[wasm-streaming] Fix function index calculation

The index of a function in the WasmModule data structure is offset by
the number of imported functions in the module. The {DecodeFunctionBody}
function of the module decoder, however, requires the function index
without this offset. The streaming processor mixed up these two ranges
of function indices. This is fixed in this CL.

R=clemensh@chromium.org

Bug: chromium:781507
Change-Id: Ie3e0c4703b06ecb923c98ffb961844915323197c
Reviewed-on: https://chromium-review.googlesource.com/776680
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49523}

src/wasm/module-compiler.cc

@@ -3413,8 +3413,6 @@ bool AsyncStreamingProcessor::ProcessCodeSectionHeader(size_t functions_count,
   // Set outstanding_finishers_ to 2, because both the AsyncCompileJob and the
   // AsyncStreamingProcessor have to finish.
   job_->outstanding_finishers_.SetValue(2);
-  next_function_ = decoder_.module()->num_imported_functions +
-                   FLAG_skip_compiling_wasm_funcs;
   compilation_unit_builder_.reset(
       new ModuleCompiler::CompilationUnitBuilder(job_->compiler_.get()));
   return true;
@@ -3425,11 +3423,12 @@ bool AsyncStreamingProcessor::ProcessFunctionBody(Vector<const uint8_t> bytes,
                                                   uint32_t offset) {
   TRACE_STREAMING("Process function body %d ...\n", next_function_);
 
-  decoder_.DecodeFunctionBody(
-      next_function_, static_cast<uint32_t>(bytes.length()), offset, false);
-  if (next_function_ >= decoder_.module()->num_imported_functions +
-                            FLAG_skip_compiling_wasm_funcs) {
-    const WasmFunction* func = &decoder_.module()->functions[next_function_];
+  if (next_function_ >= FLAG_skip_compiling_wasm_funcs) {
+    decoder_.DecodeFunctionBody(
+        next_function_, static_cast<uint32_t>(bytes.length()), offset, false);
+
+    uint32_t index = next_function_ + decoder_.module()->num_imported_functions;
+    const WasmFunction* func = &decoder_.module()->functions[index];
     WasmName name = {nullptr, 0};
     compilation_unit_builder_->AddUnit(job_->module_env_.get(), func, offset,
                                        bytes, name);

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

@@ -871,6 +871,28 @@ STREAM_TEST(TestModuleWithZeroFunctions) {
   CHECK(tester.IsPromiseFulfilled());
 }
 
+// Test that all bytes arrive before doing any compilation. FinishStream is
+// called immediately.
+STREAM_TEST(TestModuleWithImportedFunction) {
+  StreamTester tester;
+  ZoneBuffer buffer(tester.zone());
+  TestSignatures sigs;
+  WasmModuleBuilder builder(tester.zone());
+  builder.AddImport(ArrayVector("Test"), sigs.i_iii());
+  {
+    WasmFunctionBuilder* f = builder.AddFunction(sigs.i_iii());
+    uint8_t code[] = {kExprGetLocal, 0, kExprEnd};
+    f->EmitCode(code, arraysize(code));
+  }
+  builder.WriteTo(buffer);
+
+  tester.OnBytesReceived(buffer.begin(), buffer.end() - buffer.begin());
+  tester.FinishStream();
+
+  tester.RunCompilerTasks();
+
+  CHECK(tester.IsPromiseFulfilled());
+}
 #undef STREAM_TEST
 
 }  // namespace wasm