4 files changed, 34 insertions, 3 deletions
diff --git a/js/loader/ModuleLoadRequest.cpp b/js/loader/ModuleLoadRequest.cpp
index 7313563df9..1a02ba6dbb 100644
--- a/js/loader/ModuleLoadRequest.cpp
+++ b/js/loader/ModuleLoadRequest.cpp
@@ -108,8 +108,6 @@ void ModuleLoadRequest::SetReady() {
   // dependencies have had their source loaded, parsed as a module and the
   // modules instantiated.
 
-  AssertAllImportsFinished();
-
   ScriptLoadRequest::SetReady();
 
   if (mWaitingParentRequest) {
@@ -162,7 +160,7 @@ void ModuleLoadRequest::ModuleErrored() {
 
   LOG(("ScriptLoadRequest (%p): Module errored", this));
 
-  if (IsCanceled()) {
+  if (IsCanceled() || IsCancelingImports()) {
     return;
   }
 
@@ -195,6 +193,7 @@ void ModuleLoadRequest::DependenciesLoaded() {
   MOZ_ASSERT(!IsErrored());
 
   CheckModuleDependenciesLoaded();
+  AssertAllImportsFinished();
   SetReady();
   LoadFinished();
 }
@@ -223,9 +222,22 @@ void ModuleLoadRequest::CheckModuleDependenciesLoaded() {
 }
 
 void ModuleLoadRequest::CancelImports() {
+  State origState = mState;
+
+  // To prevent reentering ModuleErrored() for this request via mImports[i]'s
+  // ChildLoadComplete().
+  mState = State::CancelingImports;
+
   for (size_t i = 0; i < mImports.Length(); i++) {
+    if (mLoader->IsFetchingAndHasWaitingRequest(mImports[i])) {
+      LOG(("CancelImports import %p is fetching and has waiting\n",
+           mImports[i].get()));
+      continue;
+    }
     mImports[i]->Cancel();
   }
+
+  mState = origState;
 }
 
 void ModuleLoadRequest::LoadFinished() {
diff --git a/js/loader/ModuleLoaderBase.cpp b/js/loader/ModuleLoaderBase.cpp
index 020542754d..506ad22edd 100644
--- a/js/loader/ModuleLoaderBase.cpp
+++ b/js/loader/ModuleLoaderBase.cpp
@@ -623,11 +623,13 @@ nsresult ModuleLoaderBase::OnFetchComplete(ModuleLoadRequest* aRequest,
   if (NS_SUCCEEDED(rv)) {
     rv = CreateModuleScript(aRequest);
 
+#if defined(MOZ_DIAGNOSTIC_ASSERT_ENABLED)
     // If a module script was created, it should either have a module record
     // object or a parse error.
     if (ModuleScript* ms = aRequest->mModuleScript) {
       MOZ_DIAGNOSTIC_ASSERT(bool(ms->ModuleRecord()) != ms->HasParseError());
     }
+#endif
 
     aRequest->ClearScriptSource();
 
@@ -1445,9 +1447,11 @@ void ModuleLoaderBase::RegisterImportMap(UniquePtr<ImportMap> aImportMap) {
           "Non-preload module loads should block import maps");
       MOZ_DIAGNOSTIC_ASSERT(!script->HadImportMap(),
                             "Only one import map can be registered");
+#if defined(MOZ_DIAGNOSTIC_ASSERT_ENABLED)
       if (JSObject* module = script->ModuleRecord()) {
         MOZ_DIAGNOSTIC_ASSERT(!JS::ModuleIsLinked(module));
       }
+#endif
       script->Shutdown();
     }
   }
@@ -1490,6 +1494,17 @@ void ModuleLoaderBase::MoveModulesTo(ModuleLoaderBase* aDest) {
   mFetchedModules.Clear();
 }
 
+bool ModuleLoaderBase::IsFetchingAndHasWaitingRequest(
+    ModuleLoadRequest* aRequest) {
+  auto entry = mFetchingModules.Lookup(aRequest->mURI);
+  if (!entry) {
+    return false;
+  }
+
+  RefPtr<LoadingRequest> loadingRequest = entry.Data();
+  return !loadingRequest->mWaiting.IsEmpty();
+}
+
 #undef LOG
 #undef LOG_ENABLED
 
diff --git a/js/loader/ModuleLoaderBase.h b/js/loader/ModuleLoaderBase.h
index 89d23e12bc..f902db18a4 100644
--- a/js/loader/ModuleLoaderBase.h
+++ b/js/loader/ModuleLoaderBase.h
@@ -480,6 +480,8 @@ class ModuleLoaderBase : public nsISupports {
 
   nsresult CreateModuleScript(ModuleLoadRequest* aRequest);
 
+  bool IsFetchingAndHasWaitingRequest(ModuleLoadRequest* aRequest);
+
   // The slot stored in ImportMetaResolve function.
   enum { ModulePrivateSlot = 0, SlotCount };
 
diff --git a/js/loader/ScriptLoadRequest.h b/js/loader/ScriptLoadRequest.h
index 442a456b15..f255b65228 100644
--- a/js/loader/ScriptLoadRequest.h
+++ b/js/loader/ScriptLoadRequest.h
@@ -124,6 +124,7 @@ class ScriptLoadRequest : public nsISupports,
     Fetching,
     Compiling,
     LoadingImports,
+    CancelingImports,
     Ready,
     Canceled
   };
@@ -139,6 +140,7 @@ class ScriptLoadRequest : public nsISupports,
   bool IsFetching() const { return mState == State::Fetching; }
   bool IsCompiling() const { return mState == State::Compiling; }
   bool IsLoadingImports() const { return mState == State::LoadingImports; }
+  bool IsCancelingImports() const { return mState == State::CancelingImports; }
   bool IsCanceled() const { return mState == State::Canceled; }
 
   bool IsPendingFetchingError() const {