fix cancellation test flake

src/ruby/spec/generic/rpc_server_spec.rb

@@ -111,6 +111,32 @@ end
 
 SlowStub = SlowService.rpc_stub_class
 
+# A test service that allows a synchronized RPC cancellation
+class SynchronizedCancellationService
+  include GRPC::GenericService
+  rpc :an_rpc, EchoMsg, EchoMsg
+  attr_reader :received_md, :delay
+
+  # notify_request_received and wait_until_rpc_cancelled are
+  # callbacks to synchronously allow the client to proceed with
+  # cancellation (after the unary request has been received),
+  # and to synchronously wait until the client has cancelled the
+  # current RPC.
+  def initialize(notify_request_received, wait_until_rpc_cancelled)
+    @notify_request_received = notify_request_received
+    @wait_until_rpc_cancelled = wait_until_rpc_cancelled
+  end
+
+  def an_rpc(req, _call)
+    GRPC.logger.info('starting a synchronusly cancelled rpc')
+    @notify_request_received.call(req)
+    @wait_until_rpc_cancelled.call
+    req  # send back the req as the response
+  end
+end
+
+SynchronizedCancellationStub = SynchronizedCancellationService.rpc_stub_class
+
 # a test service that hangs onto call objects
 # and uses them after the server-side call has been
 # finished
@@ -384,20 +410,64 @@ describe GRPC::RpcServer do
       end
 
       it 'should handle cancellation correctly', server: true do
-        service = SlowService.new
+        request_received = false
+        request_received_mu = Mutex.new
+        request_received_cv = ConditionVariable.new
+        notify_request_received = proc do |req|
+          request_received_mu.synchronize do
+            fail 'req is nil' if req.nil?
+            expect(req.is_a?(EchoMsg)).to be true
+            fail 'test bug - already set' if request_received
+            request_received = true
+            request_received_cv.signal
+          end
+        end
+
+        rpc_cancelled = false
+        rpc_cancelled_mu = Mutex.new
+        rpc_cancelled_cv = ConditionVariable.new
+        wait_until_rpc_cancelled = proc do
+          rpc_cancelled_mu.synchronize do
+            loop do
+              break if rpc_cancelled
+              rpc_cancelled_cv.wait(rpc_cancelled_mu)
+            end
+          end
+        end
+
+        service = SynchronizedCancellationService.new(notify_request_received,
+                                                      wait_until_rpc_cancelled)
         @srv.handle(service)
-        t = Thread.new { @srv.run }
+        srv_thd = Thread.new { @srv.run }
         @srv.wait_till_running
         req = EchoMsg.new
-        stub = SlowStub.new(@host, :this_channel_is_insecure, **client_opts)
-        op = stub.an_rpc(req, metadata: { k1: 'v1', k2: 'v2' }, return_op: true)
-        Thread.new do  # cancel the call
-          sleep 0.1
-          op.cancel
+        stub = SynchronizedCancellationStub.new(@host,
+                                                :this_channel_is_insecure,
+                                                **client_opts)
+        op = stub.an_rpc(req, return_op: true)
+
+        client_thd = Thread.new do
+          expect { op.execute }.to raise_error GRPC::Cancelled
         end
-        expect { op.execute }.to raise_error GRPC::Cancelled
+
+        request_received_mu.synchronize do
+          loop do
+            break if request_received
+            request_received_cv.wait(request_received_mu)
+          end
+        end
+
+        op.cancel
+
+        rpc_cancelled_mu.synchronize do
+          fail 'test bug - already set' if rpc_cancelled
+          rpc_cancelled = true
+          rpc_cancelled_cv.signal
+        end
+
+        client_thd.join
         @srv.stop
-        t.join
+        srv_thd.join
       end
 
       it 'should handle multiple parallel requests', server: true do