[Compile] Ensure CollectSourcePositions is context independent.

Also adds a NullContextScope for code which wants to ensure it is context
independent. Removes a workaround in V8ProfilerAgentImpl::startProfiling
which created a context due to CollectSourcePositions not being context
indpendent.

BUG=chromium:992063

Change-Id: I94c7eea6416dc64bc61fb8ff9cd945449a791a77
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1748693
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63176}

src/codegen/compiler.cc

@@ -1181,6 +1181,9 @@ bool Compiler::CollectSourcePositions(Isolate* isolate,
   DCHECK(shared_info->HasBytecodeArray());
   DCHECK(!shared_info->GetBytecodeArray().HasSourcePositionTable());
 
+  // Source position collection should be context independent.
+  NullContextScope null_context_scope(isolate);
+
   // Collecting source positions requires allocating a new source position
   // table.
   DCHECK(AllowHeapAllocation::IsAllowed());

src/execution/isolate.h

@@ -1936,6 +1936,14 @@ class V8_EXPORT_PRIVATE SaveAndSwitchContext : public SaveContext {
   SaveAndSwitchContext(Isolate* isolate, Context new_context);
 };
 
+// A scope which sets the given isolate's context to null for its lifetime to
+// ensure that code does not make assumptions on a context being available.
+class NullContextScope : public SaveAndSwitchContext {
+ public:
+  explicit NullContextScope(Isolate* isolate)
+      : SaveAndSwitchContext(isolate, Context()) {}
+};
+
 class AssertNoContextChange {
 #ifdef DEBUG
  public:

src/inspector/v8-profiler-agent-impl.cc

@@ -502,13 +502,6 @@ void V8ProfilerAgentImpl::startProfiling(const String16& title) {
         m_state->integerProperty(ProfilerAgentState::samplingInterval, 0);
     if (interval) m_profiler->SetSamplingInterval(interval);
   }
-  // Create a new temporary context and enter it while starting profiling
-  // since this might involve collecting source positions and requires a
-  // context.
-  // TODO(992063): Remove this once parsing / collecting source positions has
-  // been properly made context independent.
-  v8::Local<v8::Context> tmpContext = v8::Context::New(m_isolate);
-  v8::Context::Scope scope(tmpContext);
   ++m_startedProfilesCount;
   m_profiler->StartProfiling(toV8String(m_isolate, title), true);
 }

src/interpreter/bytecode-generator.cc

@@ -1034,7 +1034,7 @@ Handle<BytecodeArray> BytecodeGenerator::FinalizeBytecode(
 #ifdef DEBUG
   // Unoptimized compilation should be context-independent. Verify that we don't
   // access the native context by nulling it out during finalization.
-  SaveAndSwitchContext save(isolate, Context());
+  NullContextScope null_context_scope(isolate);
 #endif
 
   AllocateDeferredConstants(isolate, script);

src/profiler/heap-snapshot-generator.cc

@@ -1997,13 +1997,13 @@ HeapSnapshotGenerator::HeapSnapshotGenerator(
 }
 
 namespace {
-class NullContextScope {
+class NullContextForSnapshotScope {
  public:
-  explicit NullContextScope(Isolate* isolate)
+  explicit NullContextForSnapshotScope(Isolate* isolate)
       : isolate_(isolate), prev_(isolate->context()) {
     isolate_->set_context(Context());
   }
-  ~NullContextScope() { isolate_->set_context(prev_); }
+  ~NullContextForSnapshotScope() { isolate_->set_context(prev_); }
 
  private:
   Isolate* isolate_;
@@ -2023,7 +2023,7 @@ bool HeapSnapshotGenerator::GenerateSnapshot() {
   heap_->PreciseCollectAllGarbage(Heap::kNoGCFlags,
                                   GarbageCollectionReason::kHeapProfiler);
 
-  NullContextScope null_context_scope(Isolate::FromHeap(heap_));
+  NullContextForSnapshotScope null_context_scope(Isolate::FromHeap(heap_));
 
 #ifdef VERIFY_HEAP
   Heap* debug_heap = heap_;