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)
+}