fix(p2p): make Client.Stop idempotent (#105)

docs/hardening-backlog.md

@@ -61,7 +61,7 @@ All items below survived an independent skeptic re-reading the cited code.
 | 31 | ⚪ low | small | operational | `report-falsely-reassuring` | open |
 | 32 | ⚪ low | small | data-integrity | `cas-no-cid-content-verification` | open |
 | 33 | ⚪ low | trivial | test-gap | `merkle-no-segwit-witness-block` | open |
-| 34 | ⚪ low | trivial | reliability | `p2p-stop-not-idempotent` | open |
+| 34 | ⚪ low | trivial | reliability | `p2p-stop-not-idempotent` | ✅ done (#105) |
 | 35 | ⚪ low | small | test-gap | `walletstore-no-differential-fuzz` | open |
 | 36 | ⚪ low | small | correctness | `headers-no-future-time-and-false-ban` | open |
 | 37 | ⚪ low | small | test-gap | `cmd-ion-node-zero-tests` | open |

internal/p2p/client_test.go

@@ -132,3 +132,19 @@ func TestPeerConfigBasics(t *testing.T) {
 		t.Errorf("NewestBlock() = %v,%d,%v; want genesis hash, 0, nil", h, height, err)
 	}
 }
+
+// TestStopIsIdempotent: a double Stop (defer + explicit, or concurrent shutdown
+// paths) must not panic on a closed quit channel.
+func TestStopIsIdempotent(t *testing.T) {
+	cfg, err := config.New("simnet")
+	if err != nil {
+		t.Fatalf("config: %v", err)
+	}
+	cl := New(cfg, chain.NewHeaderChain(cfg.Params), log.Noop())
+	if err := cl.Stop(); err != nil {
+		t.Fatalf("first Stop: %v", err)
+	}
+	if err := cl.Stop(); err != nil { // must not panic (close of closed channel)
+		t.Fatalf("second Stop: %v", err)
+	}
+}

internal/p2p/peer.go

@@ -49,6 +49,7 @@ type Client struct {
 	syncedCh chan struct{}
 	syncOnce sync.Once
 	quit     chan struct{}
+	stopOnce sync.Once
 
 	// allowSelfConns relaxes btcd's self-connection (nonce) guard. It is only
 	// set in tests that run two peers in the same process (which share btcd's
@@ -282,11 +283,15 @@ func (c *Client) Start() error {
 }
 
 // Stop tears down peering.
+// Stop is idempotent: a double Stop (e.g. a defer plus an explicit shutdown, or two
+// concurrent shutdown paths) must not panic on a closed quit channel.
 func (c *Client) Stop() error {
-	close(c.quit)
-	if c.cm != nil {
-		c.cm.Stop()
-	}
+	c.stopOnce.Do(func() {
+		close(c.quit)
+		if c.cm != nil {
+			c.cm.Stop()
+		}
+	})
 	return nil
 }