Code review:

- harden relay backoff fields for cross-thread access
- reuse EmptyConnectionListener in backoff test
- extract shared relay-client test fakes

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip01Core/relay/client/single/basic/BasicRelayClient.kt

@@ -31,6 +31,7 @@ import com.vitorpamplona.quartz.nip01Core.relay.sockets.WebSocketListener
 import com.vitorpamplona.quartz.nip01Core.relay.sockets.WebsocketBuilder
 import com.vitorpamplona.quartz.utils.Log
 import com.vitorpamplona.quartz.utils.TimeUtils
+import kotlin.concurrent.Volatile
 import kotlin.concurrent.atomics.AtomicBoolean
 import kotlin.concurrent.atomics.ExperimentalAtomicApi
 import kotlin.coroutines.cancellation.CancellationException
@@ -75,18 +76,22 @@ open class BasicRelayClient(
     private var socket: WebSocket? = null
 
     // True if it has received the onOpen call from the socket.
-    private var isReady: Boolean = false
+    // @Volatile: written on the serialized socket-callback thread, read from the
+    // relay-pool/timer thread (see RelayLoadingCursors for the same pattern).
+    @Volatile private var isReady: Boolean = false
     private var usingCompression: Boolean = false
 
     // keeps increasing the delay to connect when errors happen.
     // This avoids the constant desire to connect when the server is
-    // having trouble or offline.
-    private var lastConnectTentativeInSeconds: Long = 0L // the beginning of time.
-    private var delayToConnectInSeconds = DELAY_TO_RECONNECT_IN_SECS
+    // having trouble or offline. @Volatile: read on the pool thread in
+    // connectAndSyncFiltersIfDisconnected, written on the socket-callback thread.
+    @Volatile private var lastConnectTentativeInSeconds: Long = 0L // the beginning of time.
+
+    @Volatile private var delayToConnectInSeconds = DELAY_TO_RECONNECT_IN_SECS
 
     // when the current connection became ready; used to decide if the
     // connection was stable enough to reset the backoff on disconnect.
-    private var connectedAtInSeconds: Long = 0L
+    @Volatile private var connectedAtInSeconds: Long = 0L
 
     // Makes sure only one socket is open for each url
     private var connectingMutex = AtomicBoolean(false)
@@ -230,6 +235,7 @@ open class BasicRelayClient(
     override fun disconnect() {
         lastConnectTentativeInSeconds = 0L // this is not an error, so prepare to reconnect as soon as requested.
         delayToConnectInSeconds = DELAY_TO_RECONNECT_IN_SECS
+        connectedAtInSeconds = 0L
         socket?.disconnect()
         socket = null
         isReady = false

quartz/src/commonTest/kotlin/com/vitorpamplona/quartz/nip01Core/relay/client/single/basic/BasicRelayClientBackoffTest.kt

@@ -20,11 +20,9 @@
  */
 package com.vitorpamplona.quartz.nip01Core.relay.client.single.basic
 
-import com.vitorpamplona.quartz.nip01Core.relay.client.listeners.RelayConnectionListener
+import com.vitorpamplona.quartz.nip01Core.relay.client.listeners.EmptyConnectionListener
 import com.vitorpamplona.quartz.nip01Core.relay.normalizer.NormalizedRelayUrl
-import com.vitorpamplona.quartz.nip01Core.relay.sockets.WebSocket
 import com.vitorpamplona.quartz.nip01Core.relay.sockets.WebSocketListener
-import com.vitorpamplona.quartz.nip01Core.relay.sockets.WebsocketBuilder
 import kotlin.test.Test
 import kotlin.test.assertTrue
 
@@ -38,32 +36,6 @@ import kotlin.test.assertTrue
 class BasicRelayClientBackoffTest {
     private val url = NormalizedRelayUrl("wss://flaky.example.com")
 
-    class FakeWebSocket : WebSocket {
-        override fun needsReconnect() = false
-
-        override fun connect() {}
-
-        override fun disconnect() {}
-
-        override fun send(msg: String) = true
-    }
-
-    class FakeWebsocketBuilder : WebsocketBuilder {
-        var connectAttempts = 0
-        lateinit var lastListener: WebSocketListener
-
-        override fun build(
-            url: NormalizedRelayUrl,
-            out: WebSocketListener,
-        ): WebSocket {
-            connectAttempts++
-            lastListener = out
-            return FakeWebSocket()
-        }
-    }
-
-    class NoopListener : RelayConnectionListener
-
     class MutableClock(
         var now: Long = 1_000_000L,
     )
@@ -99,7 +71,7 @@ class BasicRelayClientBackoffTest {
     ) = BasicRelayClient(
         url = url,
         socketBuilder = builder,
-        listener = NoopListener(),
+        listener = EmptyConnectionListener,
         nowInSeconds = { clock.now },
     )
 
