Fix a race condition between stdout/stderr prints vs exit() when using --emrun: fully wait until all stdout/stderr messages have been sent before closing the Firefox browser window.

src/emrun_postjs.js

@@ -1,10 +1,10 @@
 if (typeof window === "object" && (typeof ENVIRONMENT_IS_PTHREAD === 'undefined' || !ENVIRONMENT_IS_PTHREAD)) {
   function emrun_register_handlers() {
-    function post(msg) {
-      var http = new XMLHttpRequest();
-      http.open("POST", "stdio.html", true);
-      http.send(msg);
-    }
+    // When C code exit()s, we may still have remaining stdout and stderr messages in flight. In that case, we can't close
+    // the browser until all those XHRs have finished, so the following state variables track that all communication is done,
+    // after which we can close.
+    var emrun_num_post_messages_in_flight = 0;
+    var emrun_should_close_itself = false;
     function postExit(msg) {
       var http = new XMLHttpRequest();
       http.onreadystatechange = function() {
@@ -19,12 +19,23 @@ if (typeof window === "object" && (typeof ENVIRONMENT_IS_PTHREAD === 'undefined'
       http.open("POST", "stdio.html", true);
       http.send(msg);
     }
+    function post(msg) {
+      var http = new XMLHttpRequest();
+      ++emrun_num_post_messages_in_flight;
+      http.onreadystatechange = function() {
+        if (http.readyState == 4 /*DONE*/) {
+          if (--emrun_num_post_messages_in_flight == 0 && emrun_should_close_itself) postExit('^exit^'+EXITSTATUS);
+        }
+      }
+      http.open("POST", "stdio.html", true);
+      http.send(msg);
+    }
     // If the address contains localhost, or we are running the page from port 6931, we can assume we're running the test runner and should post stdout logs.
     if (document.URL.search("localhost") != -1 || document.URL.search(":6931/") != -1) {
       var emrun_http_sequence_number = 1;
       var prevPrint = Module['print'];
       var prevErr = Module['printErr'];
-      function emrun_exit() { postExit('^exit^'+EXITSTATUS); };	
+      function emrun_exit() { if (emrun_num_post_messages_in_flight == 0) postExit('^exit^'+EXITSTATUS); else emrun_should_close_itself = true; };
       Module['addOnExit'](emrun_exit);
       Module['print'] = function emrun_print(text) { post('^out^'+(emrun_http_sequence_number++)+'^'+encodeURIComponent(text)); prevPrint(text); }
       Module['printErr'] = function emrun_printErr(text) { post('^err^'+(emrun_http_sequence_number++)+'^'+encodeURIComponent(text)); prevErr(text); }