itest: document the flake found in preimage extraction

We now use dedicated methods to properly document the flakes in *one*
place.

itest/flakes.go

@@ -0,0 +1,33 @@
+package itest
+
+import (
+	"time"
+
+	"github.com/lightningnetwork/lnd/lntest"
+)
+
+// flakePreimageSettlement documents a flake found when testing the preimage
+// extraction logic in a force close. The scenario is,
+//   - Alice and Bob have a channel.
+//   - Alice sends an HTLC to Bob, and Bob won't settle it.
+//   - Alice goes offline.
+//   - Bob force closes the channel and claims the HTLC using the preimage using
+//     a sweeping tx.
+//
+// TODO(yy): Expose blockbeat to the link layer so the preimage extraction
+// happens in the same block where it's spent.
+func flakePreimageSettlement(ht *lntest.HarnessTest) {
+	// Mine a block to trigger the sweep. This is needed because the
+	// preimage extraction logic from the link is not managed by the
+	// blockbeat, which means the preimage may be sent to the contest
+	// resolver after it's launched, which means Bob would miss the block to
+	// launch the resolver.
+	ht.MineEmptyBlocks(1)
+
+	// Sleep for 2 seconds, which is needed to make sure the mempool has the
+	// correct tx. The above mined block can cause an RBF, if the preimage
+	// extraction has already been finished before the block is mined. This
+	// means Bob would have created the sweeping tx - mining another block
+	// would cause the sweeper to RBF it.
+	time.Sleep(2 * time.Second)
+}

itest/lnd_channel_force_close_test.go

@@ -1371,13 +1371,7 @@ func testFailingChannel(ht *lntest.HarnessTest) {
 	// Carol will use the correct preimage to resolve the HTLC on-chain.
 	ht.AssertNumPendingSweeps(carol, 1)
 
-	// Mine a block to trigger the sweep. This is needed because the
+	flakePreimageSettlement(ht)
-	// preimage extraction logic from the link is not managed by the
-	// blockbeat, which means the preimage may be sent to the contest
-	// resolver after it's launched.
-	//
-	// TODO(yy): Expose blockbeat to the link layer.
-	ht.MineEmptyBlocks(1)
 
 	// Carol should have broadcast her sweeping tx.
 	ht.AssertNumTxsInMempool(1)

itest/lnd_multi-hop_force_close_test.go

@@ -1842,13 +1842,7 @@ func runLocalClaimIncomingHTLC(ht *lntest.HarnessTest,
 	// - the anchor output from channel Bob=>Carol.
 	ht.AssertNumPendingSweeps(bob, 3)
 
-	// Mine a block to trigger the sweep. This is needed because the
+	flakePreimageSettlement(ht)
-	// preimage extraction logic from the link is not managed by the
-	// blockbeat, which means the preimage may be sent to the contest
-	// resolver after it's launched.
-	//
-	// TODO(yy): Expose blockbeat to the link layer.
-	ht.MineEmptyBlocks(1)
 
 	// At this point, Bob should have broadcast his second layer success
 	// tx, and should have sent it to his sweeper.
@@ -2126,13 +2120,7 @@ func runLocalClaimIncomingHTLCLeased(ht *lntest.HarnessTest,
 	// - the anchor output from channel Bob=>Carol.
 	ht.AssertNumPendingSweeps(bob, 3)
 
-	// Mine a block to trigger the sweep. This is needed because the
+	flakePreimageSettlement(ht)
-	// preimage extraction logic from the link is not managed by the
-	// blockbeat, which means the preimage may be sent to the contest
-	// resolver after it's launched.
-	//
-	// TODO(yy): Expose blockbeat to the link layer.
-	ht.MineEmptyBlocks(1)
 
 	// At this point, Bob should have broadcast his second layer success
 	// tx, and should have sent it to his sweeper.
@@ -2527,13 +2515,7 @@ func runLocalPreimageClaim(ht *lntest.HarnessTest,
 		ht.MineEmptyBlocks(1)
 	}
 
-	// Mine a block to trigger the sweep. This is needed because the
+	flakePreimageSettlement(ht)
-	// preimage extraction logic from the link is not managed by the
-	// blockbeat, which means the preimage may be sent to the contest
-	// resolver after it's launched.
-	//
-	// TODO(yy): Expose blockbeat to the link layer.
-	ht.MineEmptyBlocks(1)
 
 	// Bob should broadcast the sweeping of the direct preimage spent now.
 	bobHtlcSweep := ht.GetNumTxsFromMempool(1)[0]
@@ -2791,13 +2773,7 @@ func runLocalPreimageClaimLeased(ht *lntest.HarnessTest,
 	// - the htlc sweeping tx.
 	ht.AssertNumPendingSweeps(bob, 3)
 
-	// Mine a block to trigger the sweep. This is needed because the
+	flakePreimageSettlement(ht)
-	// preimage extraction logic from the link is not managed by the
-	// blockbeat, which means the preimage may be sent to the contest
-	// resolver after it's launched.
-	//
-	// TODO(yy): Expose blockbeat to the link layer.
-	ht.MineEmptyBlocks(1)
 
 	// Bob should broadcast the sweeping of the direct preimage spent now.
 	bobHtlcSweep := ht.GetNumTxsFromMempool(1)[0]
@@ -3223,13 +3199,7 @@ func runHtlcAggregation(ht *lntest.HarnessTest,
 	// txns.
 	ht.AssertNumPendingSweeps(bob, numInvoices*2)
 
-	// Mine a block to trigger the sweep. This is needed because the
+	flakePreimageSettlement(ht)
-	// preimage extraction logic from the link is not managed by the
-	// blockbeat, which means the preimage may be sent to the contest
-	// resolver after it's launched.
-	//
-	// TODO(yy): Expose blockbeat to the link layer.
-	ht.MineEmptyBlocks(1)
 
 	// We expect to see three sweeping txns:
 	// 1. Bob's sweeping tx for all timeout HTLCs.

itest/lnd_payment_test.go

@@ -152,13 +152,7 @@ func testPaymentSucceededHTLCRemoteSwept(ht *lntest.HarnessTest) {
 	// Alice should have a pending force close channel.
 	ht.AssertNumPendingForceClose(alice, 1)
 
-	// Mine a block to trigger the sweep. This is needed because the
+	flakePreimageSettlement(ht)
-	// preimage extraction logic from the link is not managed by the
-	// blockbeat, which means the preimage may be sent to the contest
-	// resolver after it's launched.
-	//
-	// TODO(yy): Expose blockbeat to the link layer.
-	ht.MineEmptyBlocks(1)
 
 	// Mine Bob's sweeping tx.
 	ht.MineBlocksAndAssertNumTxes(1, 1)