fix: add thread safety to NostrTransport read receipt queue

Synchronized access to readQueue and isSendingReadAcks using the
existing concurrent DispatchQueue with barrier flags:

- sendReadReceipt(): wrap enqueue in barrier async
- processReadQueueIfNeeded(): extract item within barrier context
- scheduleNextReadAck(): wrap callback in barrier async

This fixes race conditions where concurrent calls could corrupt the
read queue or cause check-then-act bugs on isSendingReadAcks.

Also adds thread safety tests:
- concurrentReadReceiptEnqueue: 100 concurrent enqueue operations
- readQueueProcessingUnderLoad: concurrent enqueue during processing
- isPeerReachableThreadSafety: concurrent read access test

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

bitchat/Services/NostrTransport.swift

@@ -149,8 +149,11 @@ final class NostrTransport: Transport, @unchecked Sendable {
 
     func sendReadReceipt(_ receipt: ReadReceipt, to peerID: PeerID) {
         // Enqueue and process with throttling to avoid relay rate limits
-        readQueue.append(QueuedRead(receipt: receipt, peerID: peerID))
-        processReadQueueIfNeeded()
+        // Use barrier to synchronize access to readQueue
+        queue.async(flags: .barrier) { [weak self] in
+            self?.readQueue.append(QueuedRead(receipt: receipt, peerID: peerID))
+            self?.processReadQueueIfNeeded()
+        }
     }
 
     func sendFavoriteNotification(to peerID: PeerID, isFavorite: Bool) {
@@ -260,16 +263,17 @@ extension NostrTransport {
 // MARK: - Private Helpers
 
 extension NostrTransport {
+    /// Must be called within a barrier on `queue`
     private func processReadQueueIfNeeded() {
         guard !isSendingReadAcks else { return }
         guard !readQueue.isEmpty else { return }
         isSendingReadAcks = true
-        sendNextReadAck()
+        let item = readQueue.removeFirst()
+        sendReadAckItem(item)
     }
 
-    private func sendNextReadAck() {
-        guard !readQueue.isEmpty else { isSendingReadAcks = false; return }
-        let item = readQueue.removeFirst()
+    /// Sends a single read ack item (called after extraction from queue within barrier)
+    private func sendReadAckItem(_ item: QueuedRead) {
         Task { @MainActor in
             guard let recipientNpub = resolveRecipientNpub(for: item.peerID) else { scheduleNextReadAck(); return }
             guard let senderIdentity = try? idBridge.getCurrentNostrIdentity() else { scheduleNextReadAck(); return }
@@ -301,9 +305,10 @@ extension NostrTransport {
 
     private func scheduleNextReadAck() {
         DispatchQueue.main.asyncAfter(deadline: .now() + readAckInterval) { [weak self] in
-            guard let self = self else { return }
-            self.isSendingReadAcks = false
-            self.processReadQueueIfNeeded()
+            self?.queue.async(flags: .barrier) { [weak self] in
+                self?.isSendingReadAcks = false
+                self?.processReadQueueIfNeeded()
+            }
         }
     }
 

bitchatTests/Services/NostrTransportTests.swift

@@ -0,0 +1,108 @@
+//
+// NostrTransportTests.swift
+// bitchat
+//
+// This is free and unencumbered software released into the public domain.
+// For more information, see <https://unlicense.org>
+//
+
+import Foundation
+import Testing
+@testable import bitchat
+
+@Suite("NostrTransport Thread Safety Tests")
+struct NostrTransportTests {
+
+    @Test("Concurrent read receipt enqueue does not crash")
+    @MainActor
+    func concurrentReadReceiptEnqueue() async throws {
+        let keychain = MockKeychain()
+        let idBridge = NostrIdentityBridge(keychain: keychain)
+        let transport = NostrTransport(keychain: keychain, idBridge: idBridge)
+
+        // Create 100 concurrent read receipt submissions
+        let iterations = 100
+
+        await withTaskGroup(of: Void.self) { group in
+            for i in 0..<iterations {
+                group.addTask {
+                    let receipt = ReadReceipt(
+                        originalMessageID: UUID().uuidString,
+                        readerID: PeerID(str: String(format: "%016x", i)),
+                        readerNickname: "Reader\(i)"
+                    )
+                    let peerID = PeerID(str: String(format: "%016x", i))
+                    transport.sendReadReceipt(receipt, to: peerID)
+                }
+            }
+        }
+
+        // If we reach here without crashing, the test passes
+        // The concurrent enqueue operations completed without data races
+    }
+
+    @Test("Read queue processes under concurrent load")
+    @MainActor
+    func readQueueProcessingUnderLoad() async throws {
+        let keychain = MockKeychain()
+        let idBridge = NostrIdentityBridge(keychain: keychain)
+        let transport = NostrTransport(keychain: keychain, idBridge: idBridge)
+
+        // Rapidly enqueue many receipts from multiple concurrent sources
+        let iterations = 50
+
+        // First batch - rapid fire
+        for i in 0..<iterations {
+            let receipt = ReadReceipt(
+                originalMessageID: UUID().uuidString,
+                readerID: PeerID(str: String(format: "%016x", i)),
+                readerNickname: "Reader\(i)"
+            )
+            transport.sendReadReceipt(receipt, to: PeerID(str: String(format: "%016x", i)))
+        }
+
+        // Give some time for processing to start
+        try await Task.sleep(nanoseconds: 100_000_000) // 100ms
+
+        // Second batch - while first might be processing
+        await withTaskGroup(of: Void.self) { group in
+            for i in iterations..<(iterations * 2) {
+                group.addTask {
+                    let receipt = ReadReceipt(
+                        originalMessageID: UUID().uuidString,
+                        readerID: PeerID(str: String(format: "%016x", i)),
+                        readerNickname: "Reader\(i)"
+                    )
+                    transport.sendReadReceipt(receipt, to: PeerID(str: String(format: "%016x", i)))
+                }
+            }
+        }
+
+        // If we reach here without crashing or deadlocking, test passes
+    }
+
+    @Test("isPeerReachable is thread safe")
+    @MainActor
+    func isPeerReachableThreadSafety() async throws {
+        let keychain = MockKeychain()
+        let idBridge = NostrIdentityBridge(keychain: keychain)
+        let transport = NostrTransport(keychain: keychain, idBridge: idBridge)
+
+        let iterations = 100
+
+        // Concurrent reads on isPeerReachable
+        await withTaskGroup(of: Bool.self) { group in
+            for i in 0..<iterations {
+                group.addTask {
+                    let peerID = PeerID(str: String(format: "%016x", i))
+                    return transport.isPeerReachable(peerID)
+                }
+            }
+
+            // Collect results (all should be false since no favorites configured)
+            for await result in group {
+                #expect(result == false)
+            }
+        }
+    }
+}