Firestore C++: compile ExecutorLibdispatch in Objective-C++ mode (#1237)

dispatch_queue_t is defined differently in libdispatch depending on
whether the library header is being include from Objective-C (or
Objective-C++) code, or else from C or C++ code. Make sure that all
source files in Firestore that include executor_libdispatch.h are
compiled in the same mode (Objective-C++) to avoid linker errors.

3 files changed, 9 insertions, 3 deletions

diff --git a/Firestore/core/src/firebase/firestore/util/CMakeLists.txt b/Firestore/core/src/firebase/firestore/util/CMakeLists.txt
index 29d91c7..b2b015b 100644
--- a/Firestore/core/src/firebase/firestore/util/CMakeLists.txt
+++ b/Firestore/core/src/firebase/firestore/util/CMakeLists.txt
@@ -130,7 +130,7 @@ if(HAVE_LIBDISPATCH)
 cc_library(
   firebase_firestore_util_executor_libdispatch
   SOURCES
-    executor_libdispatch.cc
+    executor_libdispatch.mm
     executor_libdispatch.h
     executor.h
   DEPENDS
diff --git a/Firestore/core/src/firebase/firestore/util/executor_libdispatch.h b/Firestore/core/src/firebase/firestore/util/executor_libdispatch.h
index b32dbff..85c34f8 100644
--- a/Firestore/core/src/firebase/firestore/util/executor_libdispatch.h
+++ b/Firestore/core/src/firebase/firestore/util/executor_libdispatch.h
@@ -17,7 +17,6 @@
 #ifndef FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_EXECUTOR_LIBDISPATCH_H_
 #define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_EXECUTOR_LIBDISPATCH_H_
 
-#include <atomic>
 #include <chrono>  // NOLINT(build/c++11)
 #include <functional>
 #include <memory>
@@ -30,6 +29,13 @@
 #include "Firestore/core/src/firebase/firestore/util/firebase_assert.h"
 #include "absl/strings/string_view.h"
 
+#if !defined(__OBJC__)
+// `dispatch_queue_t` gets defined to different types when compiled in C++ or
+// Objective-C mode. Source files including this header should all be compiled
+// in the same mode to avoid linker errors.
+#error "This header only supports Objective-C++ (see comment for more info)."
+#endif  // !defined(__OBJC__)
+
 namespace firebase {
 namespace firestore {
 namespace util {
@@ -78,7 +84,7 @@ class ExecutorLibdispatch : public Executor {
   absl::string_view GetCurrentQueueLabel() const;
   absl::string_view GetTargetQueueLabel() const;
 
-  std::atomic<dispatch_queue_t> dispatch_queue_;
+  dispatch_queue_t dispatch_queue_;
   // Stores non-owned pointers to `TimeSlot`s.
   // Invariant: if a `TimeSlot` is in `schedule_`, it's a valid pointer.
   std::vector<TimeSlot*> schedule_;
diff --git a/Firestore/core/src/firebase/firestore/util/executor_libdispatch.cc b/Firestore/core/src/firebase/firestore/util/executor_libdispatch.mm
index b40f0dd..b40f0dd 100644
--- a/Firestore/core/src/firebase/firestore/util/executor_libdispatch.cc
+++ b/Firestore/core/src/firebase/firestore/util/executor_libdispatch.mm