SERVER-42559 Make the WaitWithOpTimeEarlierThanLowestQueued test run more predictably

by making the test thread wait for the WaitForMajorityService thread pick up the 1st request before proceeding

Author: renctan

src/mongo/db/s/wait_for_majority_service_test.cpp

@@ -76,6 +76,9 @@ class WaitForMajorityServiceTest : public ServiceContextTest {
     Status waitForWriteConcernStub(OperationContext* opCtx, const repl::OpTime& opTime) {
         stdx::unique_lock<stdx::mutex> lk(_mutex);
 
+        _waitForMajorityCallCount++;
+        _callCountChangedCV.notify_one();
+
         while (!_isTestReady) {
             auto status = opCtx->waitForConditionOrInterruptNoAssert(_isTestReadyCV, lk);
             if (!status.isOK()) {
@@ -98,15 +101,22 @@ class WaitForMajorityServiceTest : public ServiceContextTest {
         return _lastOpTimeWaited;
     }
 
+    void waitForMajorityCallCountGreaterThan(int expectedCount) {
+        stdx::unique_lock lk(_mutex);
+        _callCountChangedCV.wait(lk, [&] { return _waitForMajorityCallCount > expectedCount; });
+    }
+
 private:
     WaitForMajorityService _waitForMajorityService;
 
     stdx::mutex _mutex;
     stdx::condition_variable _isTestReadyCV;
     stdx::condition_variable _finishWaitingOneOpTimeCV;
+    stdx::condition_variable _callCountChangedCV;
 
     bool _isTestReady{false};
     repl::OpTime _lastOpTimeWaited;
+    int _waitForMajorityCallCount{0};
 };
 
 TEST_F(WaitForMajorityServiceTest, WaitOneOpTime) {
@@ -144,8 +154,17 @@ TEST_F(WaitForMajorityServiceTest, WaitWithOpTimeEarlierThanLowestQueued) {
     repl::OpTime earlierOpTime(Timestamp(1, 0), 2);
 
     auto laterFuture = waitService()->waitUntilMajority(laterOpTime);
+
+    // Wait until the background thread picks up the queued opTime.
+    waitForMajorityCallCountGreaterThan(0);
+
+    // The 2nd request has an earlier time, so it will interrupt 'laterOpTime' and skip the line.
     auto earlierFuture = waitService()->waitUntilMajority(earlierOpTime);
 
+    // Wait for background thread to finish transitioning from waiting on laterOpTime to
+    // earlierOpTime.
+    waitForMajorityCallCountGreaterThan(1);
+
     ASSERT_FALSE(laterFuture.isReady());
     ASSERT_FALSE(earlierFuture.isReady());