Serialize inclusions by the CountingIncluder

Should fix ASAN failures in the CountingIncluderTest:
The ConcreteCountingIncluder include delegate is not safe
to call concurrently.  It's just simpler/nicer for the base
class to protect us in this way.

libshaderc_util/include/libshaderc_util/counting_includer.h

@@ -19,9 +19,13 @@
 
 #include "glslang/Public/ShaderLang.h"
 
+#include "libshaderc_util/mutex.h"
+
 namespace shaderc_util {
 
 // An Includer that counts how many #include directives it saw.
+// Inclusions are internally serialized, but releasing a previous result
+// can occur concurrently.
 class CountingIncluder : public glslang::TShader::Includer {
  public:
  // Done as .store(0) instead of in the initializer list for the following
@@ -42,22 +46,28 @@ class CountingIncluder : public glslang::TShader::Includer {
  // requesting source. For the semantics of the result, see the base class.
  // Also increments num_include_directives and returns the results of
  // include_delegate(filename). Subclasses should override include_delegate()
- // instead of this method.
+ // instead of this method. Inclusions are serialized.
  glslang::TShader::Includer::IncludeResult* includeSystem(
  const char* requested_source, const char* requesting_source,
  size_t include_depth) final {
  ++num_include_directives_;
- return include_delegate(requested_source, requesting_source,
- IncludeType::System, include_depth);
+ include_mutex_.lock();
+ auto result = include_delegate(requested_source, requesting_source,
+ IncludeType::System, include_depth);
+ include_mutex_.unlock();
+ return result;
  }
 
  // Like includeSystem, but for "local" include search.
  glslang::TShader::Includer::IncludeResult* includeLocal(
  const char* requested_source, const char* requesting_source,
  size_t include_depth) final {
  ++num_include_directives_;
- return include_delegate(requested_source, requesting_source,
- IncludeType::Local, include_depth);
+ include_mutex_.lock();
+ auto result = include_delegate(requested_source, requesting_source,
+ IncludeType::Local, include_depth);
+ include_mutex_.unlock();
+ return result;
  }
 
  // Releases the given IncludeResult.
@@ -81,6 +91,10 @@ class CountingIncluder : public glslang::TShader::Includer {
 
  // The number of #include directive encountered.
  std::atomic_int num_include_directives_;
+
+ // A mutex to protect against concurrent inclusions. We can't trust
+ // our delegates to be safe for concurrent inclusions.
+ shaderc_util::mutex include_mutex_;
 };
 }