fix(os): avoid recursive signal cancellation lock

Author: stephenlclarke

Sources/ContainerizationOS/AsyncSignalHandler.swift

@@ -43,19 +43,22 @@ public final class AsyncSignalHandler: Sendable {
     /// Cancel every AsyncStream of signals, as well as the underlying
     /// DispatchSignalSource's for each registered signal.
     public func cancel() {
-        self.state.withLock {
-            if $0.conts.isEmpty {
-                return
-            }
-
-            for cont in $0.conts {
-                cont.finish()
-            }
-            for source in $0.sources {
-                source.cancel()
-            }
+        let resources = self.state.withLock {
+            let continuations = $0.conts
+            let sources = $0.sources
             $0.conts.removeAll()
             $0.sources.removeAll()
+            return (continuations, sources)
+        }
+
+        // Finishing a continuation invokes its onTermination callback, which
+        // calls cancel() again. Release the state lock before doing so to keep
+        // cancellation idempotent without recursively acquiring the mutex.
+        for continuation in resources.0 {
+            continuation.finish()
+        }
+        for source in resources.1 {
+            source.cancel()
         }
     }
 

Tests/ContainerizationOSTests/AsyncSignalHandlerTests.swift

@@ -0,0 +1,41 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2026 Apple Inc. and the Containerization project authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//===----------------------------------------------------------------------===//
+
+import Testing
+
+@testable import ContainerizationOS
+
+@Suite("Async signal handler tests")
+struct AsyncSignalHandlerTests {
+    @Test
+    func cancelFinishesStreamsWithoutRecursivelyLocking() async {
+        // Stream cancellation exercises the recursive callback without installing
+        // a process-wide signal disposition in the shared test runner.
+        let handler = AsyncSignalHandler.create(notify: [])
+        let firstStream = handler.signals
+        let secondStream = handler.signals
+
+        handler.cancel()
+
+        var firstIterator = firstStream.makeAsyncIterator()
+        var secondIterator = secondStream.makeAsyncIterator()
+        #expect(await firstIterator.next() == nil)
+        #expect(await secondIterator.next() == nil)
+
+        // Cancellation remains safe after all resources have been drained.
+        handler.cancel()
+    }
+}