[web snapshots] Support concatenating snapshots

After the snapshot data ends, interpret the rest as normal JS code.

Bug: v8:11525
Change-Id: I8de4cf4027e582e6b8e330dda7708623373d2675
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3263896Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77741}

src/web-snapshot/web-snapshot.cc

@@ -834,9 +834,36 @@ bool WebSnapshotDeserializer::Deserialize() {
   ProcessDeferredReferences();
   DeserializeExports();
   DCHECK_EQ(deferred_references_->Length(), 0);
-  if (deserializer_->position_ != deserializer_->end_) {
-    Throw("Web snapshot: Snapshot length mismatch");
-    return false;
+
+  // If there is more data, treat it as normal JavaScript.
+  DCHECK_LE(deserializer_->position_, deserializer_->end_);
+  auto remaining_bytes = deserializer_->end_ - deserializer_->position_;
+  if (remaining_bytes > 0 && remaining_bytes < v8::String::kMaxLength) {
+    v8::Isolate* v8_isolate = reinterpret_cast<v8::Isolate*>(isolate_);
+    v8::Local<v8::String> source =
+        v8::String::NewFromUtf8(
+            v8_isolate, reinterpret_cast<const char*>(deserializer_->position_),
+            NewStringType::kNormal, static_cast<int>(remaining_bytes))
+            .ToLocalChecked();
+
+    ScriptOrigin origin(v8_isolate, v8::String::NewFromUtf8Literal(
+                                        v8_isolate, "(web snapshot)",
+                                        NewStringType::kInternalized));
+
+    ScriptCompiler::Source script_source(source, origin);
+    Local<UnboundScript> script;
+    if (!ScriptCompiler::CompileUnboundScript(v8_isolate, &script_source)
+             .ToLocal(&script)) {
+      DCHECK(isolate_->has_pending_exception());
+      return false;
+    }
+    Local<Value> result;
+    if (!script->BindToCurrentContext()
+             ->Run(v8_isolate->GetCurrentContext())
+             .ToLocal(&result)) {
+      DCHECK(isolate_->has_pending_exception());
+      return false;
+    }
   }
 
   if (FLAG_trace_web_snapshot) {

test/cctest/test-web-snapshots.cc

@@ -700,5 +700,53 @@ TEST(FunctionKinds) {
                            kObjectCount);
 }
 
+// Test that concatenating JS code to the snapshot works.
+TEST(Concatenation) {
+  CcTest::InitializeVM();
+  v8::Isolate* isolate = CcTest::isolate();
+
+  const char* snapshot_source = "var foo = {a: 1};\n";
+  const char* source_to_append = "var bar = {a: 10};";
+  const char* test_source = "foo.a + bar.a";
+  uint32_t kObjectCount = 1;
+
+  WebSnapshotData snapshot_data;
+  {
+    v8::HandleScope scope(isolate);
+    v8::Local<v8::Context> new_context = CcTest::NewContext();
+    v8::Context::Scope context_scope(new_context);
+
+    CompileRun(snapshot_source);
+    v8::Local<v8::PrimitiveArray> exports = v8::PrimitiveArray::New(isolate, 1);
+    v8::Local<v8::String> str =
+        v8::String::NewFromUtf8(isolate, "foo").ToLocalChecked();
+    exports->Set(isolate, 0, str);
+    WebSnapshotSerializer serializer(isolate);
+    CHECK(serializer.TakeSnapshot(new_context, exports, snapshot_data));
+    CHECK(!serializer.has_error());
+    CHECK_NOT_NULL(snapshot_data.buffer);
+    CHECK_EQ(kObjectCount, serializer.object_count());
+  }
+
+  auto buffer_size = snapshot_data.buffer_size + strlen(source_to_append);
+  std::unique_ptr<uint8_t[]> buffer(new uint8_t[buffer_size]);
+  memcpy(buffer.get(), snapshot_data.buffer, snapshot_data.buffer_size);
+  memcpy(buffer.get() + snapshot_data.buffer_size, source_to_append,
+         strlen(source_to_append));
+
+  {
+    v8::HandleScope scope(isolate);
+    v8::Local<v8::Context> new_context = CcTest::NewContext();
+    v8::Context::Scope context_scope(new_context);
+    WebSnapshotDeserializer deserializer(isolate);
+    CHECK(deserializer.UseWebSnapshot(buffer.get(), buffer_size));
+    CHECK(!deserializer.has_error());
+    CHECK_EQ(kObjectCount, deserializer.object_count());
+
+    v8::Local<v8::Number> result = CompileRun(test_source).As<v8::Number>();
+    CHECK_EQ(11, result->Value());
+  }
+}
+
 }  // namespace internal
 }  // namespace v8