[wasm] Handle modules without code in streaming compilation

Streaming compilation started the compilation of a module at the
beginning of the code section. However, there exist valid modules which
do not contain a code section. In this CL we check for the existence of
a code section when we finish the stream. We do this by checking if the
module compiler in the AsyncCompileJob exists, because the module
compiler gets initialized at the beginning of the code section.

If we detect that compilation has not been started because there was no
code section, then we start compilation when the stream finishes.

R=clemensh@chromium.org

Bug: chromium:771973
Change-Id: I7c95a7a791d02254f086961e7cd81885eec27382
Reviewed-on: https://chromium-review.googlesource.com/778541
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49494}

src/wasm/module-compiler.cc

@@ -3489,8 +3489,18 @@ void AsyncStreamingProcessor::OnFinishedStream(std::unique_ptr<uint8_t[]> bytes,
   ModuleResult result = decoder_.FinishDecoding(false);
   DCHECK(result.ok());
   job_->module_ = std::move(result.val);
-  if (job_->DecrementAndCheckFinisherCount())
-    job_->DoSync<AsyncCompileJob::FinishCompile>();
+  if (job_->DecrementAndCheckFinisherCount()) {
+    if (!job_->compiler_) {
+      // We are processing a WebAssembly module without code section. We need to
+      // prepare compilation first before we can finish it.
+      // {PrepareAndStartCompile} will call {FinishCompile} by itself if there
+      // is no code section.
+      job_->DoSync<AsyncCompileJob::PrepareAndStartCompile>(job_->module_.get(),
+                                                            true);
+    } else {
+      job_->DoSync<AsyncCompileJob::FinishCompile>();
+    }
+  }
 }
 
 // Report an error detected in the StreamingDecoder.

src/wasm/streaming-decoder.cc

@@ -385,8 +385,7 @@ StreamingDecoder::DecodeNumberOfFunctions::NextWithValue(
         section_buffer(), section_buffer()->payload_offset() + bytes_needed(),
         value());
   } else {
-    if (section_buffer()->payload_offset() + size() !=
-        section_buffer()->length()) {
+    if (section_buffer()->payload_length() != bytes_needed()) {
       return streaming->Error("not all code section bytes were used");
     }
     return base::make_unique<DecodeSectionID>(streaming->module_offset());

test/cctest/wasm/test-streaming-compilation.cc

@@ -835,6 +835,42 @@ STREAM_TEST(TestAbortAfterCompilationError2) {
   tester.RunCompilerTasks();
 }
 
+STREAM_TEST(TestOnlyModuleHeader) {
+  StreamTester tester;
+
+  const uint8_t bytes[] = {
+      WASM_MODULE_HEADER,  // module header
+  };
+
+  tester.OnBytesReceived(bytes, arraysize(bytes));
+  tester.FinishStream();
+  tester.RunCompilerTasks();
+
+  CHECK(tester.IsPromiseFulfilled());
+}
+
+STREAM_TEST(TestModuleWithZeroFunctions) {
+  StreamTester tester;
+
+  const uint8_t bytes[] = {
+      WASM_MODULE_HEADER,    // module header
+      kTypeSectionCode,      // section code
+      U32V_1(1),             // section size
+      U32V_1(0),             // type count
+      kFunctionSectionCode,  // section code
+      U32V_1(1),             // section size
+      U32V_1(0),             // functions count
+      kCodeSectionCode,      // section code
+      U32V_1(1),             // section size
+      U32V_1(0),             // functions count
+  };
+
+  tester.OnBytesReceived(bytes, arraysize(bytes));
+  tester.FinishStream();
+  tester.RunCompilerTasks();
+  CHECK(tester.IsPromiseFulfilled());
+}
+
 #undef STREAM_TEST
 
 }  // namespace wasm