Make sure ports are closed when they're no longer used.

chrome/common/extensions/docs/static/messaging.html

 <div id="pageData-name" class="pageData">Message Passing</div>
 <div id="pageData-showTOC" class="pageData">true</div>
 
 <p>
 Since content scripts run in the context of a web page and not the extension,
 they often need some way of communicating with the rest of the extension. For
 example, an RSS reader extension might use content scripts to detect the
 presence of an RSS feed on a page, then notify the background page in order to
 display a page action icon for that page.
 
@@ skipping 45 matching lines @@
   chrome.tabs.sendRequest(tab.id, {greeting: "hello"}, function(response) {
     console.log(response.farewell);
   });
 });
 </pre>
 
 <p>
 On the receiving end, you need to set up an
 <a href="extension.html#event-onRequest">chrome.extension.onRequest</a>
 event listener to handle the message. This looks the same from a content
-script or extension page. The request will remain open until you call
-sendResponse, so it is good practice to call sendResponse with an empty
-object to allow the request to be cleaned up.
+script or extension page.
 <pre>
 chrome.extension.onRequest.addListener(
   function(request, sender, sendResponse) {
     console.log(sender.tab ?
                 "from a content script:" + sender.tab.url :
                 "from the extension");
     if (request.greeting == "hello")
       sendResponse({farewell: "goodbye"});
-    else
-      sendResponse({}); // snub them.
   });
 </pre>
 
 <p class="note">
 <b>Note:</b> If multiple pages are listening for onRequest events, only the
 first to call sendResponse() for a particular event will succeed in sending the
 response. All other responses to that event will be ignored.
 </p>
 
 
@@ skipping 89 matching lines @@
 case, except you use the
 <a href="extension.html#event-onRequestExternal">chrome.extension.onRequestExternal</a>
 or
 <a href="extension.html#event-onConnectExternal">chrome.extension.onConnectExternal</a>
 methods. Here's an example of each:
 <pre>
 // For simple requests:
 chrome.extension.onRequestExternal.addListener(
   function(request, sender, sendResponse) {
     if (sender.id == blacklistedExtension)
-      sendResponse({});  // don't allow this extension access
+      return;  // don't allow this extension access
     else if (request.getTargetData)
       sendResponse({targetData: targetData});
     else if (request.activateLasers) {
       var success = activateLasers();
       sendResponse({activateLasers: success});
     }
   });
 
 // For long-lived connections:
 chrome.extension.onConnectExternal.addListener(function(port) {
@@ skipping 71 matching lines @@
 <a href="http://src.chromium.org/viewvc/chrome/trunk/src/chrome/common/extensions/docs/examples/api/messaging/">examples/api/messaging</a>
 directory.
 Also see the
 <a href="http://src.chromium.org/viewvc/chrome/trunk/src/chrome/common/extensions/docs/examples/howto/contentscript_xhr">contentscript_xhr</a> example,
 in which a content script and its parent extension exchange messages,
 so that the parent extension can perform
 cross-site requests on behalf of the content script.
 For more examples and for help in viewing the source code, see
 <a href="samples.html">Samples</a>.
 </p>