@@ -167,7 +139,7 @@ class BasicRelayClientBackoffTest {
         runTicks(client, builder, clock, totalSeconds = 15 * 60) { listener ->
             listener.onOpen(50, false)
         }
-        check(client.isConnected()) { "Test setup: relay should have reconnected and stayed up" }
+        assertTrue(client.isConnected(), "Test setup: relay should have reconnected and stayed up")
         clock.now += 10 * 60
         builder.lastListener.onClosed(1000, "server restart")
 

quartz/src/commonTest/kotlin/com/vitorpamplona/quartz/nip01Core/relay/client/single/basic/BasicRelayClientTest.kt

@@ -23,36 +23,11 @@ package com.vitorpamplona.quartz.nip01Core.relay.client.single.basic
 import com.vitorpamplona.quartz.nip01Core.relay.client.listeners.RelayConnectionListener
 import com.vitorpamplona.quartz.nip01Core.relay.client.single.IRelayClient
 import com.vitorpamplona.quartz.nip01Core.relay.normalizer.NormalizedRelayUrl
-import com.vitorpamplona.quartz.nip01Core.relay.sockets.WebSocket
 import com.vitorpamplona.quartz.nip01Core.relay.sockets.WebSocketListener
-import com.vitorpamplona.quartz.nip01Core.relay.sockets.WebsocketBuilder
 import kotlin.test.Test
 import kotlin.test.assertEquals
-import kotlin.test.assertNotNull
 
 class BasicRelayClientTest {
-    private class FakeWebSocket : WebSocket {
-        override fun needsReconnect() = false
-
-        override fun connect() {}
-
-        override fun disconnect() {}
-
-        override fun send(msg: String) = true
-    }
-
-    private class FakeWebsocketBuilder : WebsocketBuilder {
-        var capturedListener: WebSocketListener? = null
-
-        override fun build(
-            url: NormalizedRelayUrl,
-            out: WebSocketListener,
-        ): WebSocket {
-            capturedListener = out
-            return FakeWebSocket()
-        }
-    }
-
     private class RecordingConnectionListener : RelayConnectionListener {
         val cannotConnectMessages = mutableListOf<String>()
 
@@ -81,9 +56,7 @@ class BasicRelayClientTest {
                 listener,
             )
         client.connect()
-        val socketListener = builder.capturedListener
-        assertNotNull(socketListener)
-        return Harness(socketListener, listener)
+        return Harness(builder.lastListener, listener)
     }
 
     @Test

quartz/src/commonTest/kotlin/com/vitorpamplona/quartz/nip01Core/relay/client/single/basic/RelayClientTestFakes.kt

@@ -0,0 +1,61 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.nip01Core.relay.client.single.basic
+
+import com.vitorpamplona.quartz.nip01Core.relay.normalizer.NormalizedRelayUrl
+import com.vitorpamplona.quartz.nip01Core.relay.sockets.WebSocket
+import com.vitorpamplona.quartz.nip01Core.relay.sockets.WebSocketListener
+import com.vitorpamplona.quartz.nip01Core.relay.sockets.WebsocketBuilder
+
+/**
+ * No-op [WebSocket] for tests. connect/disconnect/send do nothing; the relay's
+ * behavior is driven by invoking the [WebSocketListener] callbacks captured by
+ * [FakeWebsocketBuilder].
+ */
+class FakeWebSocket : WebSocket {
+    override fun needsReconnect() = false
+
+    override fun connect() {}
+
+    override fun disconnect() {}
+
+    override fun send(msg: String) = true
+}
+
+/**
+ * [WebsocketBuilder] that records how many sockets were built ([connectAttempts])
+ * and exposes the most recently captured [WebSocketListener] so a test can drive
+ * onOpen/onFailure/onClosed. [lastListener] is set on every [build]; read it only
+ * after the client has attempted to connect.
+ */
+class FakeWebsocketBuilder : WebsocketBuilder {
+    var connectAttempts = 0
+    lateinit var lastListener: WebSocketListener
+
+    override fun build(
+        url: NormalizedRelayUrl,
+        out: WebSocketListener,
+    ): WebSocket {
+        connectAttempts++
+        lastListener = out
+        return FakeWebSocket()
+    }
+}