In Rados notify based locking, prevented multiple promise setting.

This happens when the notification is received multiple times. This did not show
before full scale test.

objectstore/BackendRados.cpp

@@ -19,6 +19,7 @@
 #include "BackendRados.hpp"
 #include "common/exception/Errnum.hpp"
 #include "common/Timer.hpp"
+#include "common/threading/MutexLocker.hpp"
 #include <rados/librados.hpp>
 #include <sys/syscall.h>
 #include <errno.h>
@@ -210,13 +211,17 @@ BackendRados::LockWatcher::LockWatcher(librados::IoCtx& context, const std::stri
 }
 
 void BackendRados::LockWatcher::handle_error(uint64_t cookie, int err) {
+  threading::MutexLocker ml(m_promiseMutex);
   m_promise.set_value();
-  TIMESTAMPEDPRINT("");
+  TIMESTAMPEDPRINT("Handled notify");
+  m_promiseSet = true;
 }
 
 void BackendRados::LockWatcher::handle_notify(uint64_t notify_id, uint64_t cookie, uint64_t notifier_id, librados::bufferlist& bl) {
+  threading::MutexLocker ml(m_promiseMutex);
   m_promise.set_value();
-  TIMESTAMPEDPRINT(""); 
+  TIMESTAMPEDPRINT("Handled notify");
+  m_promiseSet = true;
 }
 
 void BackendRados::LockWatcher::wait(const durationUs& timeout) {

objectstore/BackendRados.hpp

@@ -20,6 +20,7 @@
 
 #include "Backend.hpp"
 #include "rados/librados.hpp"
+#include "common/threading/Mutex.hpp"
 #include <future>
 
 
@@ -105,6 +106,10 @@ private:
     typedef std::chrono::microseconds durationUs;
     void wait(const durationUs & timeout);
   private:
+    // We could receive several notifications. The promise should be set only
+    // on the first occurrence.
+    threading::Mutex m_promiseMutex;
+    bool m_promiseSet = false;
     std::promise<void> m_promise;
     std::future<void> m_future;
     librados::IoCtx & m_context;