[wasm][gc] Fix NativeModule::GetCode for nonexisting code

{NativeModule::GetCode} can actually return {nullptr} if no code was
compiled yet for a function, e.g. in asm.js where we use lazy
compilation. In that case, we must not try to increment the ref count
on the nonexisting code object.

We had a few errors recently that were hard to reproduce because we do
not have a flag to enable code logging. Clusterfuzz managed to
accomplish this by passing --trace-ic.
In order to test bugs in code logging properly, this CL introduces a
new runtime function called "EnableCodeLoggingForTesting". It registers
a noop {CodeEventListener} and enables code logging in the wasm engine.
We should whitelist this flag in ClusterFuzz to potentially flush out
more bugs.

R=mstarzinger@chromium.org
CC=frgossen@chromium.org

Bug: v8:8217, chromium:961129, chromium:961245, chromium:961128
Change-Id: I2f97c109db70b41531d58580b71f6781beeb8dcb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1602700
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61404}

src/runtime/runtime-test.cc

@@ -1284,5 +1284,41 @@ RUNTIME_FUNCTION(Runtime_StaticAssert) {
   return ReadOnlyRoots(isolate).undefined_value();
 }
 
+RUNTIME_FUNCTION(Runtime_EnableCodeLoggingForTesting) {
+  // The {NoopListener} currently does nothing on any callback, but reports
+  // {true} on {is_listening_to_code_events()}. Feel free to add assertions to
+  // any method to further test the code logging callbacks.
+  class NoopListener final : public CodeEventListener {
+    void CodeCreateEvent(LogEventsAndTags tag, AbstractCode code,
+                         const char* comment) final {}
+    void CodeCreateEvent(LogEventsAndTags tag, AbstractCode code,
+                         Name name) final {}
+    void CodeCreateEvent(LogEventsAndTags tag, AbstractCode code,
+                         SharedFunctionInfo shared, Name source) final {}
+    void CodeCreateEvent(LogEventsAndTags tag, AbstractCode code,
+                         SharedFunctionInfo shared, Name source, int line,
+                         int column) final {}
+    void CodeCreateEvent(LogEventsAndTags tag, const wasm::WasmCode* code,
+                         wasm::WasmName name) final {}
+    void CallbackEvent(Name name, Address entry_point) final {}
+    void GetterCallbackEvent(Name name, Address entry_point) final {}
+    void SetterCallbackEvent(Name name, Address entry_point) final {}
+    void RegExpCodeCreateEvent(AbstractCode code, String source) final {}
+    void CodeMoveEvent(AbstractCode from, AbstractCode to) final {}
+    void SharedFunctionInfoMoveEvent(Address from, Address to) final {}
+    void CodeMovingGCEvent() final {}
+    void CodeDisableOptEvent(AbstractCode code,
+                             SharedFunctionInfo shared) final {}
+    void CodeDeoptEvent(Code code, DeoptimizeKind kind, Address pc,
+                        int fp_to_sp_delta) final {}
+
+    bool is_listening_to_code_events() final { return true; }
+  };
+  static base::LeakyObject<NoopListener> noop_listener;
+  isolate->wasm_engine()->EnableCodeLogging(isolate);
+  isolate->code_event_dispatcher()->AddListener(noop_listener.get());
+  return ReadOnlyRoots(isolate).undefined_value();
+}
+
 }  // namespace internal
 }  // namespace v8

src/runtime/runtime.h

@@ -519,7 +519,8 @@ namespace internal {
   F(WasmNumInterpretedCalls, 1, 1)            \
   F(WasmTraceMemory, 1, 1)                    \
   F(SetWasmThreadsEnabled, 1, 1)              \
-  F(StaticAssert, 1, 1)
+  F(StaticAssert, 1, 1)                       \
+  F(EnableCodeLoggingForTesting, 0, 1)
 
 #define FOR_EACH_INTRINSIC_TYPEDARRAY(F, I) \
   F(ArrayBufferDetach, 1, 1)                \

src/wasm/wasm-code-manager.cc

@@ -149,7 +149,7 @@ bool WasmCode::ShouldBeLogged(Isolate* isolate) {
   // The return value is cached in {WasmEngine::IsolateData::log_codes}. Ensure
   // to call {WasmEngine::EnableCodeLogging} if this return value would change
   // for any isolate. Otherwise we might lose code events.
-  return isolate->logger()->is_listening_to_code_events() ||
+  return isolate->code_event_dispatcher()->IsListeningToCodeEvents() ||
          isolate->is_profiling();
 }
 
@@ -832,7 +832,7 @@ WasmCode* NativeModule::GetCode(uint32_t index) const {
   DCHECK_LT(index, num_functions());
   DCHECK_LE(module_->num_imported_functions, index);
   WasmCode* code = code_table_[index - module_->num_imported_functions];
-  WasmCodeRefScope::AddRef(code);
+  if (code) WasmCodeRefScope::AddRef(code);
   return code;
 }
 

test/mjsunit/regress/wasm/regress-961129.js

@@ -0,0 +1,18 @@
+// Copyright 2019 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --allow-natives-syntax
+
+%EnableCodeLoggingForTesting();
+
+function module() {
+  "use asm";
+  function f() {
+    var i = 4;
+    return i | 0;
+  }
+  return {f: f};
+}
+
+module().f();