Fix IDLE for very fast connections

When issuing commands in rapid succession, it was possible for
node-imap to send multiple "DONE" commands. This resulted in wedged
state when the server dropped out of IDLE and then didn't know what to
do with the second DONE command.

This change adds a safeguard that ensures _send never sends more than
one DONE, and queues (non-DONE/IDLE) commands up for when the server
was able to acknowledge.

lib/imap.js

@@ -27,7 +27,8 @@ var CRLF = '\r\n',
 // extension constants
 var IDLE_NONE = 1,
     IDLE_WAIT = 2,
-    IDLE_READY = 3;
+    IDLE_READY = 3,
+    IDLE_DONE = 4;
 
 function ImapConnection (options) {
   if (!(this instanceof ImapConnection))
@@ -1209,16 +1210,18 @@ ImapConnection.prototype._send = function(cmdstr, cb, bypass) {
       cbargs: []
     });
   }
-  if (this._state.ext.idle.state === IDLE_WAIT)
+  if (this._state.ext.idle.state === IDLE_WAIT ||
+      (this._state.ext.idle.state == IDLE_DONE && cmdstr !== 'DONE'))
     return;
   if ((cmdstr === undefined && this._state.requests.length)
       || this._state.requests.length === 1 || bypass) {
     var prefix = '',
         cmd = (bypass ? cmdstr : this._state.requests[0].cmdstr);
     clearTimeout(this._state.tmrKeepalive);
-    if (this._state.ext.idle.state === IDLE_READY && cmd !== 'DONE')
+    if (this._state.ext.idle.state === IDLE_READY && cmd !== 'DONE') {
+      this._state.ext.idle.state = IDLE_DONE;
       return this._send('DONE', undefined, true);
-    else if (cmd === 'IDLE') {
+    } else if (cmd === 'IDLE') {
        // we use a different prefix to differentiate and disregard the tagged
        // response the server will send us when we issue DONE
       prefix = 'IDLE ';