refactor: use most recently seen error on second monitor failure

Author: mbroadst

lib/core/sdam/monitor.js

@@ -182,15 +182,15 @@ function monitorServer(monitor) {
   // TODO: the next line is a legacy event, remove in v4
   process.nextTick(() => monitor.emit('monitoring', monitor[kServer]));
 
-  checkServer(monitor, (err, isMaster) => {
+  checkServer(monitor, (e0, isMaster) => {
     if (isMaster) {
       successHandler(monitor, start, isMaster);
       return;
     }
 
     // otherwise an error occured on initial discovery, also bail
     if (monitor[kServer].description.type === ServerType.Unknown) {
-      failureHandler(monitor, start, err);
+      failureHandler(monitor, start, e0);
       return;
     }
 
@@ -199,10 +199,9 @@ function monitorServer(monitor) {
     // change its type to `Unknown` only after retrying once.
     monitor.emit('resetConnectionPool');
 
-    checkServer(monitor, (error, isMaster) => {
-      if (error) {
-        // NOTE: using the _first_ error encountered here
-        failureHandler(monitor, start, err);
+    checkServer(monitor, (e1, isMaster) => {
+      if (e1) {
+        failureHandler(monitor, start, e1);
         return;
       }
 

test/unit/sdam/monitoring.test.js

@@ -256,5 +256,57 @@ describe('monitoring', function() {
 
       monitor.connect();
     });
+
+    it('should report the most recent error on second monitoring failure', function(done) {
+      let failedCount = 0;
+      let initialConnectCompleted = false;
+      mockServer.setMessageHandler(request => {
+        const doc = request.document;
+        if (doc.ismaster) {
+          if (!initialConnectCompleted) {
+            request.reply(mock.DEFAULT_ISMASTER_36);
+            initialConnectCompleted = true;
+            return;
+          }
+
+          if (failedCount === 0) {
+            failedCount++;
+            request.reply({ ok: 0, errmsg: 'first error message' });
+          } else {
+            failedCount++;
+            request.reply({ ok: 0, errmsg: 'second error message' });
+          }
+        }
+      });
+
+      const server = new MockServer(mockServer.address());
+      const monitor = new Monitor(server, {
+        heartbeatFrequencyMS: 250,
+        minHeartbeatFrequencyMS: 50
+      });
+      this.defer(() => monitor.close());
+
+      let resetRequested = false;
+      monitor.on('resetConnectionPool', () => (resetRequested = true));
+      monitor.on('serverHeartbeatSucceeded', () => {
+        if (server.description.type === ServerType.Unknown) {
+          // this is the first successful heartbeat, set the server type
+          server.description.type = ServerType.Standalone;
+          return;
+        }
+
+        done(new Error('unexpected heartbeat success'));
+      });
+
+      monitor.on('serverHeartbeatFailed', event => {
+        expect(resetRequested).to.be.true;
+        expect(event)
+          .property('failure')
+          .to.match(/second error message/);
+        done();
+      });
+
+      monitor.connect();
+    });
   });
 });