fix: prevent parallel monitor checks

src/sdam/monitor.ts

@@ -328,6 +328,10 @@ function checkServer(monitor: Monitor, callback: Callback<Document | null>) {
 
 function monitorServer(monitor: Monitor) {
   return (callback: Callback) => {
+    if (monitor.s.state === STATE_MONITORING) {
+      callback();
+      return;
+    }
     stateTransition(monitor, STATE_MONITORING);
     function done() {
       if (!isInCloseState(monitor)) {

test/integration/server-discovery-and-monitoring/server_discovery_and_monitoring.spec.test.ts

@@ -25,8 +25,8 @@ describe('SDAM Unified Tests', function () {
     // TODO(NODE-4573): fix socket leaks
     const LEAKY_TESTS = [
       'Command error on Monitor handshake',
-      'Network error on Monitor check',
-      'Network timeout on Monitor check',
+      // 'Network error on Monitor check',
+      // 'Network timeout on Monitor check',
       'Network error on Monitor handshake',
       'Network timeout on Monitor handshake'
     ];

test/unit/sdam/monitor.test.ts

@@ -400,41 +400,71 @@ describe('monitoring', function () {
         });
       });
 
-      context('when it has been < minHeartbeatFrequencyMS since fn() last completed', function () {
+      context(
+        'when it has been < minHeartbeatFrequencyMS and >= 0 since fn() last completed',
+        function () {
+          beforeEach(function () {
+            executor = new MonitorInterval(fnSpy, {
+              minHeartbeatFrequencyMS: 10,
+              heartbeatFrequencyMS: 30
+            });
+
+            // call fn() once
+            clock.tick(30);
+            expect(fnSpy.calledOnce).to.be.true;
+
+            fnSpy.callCount = 0;
+
+            // advance less than minHeartbeatFrequency
+            clock.tick(5);
+
+            executor.wake();
+          });
+
+          it('reschedules fn() to minHeartbeatFrequencyMS after the last call', function () {
+            expect(fnSpy.callCount).to.equal(0);
+            clock.tick(5);
+            expect(fnSpy.calledOnce).to.be.true;
+          });
+
+          context('when wake() is called more than once', function () {
+            it('schedules fn() minHeartbeatFrequencyMS after the last call to fn()', function () {
+              executor.wake();
+              executor.wake();
+              executor.wake();
+
+              expect(fnSpy.callCount).to.equal(0);
+              clock.tick(5);
+              expect(fnSpy.calledOnce).to.be.true;
+            });
+          });
+        }
+      );
+
+      context('when it has been <0 since fn() has last executed', function () {
         beforeEach(function () {
           executor = new MonitorInterval(fnSpy, {
             minHeartbeatFrequencyMS: 10,
             heartbeatFrequencyMS: 30
           });
 
-          // call fn() once
-          clock.tick(30);
-          expect(fnSpy.calledOnce).to.be.true;
-
-          fnSpy.callCount = 0;
-
-          // advance less than minHeartbeatFrequency
-          clock.tick(5);
-
+          // negative ticks aren't supported, so manually set execution time
+          executor.lastExecutionEnded = Infinity;
           executor.wake();
         });
 
-        it('reschedules fn() to minHeartbeatFrequencyMS after the last call', function () {
-          expect(fnSpy.callCount).to.equal(0);
-          clock.tick(5);
+        it('executes fn() immediately', function () {
           expect(fnSpy.calledOnce).to.be.true;
         });
 
-        context('when wake() is called more than once', function () {
-          it('schedules fn() minHeartbeatFrequencyMS after the last call to fn()', function () {
-            executor.wake();
-            executor.wake();
-            executor.wake();
+        it('reschedules fn() to minHeartbeatFrequency away', function () {
+          fnSpy.callCount = 0;
 
-            expect(fnSpy.callCount).to.equal(0);
-            clock.tick(5);
-            expect(fnSpy.calledOnce).to.be.true;
-          });
+          clock.tick(29);
+          expect(fnSpy.callCount).to.equal(0);
+
+          clock.tick(1);
+          expect(fnSpy.calledOnce).to.be.true;
         });
       });
     });