feat(sessions): track dirty state of sessions, drop after use

If a network error occurs during an operation using a session, that
session must now be marked dirty and discarded after the operation
completes (allowing for retryability).

NODE-1977

lib/core/sdam/server.js

@@ -240,6 +240,12 @@ class Server extends EventEmitter {
     }
 
     wireProtocol.command(this, ns, cmd, options, (err, result) => {
+      if (err && err instanceof MongoNetworkError) {
+        if (options.session) {
+          options.session.serverSession.isDirty = true;
+        }
+      }
+
       if (err && isSDAMUnrecoverableError(err)) {
         this.emit('error', err);
       }
@@ -258,6 +264,12 @@ class Server extends EventEmitter {
    */
   query(ns, cmd, cursorState, options, callback) {
     wireProtocol.query(this, ns, cmd, cursorState, options, (err, result) => {
+      if (err && err instanceof MongoNetworkError) {
+        if (options.session) {
+          options.session.serverSession.isDirty = true;
+        }
+      }
+
       if (err && isSDAMUnrecoverableError(err)) {
         this.emit('error', err);
       }
@@ -276,6 +288,12 @@ class Server extends EventEmitter {
    */
   getMore(ns, cursorState, batchSize, options, callback) {
     wireProtocol.getMore(this, ns, cursorState, batchSize, options, (err, result) => {
+      if (err && err instanceof MongoNetworkError) {
+        if (options.session) {
+          options.session.serverSession.isDirty = true;
+        }
+      }
+
       if (err && isSDAMUnrecoverableError(err)) {
         this.emit('error', err);
       }
@@ -406,6 +424,12 @@ function executeWriteOperation(args, options, callback) {
   }
 
   return wireProtocol[op](server, ns, ops, options, (err, result) => {
+    if (err && err instanceof MongoNetworkError) {
+      if (options.session) {
+        options.session.serverSession.isDirty = true;
+      }
+    }
+
     if (err && isSDAMUnrecoverableError(err)) {
       server.emit('error', err);
     }

lib/core/sessions.js

@@ -527,6 +527,7 @@ class ServerSession {
     this.id = { id: new Binary(uuidV4(), Binary.SUBTYPE_UUID) };
     this.lastUse = Date.now();
     this.txnNumber = 0;
+    this.isDirty = false;
   }
 
   /**
@@ -603,15 +604,20 @@ class ServerSessionPool {
   release(session) {
     const sessionTimeoutMinutes = this.topology.logicalSessionTimeoutMinutes;
     while (this.sessions.length) {
-      const session = this.sessions[this.sessions.length - 1];
-      if (session.hasTimedOut(sessionTimeoutMinutes)) {
+      const pooledSession = this.sessions[this.sessions.length - 1];
+      if (pooledSession.hasTimedOut(sessionTimeoutMinutes)) {
         this.sessions.pop();
       } else {
         break;
       }
     }
 
     if (!session.hasTimedOut(sessionTimeoutMinutes)) {
+      if (session.isDirty) {
+        return;
+      }
+
+      // otherwise, readd this session to the session pool
       this.sessions.unshift(session);
     }
   }