Deterministically crash in DefaultActorImpl::destroy() when self escapes in deinit

Author: nickolas-pohilets

stdlib/public/Concurrency/Actor.cpp

@@ -1653,6 +1653,22 @@ void ProcessOutOfLineJob::process(Job *job) {
 #endif /* !SWIFT_CONCURRENCY_ACTORS_AS_LOCKS */
 
 void DefaultActorImpl::destroy() {
+  HeapObject *object = asAbstract(this);
+  size_t retainCount = swift_retainCount(object);
+  if (SWIFT_UNLIKELY(retainCount > 1)) {
+    auto descriptor = object->metadata->getTypeContextDescriptor();
+
+    swift_Concurrency_fatalError(0,
+                      "Object %p of class %s deallocated with non-zero retain "
+                      "count %zd. This object's deinit, or something called "
+                      "from it, may have created a strong reference to self "
+                      "which outlived deinit, resulting in a dangling "
+                      "reference.\n",
+                      object,
+                      descriptor ? descriptor->Name.get() : "<unknown>",
+                      retainCount);
+  }
+
 #if SWIFT_CONCURRENCY_ACTORS_AS_LOCKS
   // TODO (rokhinip): Do something to assert that the lock is unowned
 #else

test/Concurrency/Runtime/actor_deinit_escaping_self.swift

@@ -4,13 +4,16 @@
 // REQUIRES: libdispatch
 // REQUIRES: concurrency
 // REQUIRES: concurrency_runtime
-// REQUIRES: swift_stdlib_asserts
 // UNSUPPORTED: back_deployment_runtime
 
 import _Concurrency
 import Dispatch
 import StdlibUnittest
 
+func formatAddr(_ obj: AnyObject) -> String {
+  "0x" + String(unsafeBitCast(obj, to: UInt.self), radix: 16)
+}
+
 actor EscapeLocked {
   var k: Int = 1
 
@@ -27,7 +30,7 @@ actor EscapeLocked {
     }
     let r = g.wait(timeout: .now() + .milliseconds(500))
     expectEqual(r, .timedOut)
-    expectCrashLater(withMessage: "Assertion failed: (!oldState.getFirstUnprioritisedJob() && \"actor has queued jobs at destruction\"), function destroy")
+    expectCrashLater(withMessage: "Object \(formatAddr(self)) of class EscapeLocked deallocated with non-zero retain count 2. This object's deinit, or something called from it, may have created a strong reference to self which outlived deinit, resulting in a dangling reference.")
   }
 }