feat: introduce MongoServerSelectionError

This error type subclasses the `MongoTimeout` error, but provides
more context that the error was specific to an issue with server
selection.

index.js

@@ -11,6 +11,7 @@ const connect = require('./lib/mongo_client').connect;
 connect.MongoError = core.MongoError;
 connect.MongoNetworkError = core.MongoNetworkError;
 connect.MongoTimeoutError = core.MongoTimeoutError;
+connect.MongoServerSelectionError = core.MongoServerSelectionError;
 connect.MongoParseError = core.MongoParseError;
 connect.MongoWriteConcernError = core.MongoWriteConcernError;
 connect.MongoBulkWriteError = require('./lib/bulk/common').BulkWriteError;

lib/core/error.js

@@ -96,7 +96,7 @@ class MongoParseError extends MongoError {
 class MongoTimeoutError extends MongoError {
   constructor(message, reason) {
     if (reason && reason.error) {
-      super(reason.error);
+      super(reason.error.message || reason.error);
     } else {
       super(message);
     }
@@ -108,6 +108,22 @@ class MongoTimeoutError extends MongoError {
   }
 }
 
+/**
+ * An error signifying a client-side server selection error
+ *
+ * @param {Error|string|object} message The error message
+ * @param {string|object} [reason] The reason the timeout occured
+ * @property {string} message The error message
+ * @property {string} [reason] An optional reason context for the timeout, generally an error saved during flow of monitoring and selecting servers
+ * @extends MongoError
+ */
+class MongoServerSelectionError extends MongoTimeoutError {
+  constructor(message, reason) {
+    super(message, reason);
+    this.name = 'MongoServerSelectionError';
+  }
+}
+
 function makeWriteConcernResultObject(input) {
   const output = Object.assign({}, input);
 
@@ -246,6 +262,7 @@ module.exports = {
   MongoNetworkError,
   MongoParseError,
   MongoTimeoutError,
+  MongoServerSelectionError,
   MongoWriteConcernError,
   mongoErrorContextSymbol,
   isRetryableError,

lib/core/index.js

@@ -20,6 +20,7 @@ module.exports = {
   MongoNetworkError: require('./error').MongoNetworkError,
   MongoParseError: require('./error').MongoParseError,
   MongoTimeoutError: require('./error').MongoTimeoutError,
+  MongoServerSelectionError: require('./error').MongoServerSelectionError,
   MongoWriteConcernError: require('./error').MongoWriteConcernError,
   mongoErrorContextSymbol: require('./error').mongoErrorContextSymbol,
   // Core

lib/core/sdam/server_selection.js

@@ -4,7 +4,7 @@ const TopologyType = require('./common').TopologyType;
 const ReadPreference = require('../topologies/read_preference');
 const MongoError = require('../error').MongoError;
 const calculateDurationInMs = require('../utils').calculateDurationInMs;
-const MongoTimeoutError = require('../error').MongoTimeoutError;
+const MongoServerSelectionError = require('../error').MongoServerSelectionError;
 
 const common = require('./common');
 const STATE_CLOSED = common.STATE_CLOSED;
@@ -260,7 +260,7 @@ function selectServers(topology, selector, timeout, start, callback) {
   const duration = calculateDurationInMs(start);
   if (duration >= timeout) {
     return callback(
-      new MongoTimeoutError(`Server selection timed out after ${timeout} ms`),
+      new MongoServerSelectionError(`Server selection timed out after ${timeout} ms`),
       topology.description
     );
   }
@@ -306,7 +306,7 @@ function selectServers(topology, selector, timeout, start, callback) {
     const iterationTimer = setTimeout(() => {
       topology.removeListener('topologyDescriptionChanged', descriptionChangedHandler);
       callback(
-        new MongoTimeoutError(
+        new MongoServerSelectionError(
           `Server selection timed out after ${timeout} ms`,
           topology.description
         )

test/unit/sdam/server_selection/spec.test.js

@@ -3,7 +3,7 @@ const path = require('path');
 const fs = require('fs');
 const core = require('../../../../lib/core');
 const Topology = core.Topology;
-const MongoTimeoutError = core.MongoTimeoutError;
+const MongoServerSelectionError = core.MongoServerSelectionError;
 const ReadPreference = core.ReadPreference;
 
 // TODO: these should be from `core` when legacy topologies are removed
@@ -275,7 +275,7 @@ function executeServerSelectionTest(testDefinition, options, testDone) {
   }
 
   // default to serverSelectionTimeoutMS of `100` for unit tests
-  topology.selectServer(selector, { serverSelectionTimeoutMS: 100 }, (err, server) => {
+  topology.selectServer(selector, { serverSelectionTimeoutMS: 50 }, (err, server) => {
     // are we expecting an error?
     if (testDefinition.error) {
       if (!err) {
@@ -287,7 +287,7 @@ function executeServerSelectionTest(testDefinition, options, testDone) {
 
     if (err) {
       // this is another expected error case
-      if (expectedServers.length === 0 && err instanceof MongoTimeoutError) return done();
+      if (expectedServers.length === 0 && err instanceof MongoServerSelectionError) return done();
       return done(err);
     }