Fixed Blastdoor to work with Python 3, and added tests for demangling to

ensure subprocess I/O works correctly, and renaming symbols in files.

Python3 fix includes no longer using poll() for input (the blocking read()
works just fine as long as you check for exceptions) and handling binary vs. non-binary strings properly.

Also a few small fixes in build_mpms that make it easier to test
merge_libraries:

- Now exits properly if postprocessing a library fails.
- Added a -B option so you can laBel the resulting MPM with a different label than the one in Rapid. This ensures that if you are using build_mpms.sh -b '--fetch_mpms=...' to fetch a subset of MPMs, you don't need to use the same label for your output MPM as the input MPM, so you don't pollute the normal label namespace.

PiperOrigin-RevId: 288805974

Author: jonsimantov

merge_libraries_test_file.cc

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2020 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/* This is a test file for merge_libraries.py tests. It contains some C and C++
+ * symbols that merge_libraries can rename. */
+
+extern "C" {
+int test_symbol(void) { return 1; }  // NOLINT
+
+int test_another_symbol(void) { return 2; }  // NOLINT
+
+int test_yet_one_more_symbol(void) { return 3; }  // NOLINT
+
+int global_c_symbol = 789;  // NOLINT
+
+extern int not_in_this_file();  // NOLINT
+}
+
+namespace test_namespace {
+
+class TestClass {
+ public:
+  TestClass();
+  int TestMethod();
+  int TestMethodNotInThisfile();
+  int TestStaticMethod();
+  int TestStaticMethodNotInThisFile();
+  static int test_static_field;  // NOLINT
+};
+
+int global_cpp_symbol = 12345;  // NOLINT
+
+int TestClass::test_static_field;
+
+TestClass::TestClass() {}
+
+int TestClass::TestMethod() { return not_in_this_file(); }
+
+int TestClass::TestStaticMethod() { return TestStaticMethodNotInThisFile(); }
+
+}  // namespace test_namespace