[wasm][streaming] Only clear the compilation_unit_builder_ if it exists

The CompilationUnitBuilder of the StreamingProcessor is cleared when an
error occurs in the streaming decoder. The clearing of the
CompilationUnitBuilder was guarded by the existence of the
ModuleCompiler, because this ModuleCompiler and the
CompilationUnitBuilder are created together. However, the
CompilationUnitBuilder is reset when the next section after the code
section is processed, whereas the ModuleCompiler exists until the end of
the AsyncCompileJob. With this CL the clearing of the
CompilationUnitBuilder is also guarded by its own existence.

R=clemensh@chromium.org

Bug: chromium:805346
Change-Id: I0e9e9eaff9239fadb21c0f17990da61cbfaa6856
Reviewed-on: https://chromium-review.googlesource.com/883527
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50844}

src/wasm/module-compiler.cc

@@ -3964,7 +3964,10 @@ void AsyncStreamingProcessor::FinishAsyncCompileJobWithError(ResultBase error) {
       job_->DoSync<AsyncCompileJob::DecodeFail>(std::move(result));
     }
 
-    compilation_unit_builder_->Clear();
+    // Clear the {compilation_unit_builder_} if it exists. This is needed
+    // because there is a check in the destructor of the
+    // {CompilationUnitBuilder} that it is empty.
+    if (compilation_unit_builder_) compilation_unit_builder_->Clear();
   } else {
     job_->DoSync<AsyncCompileJob::DecodeFail>(std::move(result));
   }

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

@@ -974,6 +974,40 @@ STREAM_TEST(TestModuleWithImportedFunction) {
 
   CHECK(tester.IsPromiseFulfilled());
 }
+
+STREAM_TEST(TestModuleWithErrorAfterDataSection) {
+  StreamTester tester;
+
+  const uint8_t bytes[] = {
+      WASM_MODULE_HEADER,                   // module header
+      kTypeSectionCode,                     // section code
+      U32V_1(1 + SIZEOF_SIG_ENTRY_x_x),     // section size
+      U32V_1(1),                            // type count
+      SIG_ENTRY_x_x(kLocalI32, kLocalI32),  // signature entry
+      kFunctionSectionCode,                 // section code
+      U32V_1(1 + 1),                        // section size
+      U32V_1(1),                            // functions count
+      0,                                    // signature index
+      kCodeSectionCode,                     // section code
+      U32V_1(6),                            // section size
+      U32V_1(1),                            // functions count
+      U32V_1(4),                            // body size
+      U32V_1(0),                            // locals count
+      kExprGetLocal,                        // some code
+      0,                                    // some code
+      kExprEnd,                             // some code
+      kDataSectionCode,                     // section code
+      U32V_1(1),                            // section size
+      U32V_1(0),                            // data segment count
+      kUnknownSectionCode,                  // section code
+      U32V_1(1),                            // invalid section size
+  };
+
+  tester.OnBytesReceived(bytes, arraysize(bytes));
+  tester.FinishStream();
+  tester.RunCompilerTasks();
+  CHECK(tester.IsPromiseRejected());
+}
 #undef STREAM_TEST
 
 }  // namespace wasm