Ajax: Protect against exceptions thrown synchronously by xhr.send

When xhr.send throws an exception synchronously, the onerror handler may have
been called already which, unchecked, makes the exception bubble up outside of
jQuery.ajax.

We now catch the exception pre-emptively and only rethrow if we know it hasn't
already been notified through the onerror handler.

Fixes #14683

src/ajax/xhr.js

@@ -112,10 +112,15 @@ jQuery.ajaxTransport(function( options ) {
 				// Create the abort callback
 				callback = xhrCallbacks[ id ] = callback("abort");
 
-				// Do send the request
-				// This may raise an exception which is actually
-				// handled in jQuery.ajax (so no try/catch here)
+				try {
+					// Do send the request (this may raise an exception)
 					xhr.send( options.hasContent && options.data || null );
+				} catch ( e ) {
+					// #14683: Only rethrow if this hasn't been notified as an error yet
+					if ( callback ) {
+						throw e;
+					}
+				}
 			},
 
 			abort: function() {

test/unit/ajax.js

@@ -1544,6 +1544,36 @@ module( "ajax", {
 		});
 	}
 
+	ajaxTest( "#14683 - jQuery.ajax() - Exceptions thrown synchronously by xhr.send should be caught", 4, [
+		{
+			url: "data/params_html.php",
+			method: "POST",
+			data: {
+				toString: function() {
+					throw "Can't parse";
+				}
+			},
+			processData: false,
+			done: function( data ) {
+				ok( false, "done: " + data );
+			},
+			fail: function( jqXHR, status, error ) {
+				ok( true, "exception caught: " + error );
+				strictEqual( jqXHR.status, 0, "proper status code" );
+				strictEqual( status, "error", "proper status" );
+			}
+		},
+		{
+			url: "http://domain.org:80d",
+			done: function( data ) {
+				ok( false, "done: " + data );
+			},
+			fail: function( _, status, error ) {
+				ok( true, "fail: " + status + " - " + error );
+			}
+		}
+	]);
+
 //----------- jQuery.ajaxPrefilter()
 
 	ajaxTest( "jQuery.ajaxPrefilter() - abort", 1, {