Make sure grpc::internal::Mutex has enough space for gpr_mu, std::mutex, and pthread_mutex_t.

include/grpc/impl/codegen/port_platform.h

@@ -115,6 +115,7 @@
 #define GPR_POSIX_SUBPROCESS 1
 #define GPR_POSIX_SYNC 1
 #define GPR_POSIX_TIME 1
+#define GPR_HAS_PTHREAD_H 1
 #define GPR_GETPID_IN_UNISTD_H 1
 #ifdef _LP64
 #define GPR_ARCH_64 1
@@ -144,6 +145,7 @@
 #define GPR_POSIX_SUBPROCESS 1
 #define GPR_POSIX_SYNC 1
 #define GPR_POSIX_TIME 1
+#define GPR_HAS_PTHREAD_H 1
 #define GPR_GETPID_IN_UNISTD_H 1
 #define GPR_SUPPORT_CHANNELS_FROM_FD 1
 #elif defined(__linux__)
@@ -170,6 +172,7 @@
 #define GPR_POSIX_SUBPROCESS 1
 #define GPR_POSIX_SYNC 1
 #define GPR_POSIX_TIME 1
+#define GPR_HAS_PTHREAD_H 1
 #define GPR_GETPID_IN_UNISTD_H 1
 #ifdef _LP64
 #define GPR_ARCH_64 1
@@ -235,6 +238,7 @@
 #define GPR_POSIX_SUBPROCESS 1
 #define GPR_POSIX_SYNC 1
 #define GPR_POSIX_TIME 1
+#define GPR_HAS_PTHREAD_H 1
 #define GPR_GETPID_IN_UNISTD_H 1
 #ifndef GRPC_CFSTREAM
 #define GPR_SUPPORT_CHANNELS_FROM_FD 1
@@ -260,6 +264,7 @@
 #define GPR_POSIX_SUBPROCESS 1
 #define GPR_POSIX_SYNC 1
 #define GPR_POSIX_TIME 1
+#define GPR_HAS_PTHREAD_H 1
 #define GPR_GETPID_IN_UNISTD_H 1
 #define GPR_SUPPORT_CHANNELS_FROM_FD 1
 #ifdef _LP64
@@ -283,6 +288,7 @@
 #define GPR_POSIX_SUBPROCESS 1
 #define GPR_POSIX_SYNC 1
 #define GPR_POSIX_TIME 1
+#define GPR_HAS_PTHREAD_H 1
 #define GPR_GETPID_IN_UNISTD_H 1
 #define GPR_SUPPORT_CHANNELS_FROM_FD 1
 #ifdef _LP64
@@ -303,6 +309,7 @@
 #define GPR_POSIX_SUBPROCESS 1
 #define GPR_POSIX_SYNC 1
 #define GPR_POSIX_TIME 1
+#define GPR_HAS_PTHREAD_H 1
 #define GPR_GETPID_IN_UNISTD_H 1
 #ifdef _LP64
 #define GPR_ARCH_64 1
@@ -325,6 +332,7 @@
 #define GPR_POSIX_SUBPROCESS 1
 #define GPR_POSIX_SYNC 1
 #define GPR_POSIX_TIME 1
+#define GPR_HAS_PTHREAD_H 1
 #define GPR_GETPID_IN_UNISTD_H 1
 #ifdef _LP64
 #define GPR_ARCH_64 1
@@ -353,6 +361,7 @@
 #define GPR_POSIX_SUBPROCESS 1
 #define GPR_POSIX_SYNC 1
 #define GPR_POSIX_TIME 1
+#define GPR_HAS_PTHREAD_H 1
 #define GPR_GETPID_IN_UNISTD_H 1
 #ifdef _LP64
 #define GPR_ARCH_64 1
@@ -378,6 +387,7 @@
 #define GPR_POSIX_SYNC 1
 #define GPR_POSIX_STRING 1
 #define GPR_POSIX_TIME 1
+#define GPR_HAS_PTHREAD_H 1
 #define GPR_GETPID_IN_UNISTD_H 1
 #else
 #error "Could not auto-detect platform"

include/grpcpp/impl/codegen/sync.h

@@ -19,8 +19,15 @@
 #ifndef GRPCPP_IMPL_CODEGEN_SYNC_H
 #define GRPCPP_IMPL_CODEGEN_SYNC_H
 
-#include <grpc/impl/codegen/log.h>
 #include <grpc/impl/codegen/port_platform.h>
+
+#ifdef GPR_HAS_PTHREAD_H
+#include <pthread.h>
+#endif
+
+#include <mutex>
+
+#include <grpc/impl/codegen/log.h>
 #include <grpc/impl/codegen/sync.h>
 
 #include <grpcpp/impl/codegen/core_codegen_interface.h>
@@ -49,7 +56,13 @@ class Mutex {
   const gpr_mu* get() const { return &mu_; }
 
  private:
-  gpr_mu mu_;
+  union {
+    gpr_mu mu_;
+    std::mutex do_not_use_sth_;
+#ifdef GPR_HAS_PTHREAD_H
+    pthread_mutex_t do_not_use_pth_;
+#endif
+  };
 };
 
 // MutexLock is a std::