Bug 1652719 - Stop allowing JS helper thread access to CHECK_THREAD APIs r=jandem

SpiderMonkey APIs often use CHECK_THREAD and AssertHeapIsIdle, but this can
be a footgun when they are called from helper-threads since often nothing
ensures the heap is idle. Instead, this patch updates the CHECK_THREAD assert
to disallow helper-thread access to prevent misuses. The checks in Atomize
and AtomizeChars are removed since those are (currently) allowed on helper
threads.

Differential Revision: https://phabricator.services.mozilla.com/D83492

js/src/vm/JSAtom.cpp

@@ -1048,8 +1048,6 @@ void AtomsTable::maybePinExistingAtom(JSContext* cx, JSAtom* atom) {
 
 JSAtom* js::Atomize(JSContext* cx, const char* bytes, size_t length,
                     PinningBehavior pin, const Maybe<uint32_t>& indexValue) {
-  CHECK_THREAD(cx);
-
   const Latin1Char* chars = reinterpret_cast<const Latin1Char*>(bytes);
   return AtomizeAndCopyChars(cx, chars, length, pin, indexValue);
 }
@@ -1057,7 +1055,6 @@ JSAtom* js::Atomize(JSContext* cx, const char* bytes, size_t length,
 template <typename CharT>
 JSAtom* js::AtomizeChars(JSContext* cx, const CharT* chars, size_t length,
                          PinningBehavior pin) {
-  CHECK_THREAD(cx);
   return AtomizeAndCopyChars(cx, chars, length, pin, Nothing());
 }
 
@@ -1242,8 +1239,6 @@ template JSAtom* js::ToAtom<NoGC>(JSContext* cx, const Value& v);
 static JSAtom* AtomizeLittleEndianTwoByteChars(JSContext* cx,
                                                const uint8_t* leTwoByte,
                                                size_t length) {
-  CHECK_THREAD(cx);
-
   LittleEndianChars chars(leTwoByte);
 
   if (JSAtom* s = cx->staticStrings().lookup(chars, length)) {

js/src/vm/JSContext.h

@@ -1304,7 +1304,7 @@ class MOZ_RAII AutoSuppressNurseryCellAlloc {
 } /* namespace js */
 
 #define CHECK_THREAD(cx)                            \
-  MOZ_ASSERT_IF(cx && !cx->isHelperThreadContext(), \
-                js::CurrentThreadCanAccessRuntime(cx->runtime()))
+  MOZ_ASSERT_IF(cx, !cx->isHelperThreadContext() && \
+                        js::CurrentThreadCanAccessRuntime(cx->runtime()))
 
 #endif /* vm_JSContext_h */