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diff --git a/js/src/gc/GCInternals.h b/js/src/gc/GCInternals.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * GC-internal definitions.
+ */
+
+#ifndef gc_GCInternals_h
+#define gc_GCInternals_h
+
+#include "mozilla/Maybe.h"
+
+#include "gc/GC.h"
+#include "vm/JSContext.h"
+
+namespace js {
+namespace gc {
+
+/*
+ * There are a couple of classes here that serve mostly as "tokens" indicating
+ * that a precondition holds. Some functions force the caller to possess such a
+ * token because they require the precondition to hold, and it is better to make
+ * the precondition explicit at the API entry point than to crash in an
+ * assertion later on when it is relied upon.
+ */
+
+struct MOZ_RAII AutoAssertNoNurseryAlloc {
+#ifdef DEBUG
+  AutoAssertNoNurseryAlloc();
+  ~AutoAssertNoNurseryAlloc();
+#else
+  AutoAssertNoNurseryAlloc() {}
+#endif
+};
+
+/*
+ * A class that serves as a token that the nursery in the current thread's zone
+ * group is empty.
+ */
+class MOZ_RAII AutoAssertEmptyNursery {
+ protected:
+  JSContext* cx;
+
+  mozilla::Maybe<AutoAssertNoNurseryAlloc> noAlloc;
+
+  // Check that the nursery is empty.
+  void checkCondition(JSContext* cx);
+
+  // For subclasses that need to empty the nursery in their constructors.
+  AutoAssertEmptyNursery() : cx(nullptr) {}
+
+ public:
+  explicit AutoAssertEmptyNursery(JSContext* cx) : cx(nullptr) {
+    checkCondition(cx);
+  }
+
+  AutoAssertEmptyNursery(const AutoAssertEmptyNursery& other)
+      : AutoAssertEmptyNursery(other.cx) {}
+};
+
+/*
+ * Evict the nursery upon construction. Serves as a token indicating that the
+ * nursery is empty. (See AutoAssertEmptyNursery, above.)
+ */
+class MOZ_RAII AutoEmptyNursery : public AutoAssertEmptyNursery {
+ public:
+  explicit AutoEmptyNursery(JSContext* cx);
+};
+
+class MOZ_RAII AutoCheckCanAccessAtomsDuringGC {
+#ifdef DEBUG
+  JSRuntime* runtime;
+
+ public:
+  explicit AutoCheckCanAccessAtomsDuringGC(JSRuntime* rt) : runtime(rt) {
+    // Ensure we're only used from within the GC.
+    MOZ_ASSERT(JS::RuntimeHeapIsMajorCollecting());
+
+    // Ensure there is no off-thread parsing running.
+    MOZ_ASSERT(!rt->hasHelperThreadZones());
+
+    // Set up a check to assert if we try to start an off-thread parse.
+    runtime->setOffThreadParsingBlocked(true);
+  }
+  ~AutoCheckCanAccessAtomsDuringGC() {
+    runtime->setOffThreadParsingBlocked(false);
+  }
+#else
+ public:
+  explicit AutoCheckCanAccessAtomsDuringGC(JSRuntime* rt) {}
+#endif
+};
+
+// Abstract base class for exclusive heap access for tracing or GC.
+class MOZ_RAII AutoHeapSession {
+ public:
+  ~AutoHeapSession();
+
+ protected:
+  AutoHeapSession(GCRuntime* gc, JS::HeapState state);
+
+ private:
+  AutoHeapSession(const AutoHeapSession&) = delete;
+  void operator=(const AutoHeapSession&) = delete;
+
+  GCRuntime* gc;
+  JS::HeapState prevState;
+  mozilla::Maybe<AutoGeckoProfilerEntry> profilingStackFrame;
+};
+
+class MOZ_RAII AutoGCSession : public AutoHeapSession {
+ public:
+  explicit AutoGCSession(GCRuntime* gc, JS::HeapState state)
+      : AutoHeapSession(gc, state) {}
+
+  AutoCheckCanAccessAtomsDuringGC& checkAtomsAccess() {
+    return maybeCheckAtomsAccess.ref();
+  }
+
+  // During a GC we can check that it's not possible for anything else to be
+  // using the atoms zone.
