p/host/peerstore/pstoremem/addr_book.go

@@ -46,7 +46,10 @@ type addrSegment struct {
 }
 
 func (segments *addrSegments) get(p peer.ID) *addrSegment {
-	return segments[byte(p[len(p)-1])]
+	if len(p) == 0 { // it's not terribly useful to use an empty peer ID, but at least we should not panic
+		return segments[0]
+	}
+	return segments[uint8(p[len(p)-1])]
 }
 
 type clock interface {

p2p/host/peerstore/test/addr_book_suite.go

@@ -135,6 +135,12 @@ func testAddAddress(ab pstore.AddrBook, clk *mockClock.Mock) func(*testing.T) {
 			ab.UpdateAddrs(id, 4*time.Second, 0)
 			AssertAddressesEqual(t, nil, ab.Addrs(id))
 		})
+
+		t.Run("accessing an empty peer ID", func(t *testing.T) {
+			addrs := GenerateAddrs(5)
+			ab.AddAddrs("", addrs, time.Hour)
+			AssertAddressesEqual(t, addrs, ab.Addrs(""))
+		})
 	}
 }