feat: introduce a class for tracking stream specific attributes

Similar to the `ServerDescription` we now have a `StreamDescription`
which describes details specific to a single stream connected to a
MongoDB topology

lib/cmap/connection.js

@@ -6,7 +6,7 @@ const MongoError = require('../core/error').MongoError;
 const MongoNetworkError = require('../core/error').MongoNetworkError;
 const MongoWriteConcernError = require('../core/error').MongoWriteConcernError;
 const CommandResult = require('../core/connection/command_result');
-const ServerDescription = require('../core/sdam/server_description').ServerDescription;
+const StreamDescription = require('./stream_description').StreamDescription;
 const wp = require('../core/wireprotocol');
 const apm = require('../core/connection/apm');
 const updateSessionFromResponse = require('../core/sessions').updateSessionFromResponse;
@@ -34,6 +34,7 @@ class Connection extends EventEmitter {
     this.closed = false;
     this.destroyed = false;
 
+    this[kDescription] = new StreamDescription(this.address, options);
     this[kGeneration] = options.generation;
     this[kLastUseTime] = Date.now();
 
@@ -69,12 +70,6 @@ class Connection extends EventEmitter {
     // hook the message stream up to the passed in stream
     stream.pipe(this[kMessageStream]);
     this[kMessageStream].pipe(stream);
-
-    if (options.compression) {
-      this[kDescription] = { compression: options.compression };
-    } else {
-      this[kDescription] = undefined;
-    }
   }
 
   get description() {
@@ -87,20 +82,7 @@ class Connection extends EventEmitter {
 
   // the `connect` method stores the result of the handshake ismaster on the connection
   set ismaster(response) {
-    if (response.compression) {
-      const compression = this[kDescription].compression;
-      const compressors = compression.compressors;
-      response.compression = {
-        compressor: compressors.filter(c => response.compression.indexOf(c) !== -1)[0]
-      };
-
-      if (compression.zlibCompressionLevel) {
-        response.compression.zlibCompressionLevel = compression.zlibCompressionLevel;
-      }
-    }
-
-    // TODO: This should be using a `StreamDescription`
-    this[kDescription] = new ServerDescription(this.address, response);
+    this[kDescription].receiveResponse(response);
 
     // TODO: remove this, and only use the `StreamDescription` in the future
     this[kIsMaster] = response;
@@ -284,15 +266,19 @@ function write(command, options, callback) {
     noResponse: typeof options.noResponse === 'boolean' ? options.noResponse : false,
     documentsReturnedIn: options.documentsReturnedIn,
 
-    // For BSON parsing
+    // for BSON parsing
     promoteLongs: typeof options.promoteLongs === 'boolean' ? options.promoteLongs : true,
     promoteValues: typeof options.promoteValues === 'boolean' ? options.promoteValues : true,
     promoteBuffers: typeof options.promoteBuffers === 'boolean' ? options.promoteBuffers : false,
     raw: typeof options.raw === 'boolean' ? options.raw : false
   };
 
-  if (this[kDescription] && this[kDescription].compression) {
-    operationDescription.agreedCompressor = this[kDescription].compression.compressor;
+  if (this[kDescription] && this[kDescription].compressor) {
+    operationDescription.agreedCompressor = this[kDescription].compressor;
+
+    if (this[kDescription].zlibCompressionLevel) {
+      operationDescription.zlibCompressionLevel = this[kDescription].zlibCompressionLevel;
+    }
   }
 
   if (typeof options.socketTimeout === 'number') {

lib/cmap/stream_description.js

@@ -0,0 +1,45 @@
+'use strict';
+const parseServerType = require('../core/sdam/server_description').parseServerType;
+
+const RESPONSE_FIELDS = [
+  'minWireVersion',
+  'maxWireVersion',
+  'maxBsonObjectSize',
+  'maxMessageSizeBytes',
+  'maxWriteBatchSize',
+  '__nodejs_mock_server__'
+];
+
+class StreamDescription {
+  constructor(address, options) {
+    this.address = address;
+    this.type = parseServerType(null);
+    this.minWireVersion = 1;
+    this.maxWireVersion = 2;
+    this.maxBsonObjectSize = 16777216;
+    this.maxMessageSizeBytes = 48000000;
+    this.maxWriteBatchSize = 100000;
+    this.compressors =
+      options && options.compression && Array.isArray(options.compression.compressors)
+        ? options.compression.compressors
+        : [];
+  }
+
+  receiveResponse(response) {
+    this.type = parseServerType(response);
+
+    RESPONSE_FIELDS.forEach(field => {
+      if (typeof response[field] !== 'undefined') {
+        this[field] = response[field];
+      }
+    });
+
+    if (response.compression) {
+      this.compressor = this.compressors.filter(c => response.compression.indexOf(c) !== -1)[0];
+    }
+  }
+}
+
+module.exports = {
+  StreamDescription
+};

lib/core/sdam/server_description.js

@@ -176,5 +176,6 @@ function parseServerType(ismaster) {
 }
 
 module.exports = {
-  ServerDescription
+  ServerDescription,
+  parseServerType
 };