Ajax: Support headers for script transport even when cross-domain

The AJAX script transport has two versions: XHR + `jQuery.globalEval` or
appending a script tag (note that `jQuery.globalEval` also appends a
script tag now, but inline). The former cannot support the `headers`
option which has so far not been taken into account.

For jQuery 3.x, the main consequence was the option not being respected
for cross-domain requests. Since in 4.x we use the latter way more
often, the option was being ignored in more cases.

The transport now checks whether the `headers` option is specified and
uses the XHR way unless `scriptAttrs` are specified as well.

Fixes gh-5142
Closes gh-5193

src/ajax/script.js

@@ -6,8 +6,14 @@ import "../ajax.js";
 function canUseScriptTag( s ) {
 
 	// A script tag can only be used for async, cross domain or forced-by-attrs requests.
+	// Requests with headers cannot use a script tag. However, when both `scriptAttrs` &
+	// `headers` options are specified, both are impossible to satisfy together; we
+	// prefer `scriptAttrs` then.
 	// Sync requests remain handled differently to preserve strict script ordering.
-	return s.crossDomain || s.scriptAttrs ||
+	return s.scriptAttrs || (
+		!s.headers &&
+		(
+			s.crossDomain ||
 
 			// When dealing with JSONP (`s.dataTypes` include "json" then)
 			// don't use a script tag so that error responses still may have
@@ -16,10 +22,12 @@ function canUseScriptTag( s ) {
 			//   * have `scriptAttrs` set as that's a script-only functionality
 			// Note that this means JSONP requests violate strict CSP script-src settings.
 			// A proper solution is to migrate from using JSONP to a CORS setup.
-		( s.async && jQuery.inArray( "json", s.dataTypes ) < 0 );
+			( s.async && jQuery.inArray( "json", s.dataTypes ) < 0 )
+		)
+	);
 }
 
-// Install script dataType. Don't specify `content.script` so that an explicit
+// Install script dataType. Don't specify `contents.script` so that an explicit
 // `dataType: "script"` is required (see gh-2432, gh-4822)
 jQuery.ajaxSetup( {
 	accepts: {

test/unit/ajax.js

@@ -71,11 +71,13 @@ QUnit.module( "ajax", {
 		};
 	} );
 
-	ajaxTest( "jQuery.ajax() - custom attributes for script tag", 5,
+	jQuery.each( [ " - Same Domain", " - Cross Domain" ], function( crossDomain, label ) {
+		ajaxTest( "jQuery.ajax() - custom attributes for script tag" + label, 5,
 			function( assert ) {
 				return {
 					create: function( options ) {
 						var xhr;
+						options.crossDomain = crossDomain;
 						options.method = "POST";
 						options.dataType = "script";
 						options.scriptAttrs = { id: "jquery-ajax-test", async: "async" };
@@ -96,10 +98,64 @@ QUnit.module( "ajax", {
 			}
 		);
 
+		ajaxTest( "jQuery.ajax() - headers for script transport" + label, 3,
+			function( assert ) {
+				return {
+					create: function( options ) {
+						Globals.register( "corsCallback" );
+						window.corsCallback = function( response ) {
+							assert.strictEqual( response.headers[ "x-custom-test-header" ],
+								"test value", "Custom header sent" );
+						};
+						options.crossDomain = crossDomain;
+						options.dataType = "script";
+						options.headers = { "x-custom-test-header": "test value" };
+						return jQuery.ajax( url( "mock.php?action=script&callback=corsCallback" ), options );
+					},
+					success: function() {
+						assert.ok( true, "success" );
+					},
+					complete: function() {
+						assert.ok( true, "complete" );
+					}
+				};
+			}
+		);
+
+		ajaxTest( "jQuery.ajax() - scriptAttrs winning over headers" + label, 4,
+			function( assert ) {
+				return {
+					create: function( options ) {
+						var xhr;
+						Globals.register( "corsCallback" );
+						window.corsCallback = function( response ) {
+							assert.ok( !response.headers[ "x-custom-test-header" ],
+								"headers losing with scriptAttrs" );
+						};
+						options.crossDomain = crossDomain;
+						options.dataType = "script";
+						options.scriptAttrs = { id: "jquery-ajax-test", async: "async" };
+						options.headers = { "x-custom-test-header": "test value" };
+						xhr = jQuery.ajax( url( "mock.php?action=script&callback=corsCallback" ), options );
+						assert.equal( jQuery( "#jquery-ajax-test" ).attr( "async" ), "async", "attr value" );
+						return xhr;
+					},
+					success: function() {
+						assert.ok( true, "success" );
+					},
+					complete: function() {
+						assert.ok( true, "complete" );
+					}
+				};
+			}
+		);
+	} );
+
 	ajaxTest( "jQuery.ajax() - execute JS when dataType option is provided", 3,
 		function( assert ) {
 			return {
 				create: function( options ) {
+					Globals.register( "corsCallback" );
 					options.crossDomain = true;
 					options.dataType = "script";
 					return jQuery.ajax( url( "mock.php?action=script&header=ecma" ), options );