Correctly skip unittests

Not all V8 build configs support JS shared memory features. Trying to
create a new shared Isolate on such a config DCHECKs at runtime. Make
the shared Isolate test fixture conditionally initialize the shared
Isolate. Users must explicitly check for support.

Bug: v8:12547
Change-Id: I3df1ce7eb5ae9a3c136f88ea8f44c650cc0408ab
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3687565
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80961}

test/unittests/js-atomics/js-atomics-mutex-unittest.cc

@@ -77,8 +77,7 @@ class LockingThread final : public v8::base::Thread {
 }  // namespace
 
 TEST_F(JSAtomicsMutexTest, Contention) {
-  if (!ReadOnlyHeap::IsReadOnlySpaceShared()) return;
+  if (!IsJSSharedMemorySupported()) return;
-  if (!COMPRESS_POINTERS_IN_SHARED_CAGE_BOOL) return;
 
   FLAG_harmony_struct = true;
 

test/unittests/test-utils.h

@@ -86,11 +86,10 @@ class IsolateWrapper final {
 //
 // A set of mixins from which the test fixtures will be constructed.
 //
-template <typename TMixin, CountersMode kCountersMode = kNoCounters,
+template <typename TMixin, CountersMode kCountersMode = kNoCounters>
-          IsolateSharedMode kSharedMode = kStandaloneIsolate>
 class WithIsolateMixin : public TMixin {
  public:
-  WithIsolateMixin() : isolate_wrapper_(kCountersMode, kSharedMode) {}
+  WithIsolateMixin() : isolate_wrapper_(kCountersMode, kStandaloneIsolate) {}
 
   v8::Isolate* v8_isolate() const { return isolate_wrapper_.isolate(); }
 
@@ -98,6 +97,37 @@ class WithIsolateMixin : public TMixin {
   v8::IsolateWrapper isolate_wrapper_;
 };
 
+// Warning: This is not a drop-in replacement for WithIsolateMixin!
+//
+// Users of WithMaybeSharedIsolateMixin, including TEST_F tests and classes that
+// mix this class in, must explicit check IsJSSharedMemorySupported() before
+// calling v8_isolate(). Creating shared Isolates is not supported on all build
+// configurations.
+template <typename TMixin, CountersMode kCountersMode = kNoCounters>
+class WithMaybeSharedIsolateMixin : public TMixin {
+ public:
+  WithMaybeSharedIsolateMixin() {
+    if (IsJSSharedMemorySupported()) {
+      isolate_wrapper_.emplace(kCountersMode, kSharedIsolate);
+    }
+  }
+
+  bool IsJSSharedMemorySupported() const {
+    DCHECK_IMPLIES(
+        internal::ReadOnlyHeap::IsReadOnlySpaceShared(),
+        !COMPRESS_POINTERS_BOOL || COMPRESS_POINTERS_IN_SHARED_CAGE_BOOL);
+    return internal::ReadOnlyHeap::IsReadOnlySpaceShared();
+  }
+
+  v8::Isolate* v8_isolate() const {
+    DCHECK(IsJSSharedMemorySupported());
+    return isolate_wrapper_->isolate();
+  }
+
+ private:
+  base::Optional<v8::IsolateWrapper> isolate_wrapper_;
+};
+
 template <typename TMixin>
 class WithIsolateScopeMixin : public TMixin {
  public:
@@ -396,9 +426,9 @@ using TestWithNativeContextAndZone =               //
                             ::testing::Test>>>>>>;
 
 using TestWithSharedIsolate =                       //
-    WithIsolateMixin<                               //
+    WithMaybeSharedIsolateMixin<                    //
         WithDefaultPlatformMixin<::testing::Test>,  //
-        kNoCounters, kSharedIsolate>;
+        kNoCounters>;
 
 class V8_NODISCARD SaveFlags {
  public: