Allow an alarm to be set again after firing

include/grpcpp/alarm_impl.h

@@ -16,8 +16,8 @@
  *
  */
 
-/// An Alarm posts the user provided tag to its associated completion queue upon
-/// expiry or cancellation.
+/// An Alarm posts the user-provided tag to its associated completion queue or
+/// invokes the user-provided function on expiry or cancellation.
 #ifndef GRPCPP_ALARM_IMPL_H
 #define GRPCPP_ALARM_IMPL_H
 
@@ -32,7 +32,6 @@
 
 namespace grpc_impl {
 
-/// A thin wrapper around \a grpc_alarm (see / \a / src/core/surface/alarm.h).
 class Alarm : private ::grpc::GrpcLibraryCodegen {
  public:
   /// Create an unset completion queue alarm

src/cpp/common/alarm.cc

@@ -40,12 +40,7 @@ class AlarmImpl : public ::grpc::internal::CompletionQueueTag {
     gpr_ref_init(&refs_, 1);
     grpc_timer_init_unset(&timer_);
   }
-  ~AlarmImpl() {
-    grpc_core::ExecCtx exec_ctx;
-    if (cq_ != nullptr) {
-      GRPC_CQ_INTERNAL_UNREF(cq_, "alarm");
-    }
-  }
+  ~AlarmImpl() {}
   bool FinalizeResult(void** tag, bool* status) override {
     *tag = tag_;
     Unref();
@@ -63,10 +58,15 @@ class AlarmImpl : public ::grpc::internal::CompletionQueueTag {
                         // queue the op on the completion queue
                         AlarmImpl* alarm = static_cast<AlarmImpl*>(arg);
                         alarm->Ref();
+                        // Preserve the cq and reset the cq_ so that the alarm
+                        // can be reset when the alarm tag is delivered.
+                        grpc_completion_queue* cq = alarm->cq_;
+                        alarm->cq_ = nullptr;
                         grpc_cq_end_op(
-                            alarm->cq_, alarm, error,
+                            cq, alarm, error,
                             [](void* arg, grpc_cq_completion* completion) {},
                             arg, &alarm->completion_);
+                        GRPC_CQ_INTERNAL_UNREF(cq, "alarm");
                       },
                       this, grpc_schedule_on_exec_ctx);
     grpc_timer_init(&timer_, grpc_timespec_to_millis_round_up(deadline),

test/cpp/common/alarm_test.cc

@@ -47,6 +47,44 @@ TEST(AlarmTest, RegularExpiry) {
   EXPECT_EQ(junk, output_tag);
 }
 
+TEST(AlarmTest, RegularExpiryMultiSet) {
+  CompletionQueue cq;
+  void* junk = reinterpret_cast<void*>(1618033);
+  Alarm alarm;
+
+  for (int i = 0; i < 3; i++) {
+    alarm.Set(&cq, grpc_timeout_seconds_to_deadline(1), junk);
+
+    void* output_tag;
+    bool ok;
+    const CompletionQueue::NextStatus status =
+        cq.AsyncNext(&output_tag, &ok, grpc_timeout_seconds_to_deadline(10));
+
+    EXPECT_EQ(status, CompletionQueue::GOT_EVENT);
+    EXPECT_TRUE(ok);
+    EXPECT_EQ(junk, output_tag);
+  }
+}
+
+TEST(AlarmTest, RegularExpiryMultiSetMultiCQ) {
+  void* junk = reinterpret_cast<void*>(1618033);
+  Alarm alarm;
+
+  for (int i = 0; i < 3; i++) {
+    CompletionQueue cq;
+    alarm.Set(&cq, grpc_timeout_seconds_to_deadline(1), junk);
+
+    void* output_tag;
+    bool ok;
+    const CompletionQueue::NextStatus status =
+        cq.AsyncNext(&output_tag, &ok, grpc_timeout_seconds_to_deadline(10));
+
+    EXPECT_EQ(status, CompletionQueue::GOT_EVENT);
+    EXPECT_TRUE(ok);
+    EXPECT_EQ(junk, output_tag);
+  }
+}
+
 struct Completion {
   bool completed = false;
   std::mutex mu;