+  mozilla::Maybe<AutoCheckCanAccessAtomsDuringGC> maybeCheckAtomsAccess;
+};
+
+class MOZ_RAII AutoMajorGCProfilerEntry : public AutoGeckoProfilerEntry {
+ public:
+  explicit AutoMajorGCProfilerEntry(GCRuntime* gc);
+};
+
+class MOZ_RAII AutoTraceSession : public AutoLockAllAtoms,
+                                  public AutoHeapSession {
+ public:
+  explicit AutoTraceSession(JSRuntime* rt)
+      : AutoLockAllAtoms(rt),
+        AutoHeapSession(&rt->gc, JS::HeapState::Tracing) {}
+};
+
+struct MOZ_RAII AutoFinishGC {
+  explicit AutoFinishGC(JSContext* cx, JS::GCReason reason) {
+    FinishGC(cx, reason);
+  }
+};
+
+// This class should be used by any code that needs exclusive access to the heap
+// in order to trace through it.
+class MOZ_RAII AutoPrepareForTracing : private AutoFinishGC,
+                                       public AutoTraceSession {
+ public:
+  explicit AutoPrepareForTracing(JSContext* cx)
+      : AutoFinishGC(cx, JS::GCReason::PREPARE_FOR_TRACING),
+        AutoTraceSession(cx->runtime()) {}
+};
+
+// This class should be used by any code that needs exclusive access to the heap
+// in order to trace through it.
+//
+// This version also empties the nursery after finishing any ongoing GC.
+class MOZ_RAII AutoEmptyNurseryAndPrepareForTracing : private AutoFinishGC,
+                                                      public AutoEmptyNursery,
+                                                      public AutoTraceSession {
+ public:
+  explicit AutoEmptyNurseryAndPrepareForTracing(JSContext* cx)
+      : AutoFinishGC(cx, JS::GCReason::PREPARE_FOR_TRACING),
+        AutoEmptyNursery(cx),
+        AutoTraceSession(cx->runtime()) {}
+};
+
+/*
+ * Temporarily disable incremental barriers.
+ */
+class AutoDisableBarriers {
+ public:
+  explicit AutoDisableBarriers(GCRuntime* gc);
+  ~AutoDisableBarriers();
+
+ private:
+  GCRuntime* gc;
+};
+
+GCAbortReason IsIncrementalGCUnsafe(JSRuntime* rt);
+
+#ifdef JS_GC_ZEAL
+
+class MOZ_RAII AutoStopVerifyingBarriers {
+  GCRuntime* gc;
+  bool restartPreVerifier;
+
+ public:
+  AutoStopVerifyingBarriers(JSRuntime* rt, bool isShutdown) : gc(&rt->gc) {
+    if (gc->isVerifyPreBarriersEnabled()) {
+      gc->endVerifyPreBarriers();
+      restartPreVerifier = !isShutdown;
+    } else {
+      restartPreVerifier = false;
+    }
+  }
+
+  ~AutoStopVerifyingBarriers() {
+    // Nasty special case: verification runs a minor GC, which *may* nest
+    // inside of an outer minor GC. This is not allowed by the
+    // gc::Statistics phase tree. So we pause the "real" GC, if in fact one
+    // is in progress.
