Created attachment 343468 [details]
More Logging

Using a debug build of WebKit at r233053.

Steps to reproduce:

1. Apply the attached patch.
2. Tools/Scripts/run-webkit-tests --debug --iterations 1000 --no-sample-on-timeout LayoutTests/http/tests/workers/worker-importScripts-banned-mimetype.html

I saw the assertion failure once.

Crashed Thread:        0  Dispatch queue: com.apple.main-thread

Exception Type:        EXC_BAD_ACCESS (SIGSEGV)
Exception Codes:       KERN_INVALID_ADDRESS at 0x00000000bbadbeef
Exception Note:        EXC_CORPSE_NOTIFY

Termination Signal:    Segmentation fault: 11
Termination Reason:    Namespace SIGNAL, Code 0xb
Terminating Process:   exc handler [1859]

Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0   com.apple.JavaScriptCore      	0x0000000327b84c50 WTFCrash + 16 (Assertions.cpp:267)
1   com.apple.WebCore             	0x000000031997fd6e WebCore::JSEventListener::jsFunction(WebCore::ScriptExecutionContext&) const + 398 (JSEventListener.h:108)
2   com.apple.WebCore             	0x000000031997f24d WebCore::JSEventListener::handleEvent(WebCore::ScriptExecutionContext&, WebCore::Event&) + 205 (JSEventListener.cpp:112)
3   com.apple.WebCore             	0x0000000319f095f6 WebCore::EventTarget::fireEventListeners(WebCore::Event&, WTF::Vector<WTF::RefPtr<WebCore::RegisteredEventListener, WTF::DumbPtrTraits<WebCore::RegisteredEventListener> >, 1ul, WTF::CrashOnOverflow, 16ul>) + 966 (EventTarget.cpp:293)
4   com.apple.WebCore             	0x0000000319f053ae WebCore::EventTarget::fireEventListeners(WebCore::Event&) + 318 (EventTarget.cpp:234)
5   com.apple.WebCore             	0x0000000319f091a6 WebCore::EventTarget::dispatchEvent(WebCore::Event&) + 246 (EventTarget.cpp:194)
6   com.apple.WebCore             	0x000000031b4ef9d4 WebCore::WorkerMessagingProxy::postMessageToWorkerObject(WebCore::MessageWithMessagePorts&&)::$_0::operator()(WebCore::ScriptExecutionContext&) + 340 (WorkerMessagingProxy.cpp:107)
7   com.apple.WebCore             	0x000000031b4ef7e4 WTF::Function<void (WebCore::ScriptExecutionContext&)>::CallableWrapper<WebCore::WorkerMessagingProxy::postMessageToWorkerObject(WebCore::MessageWithMessagePorts&&)::$_0>::call(WebCore::ScriptExecutionContext&) + 52 (Function.h:101)
8   com.apple.WebCore             	0x0000000319800a80 WTF::Function<void (WebCore::ScriptExecutionContext&)>::operator()(WebCore::ScriptExecutionContext&) const + 160 (Function.h:56)
9   com.apple.WebCore             	0x00000003197ee3bd WebCore::ScriptExecutionContext::Task::performTask(WebCore::ScriptExecutionContext&) + 29 (ScriptExecutionContext.h:188)
10  com.apple.WebCore             	0x0000000319eaf708 WebCore::Document::postTask(WebCore::ScriptExecutionContext::Task&&)::$_2::operator()() + 280 (Document.cpp:5836)
11  com.apple.WebCore             	0x0000000319eaf549 WTF::Function<void ()>::CallableWrapper<WebCore::Document::postTask(WebCore::ScriptExecutionContext::Task&&)::$_2>::call() + 25 (Function.h:101)
12  com.apple.JavaScriptCore      	0x0000000327ba9fcd WTF::Function<void ()>::operator()() const + 141 (Function.h:56)
13  com.apple.JavaScriptCore      	0x0000000327bd60dd WTF::dispatchFunctionsFromMainThread() + 333 (MainThread.cpp:129)
14  com.apple.JavaScriptCore      	0x0000000327bd8ce5 -[JSWTFMainThreadCaller call] + 21 (MainThreadMac.mm:56)
15  com.apple.Foundation          	0x00007fff34e16f7a __NSThreadPerformPerform + 327
16  com.apple.CoreFoundation      	0x00007fff329d2f2a __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
17  com.apple.CoreFoundation      	0x00007fff32a7137c __CFRunLoopDoSource0 + 108
18  com.apple.CoreFoundation      	0x00007fff329b9dc3 __CFRunLoopDoSources0 + 195
19  com.apple.CoreFoundation      	0x00007fff329b9352 __CFRunLoopRun + 1232
20  com.apple.CoreFoundation      	0x00007fff329b8c55 CFRunLoopRunSpecific + 467
21  com.apple.Foundation          	0x00007fff34dd1881 -[NSRunLoop(NSRunLoop) runMode:beforeDate:] + 280
22  com.apple.Foundation          	0x00007fff34dd1756 -[NSRunLoop(NSRunLoop) run] + 76
23  libxpc.dylib                  	0x00007fff60225f1e _xpc_objc_main + 567
24  libxpc.dylib                  	0x00007fff60224b75 xpc_main + 443
25  com.apple.WebKit.WebContent   	0x000000010c83406c main + 1340 (XPCServiceMain.mm:160)
26  libdyld.dylib                 	0x00007fff5ffdc151 start + 1