itest: document a flake found on macOS

itest/flakes.go

@@ -1,6 +1,7 @@
 package itest
 
 import (
+	"runtime"
 	"time"
 
 	"github.com/btcsuite/btcd/btcutil"
@@ -70,3 +71,36 @@ func flakeTxNotifierNeutrino(ht *lntest.HarnessTest) {
 		ht.MineEmptyBlocks(1)
 	}
 }
+
+// flakeSkipPendingSweepsCheckDarwin documents a flake found only in macOS
+// build. When running in macOS, we might see three anchor sweeps - one from the
+// local, one from the remote, and one from the pending remote:
+//   - the local one will be swept.
+//   - the remote one will be marked as failed due to `testmempoolaccept` check.
+//   - the pending remote one will not be attempted due to it being uneconomical
+//     since it was not used for CPFP.
+//
+// The anchor from the pending remote may or may not appear, which is a bug
+// found only in macOS - when updating the commitments, the channel state
+// machine somehow thinks we still have a pending remote commitment, causing it
+// to sweep the anchor from that version.
+//
+// TODO(yy): fix the above bug in the channel state machine.
+func flakeSkipPendingSweepsCheckDarwin(ht *lntest.HarnessTest,
+	node *node.HarnessNode, num int) {
+
+	// Skip the assertion below if it's on macOS.
+	if isDarwin() {
+		ht.Logf("Skipped AssertNumPendingSweeps for node %s",
+			node.Name())
+
+		return
+	}
+
+	ht.AssertNumPendingSweeps(node, num)
+}
+
+// isDarwin returns true if the test is running on a macOS.
+func isDarwin() bool {
+	return runtime.GOOS == "darwin"
+}