+    gcstats::PhaseKind outer = gc->stats().currentPhaseKind();
+    if (outer != gcstats::PhaseKind::NONE) {
+      gc->stats().endPhase(outer);
+    }
+    MOZ_ASSERT(gc->stats().currentPhaseKind() == gcstats::PhaseKind::NONE);
+
+    if (restartPreVerifier) {
+      gc->startVerifyPreBarriers();
+    }
+
+    if (outer != gcstats::PhaseKind::NONE) {
+      gc->stats().beginPhase(outer);
+    }
+  }
+};
+#else
+struct MOZ_RAII AutoStopVerifyingBarriers {
+  AutoStopVerifyingBarriers(JSRuntime*, bool) {}
+};
+#endif /* JS_GC_ZEAL */
+
+#ifdef JSGC_HASH_TABLE_CHECKS
+void CheckHashTablesAfterMovingGC(JSRuntime* rt);
+void CheckHeapAfterGC(JSRuntime* rt);
+#endif
+
+struct MovingTracer final : public GenericTracer {
+  explicit MovingTracer(JSRuntime* rt)
+      : GenericTracer(rt, JS::TracerKind::Moving,
+                      JS::WeakMapTraceAction::TraceKeysAndValues) {}
+
+  JSObject* onObjectEdge(JSObject* obj) override;
+  Shape* onShapeEdge(Shape* shape) override;
+  JSString* onStringEdge(JSString* string) override;
+  js::BaseScript* onScriptEdge(js::BaseScript* script) override;
+  BaseShape* onBaseShapeEdge(BaseShape* base) override;
+  Scope* onScopeEdge(Scope* scope) override;
+  RegExpShared* onRegExpSharedEdge(RegExpShared* shared) override;
+  BigInt* onBigIntEdge(BigInt* bi) override;
+  ObjectGroup* onObjectGroupEdge(ObjectGroup* group) override;
+  JS::Symbol* onSymbolEdge(JS::Symbol* sym) override;
+  jit::JitCode* onJitCodeEdge(jit::JitCode* jit) override;
+
+ private:
+  template <typename T>
+  T* onEdge(T* thingp);
+};
+
+struct SweepingTracer final : public GenericTracer {
+  explicit SweepingTracer(JSRuntime* rt)
+      : GenericTracer(rt, JS::TracerKind::Sweeping,
+                      JS::WeakMapTraceAction::TraceKeysAndValues) {}
+
+  JSObject* onObjectEdge(JSObject* obj) override;
+  Shape* onShapeEdge(Shape* shape) override;
+  JSString* onStringEdge(JSString* string) override;
+  js::BaseScript* onScriptEdge(js::BaseScript* script) override;
+  BaseShape* onBaseShapeEdge(BaseShape* base) override;
+  jit::JitCode* onJitCodeEdge(jit::JitCode* jit) override;
+  Scope* onScopeEdge(Scope* scope) override;
+  RegExpShared* onRegExpSharedEdge(RegExpShared* shared) override;
+  BigInt* onBigIntEdge(BigInt* bi) override;
+  js::ObjectGroup* onObjectGroupEdge(js::ObjectGroup* group) override;
+  JS::Symbol* onSymbolEdge(JS::Symbol* sym) override;
+
+ private:
+  template <typename T>
+  T* onEdge(T* thingp);
+};
+
+extern void DelayCrossCompartmentGrayMarking(JSObject* src);
+
+inline bool IsOOMReason(JS::GCReason reason) {
+  return reason == JS::GCReason::LAST_DITCH ||
+         reason == JS::GCReason::MEM_PRESSURE;
+}
+
+// TODO: Bug 1650075. Adding XPCONNECT_SHUTDOWN seems to cause crash.
+inline bool IsShutdownReason(JS::GCReason reason) {
+  return reason == JS::GCReason::WORKER_SHUTDOWN ||
+         reason == JS::GCReason::SHUTDOWN_CC ||
+         reason == JS::GCReason::DESTROY_RUNTIME;
+}
+
+TenuredCell* AllocateCellInGC(JS::Zone* zone, AllocKind thingKind);
+
+} /* namespace gc */
+} /* namespace js */
+
+#endif /* gc_GCInternals_h */