multi: store peer flap rate on disk on best effort basis

Since we will use peer flap rate to determine how we rate limit, we
store this value on disk per peer per channel. This allows us to
restart with memory of our peers past behaviour, so we don't give badly
behaving peers have a fresh start on restart. Last flap timestamp is
stored with our flap count so that we can degrade this all time flap
count over time for peers that have not recently flapped.

chanfitness/chaneventstore_test.go

@@ -179,6 +179,51 @@ func testEventStore(t *testing.T, generateEvents func(*chanEventStoreTestCtx),
 	require.Equal(t, expectedChannels, monitor.channelCount())
 }
 
+// TestStoreFlapCount tests flushing of flap counts to disk on timer ticks and
+// on store shutdown.
+func TestStoreFlapCount(t *testing.T) {
+	testCtx := newChanEventStoreTestCtx(t)
+	testCtx.start()
+
+	pubkey, _, _ := testCtx.createChannel()
+	testCtx.peerEvent(pubkey, false)
+
+	// Now, we tick our flap count ticker. We expect our main goroutine to
+	// flush our tick count to disk.
+	testCtx.tickFlapCount()
+
+	// Since we just tracked a offline event, we expect a single flap for
+	// our peer.
+	expectedUpdate := peerFlapCountMap{
+		pubkey: {
+			Count:    1,
+			LastFlap: testCtx.clock.Now(),
+		},
+	}
+
+	testCtx.assertFlapCountUpdated()
+	testCtx.assertFlapCountUpdates(expectedUpdate)
+
+	// Create three events for out peer, online/offline/online.
+	testCtx.peerEvent(pubkey, true)
+	testCtx.peerEvent(pubkey, false)
+	testCtx.peerEvent(pubkey, true)
+
+	// Trigger another write.
+	testCtx.tickFlapCount()
+
+	// Since we have processed 3 more events for our peer, we update our
+	// expected online map to have a flap count of 4 for this peer.
+	expectedUpdate[pubkey] = &channeldb.FlapCount{
+		Count:    4,
+		LastFlap: testCtx.clock.Now(),
+	}
+	testCtx.assertFlapCountUpdated()
+	testCtx.assertFlapCountUpdates(expectedUpdate)
+
+	testCtx.stop()
+}
+
 // TestGetChanInfo tests the GetChanInfo function for the cases where a channel
 // is known and unknown to the store.
 func TestGetChanInfo(t *testing.T) {