Fixed AMP URLs not being transformed to absolute (#12737)

refs https://github.com/TryGhost/Team/issues/467
refs https://github.com/TryGhost/Ghost/pull/12731

- AMP helper fetches HTML directly from the database rather than fetching via the API so we can't rely on the API serializers to perform transforms for us
- switched the `relativeToAbsolute(html)` call to `transformReadyToAbsolute(html)` to match the new `__GHOST_URL__` storage format

core/frontend/apps/amp/lib/helpers/amp_content.js

@@ -119,8 +119,8 @@ function getAmperizeHTML(html, post) {
 
     amperize = amperize || new Amperize();
 
-    // make relative URLs abolute
+    // make transform-ready URLs abolute
-    html = urlUtils.htmlRelativeToAbsolute(html, post.url);
+    html = urlUtils.transformReadyToAbsolute(html);
 
     if (!amperizeCache[post.id] || moment(new Date(amperizeCache[post.id].updated_at)).diff(new Date(post.updated_at)) < 0) {
         return new Promise((resolve) => {

test/unit/apps/amp/amp_content_spec.js

@@ -122,6 +122,30 @@ describe('{{amp_content}} helper', function () {
             ampContentHelper.__set__('amperizeCache', {});
         });
 
+        it('transforms URLs to absolute', function (done) {
+            const GIF1x1 = Buffer.from('R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==', 'base64');
+
+            nock('https://ghost.org/blog/')
+                .get('/image.png')
+                .reply(200, GIF1x1);
+
+            const testData = {
+                html: '<a href="__GHOST_URL__/"><img src="__GHOST_URL__/image.png" alt="Test image" /></a>',
+                updated_at: 'Wed Jul 27 2016 18:17:22 GMT+0200 (CEST)',
+                id: 1
+            };
+
+            const ampResult = ampContentHelper.call(testData);
+
+            ampResult.then(function (rendered) {
+                should.exist(rendered);
+                rendered.string.should.not.containEql('__GHOST_URL__');
+                rendered.string.should.containEql('href="https://ghost.org/blog/"');
+                rendered.string.should.containEql('src="https://ghost.org/blog/image.png"');
+                done();
+            }).catch(done);
+        });
+
         it('can transform img tags to amp-img', function (done) {
             const GIF1x1 = Buffer.from('R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==', 'base64');
 
@@ -130,7 +154,7 @@ describe('{{amp_content}} helper', function () {
                 .reply(200, GIF1x1);
 
             const testData = {
-                html: '<img src="/content/images/2019/06/test.jpg" alt="The Ghost Logo" />',
+                html: '<img src="__GHOST_URL__/content/images/2019/06/test.jpg" alt="The Ghost Logo" />',
                 updated_at: 'Wed Jul 27 2016 18:17:22 GMT+0200 (CEST)',
                 id: 1
             };
@@ -189,7 +213,7 @@ describe('{{amp_content}} helper', function () {
 
         it('removes inline style', function (done) {
             const testData = {
-                html: '<amp-img src="/content/images/2016/08/aileen_small.jpg" style="border-radius: 50%"; !important' +
+                html: '<amp-img src="__GHOST_URL__/content/images/2016/08/aileen_small.jpg" style="border-radius: 50%"; !important' +
                           'border="0" align="center" font="Arial" width="50" height="50" layout="responsive"></amp-img>' +
                           '<p align="right" style="color: red; !important" bgcolor="white">Hello</p>' +
                           '<table style="width:100%"><tr bgcolor="tomato" colspan="2"><th font="Arial">Name:</th> ' +
@@ -260,7 +284,7 @@ describe('{{amp_content}} helper', function () {
 
         it('can handle not existing img src by returning not Amperized HTML', function (done) {
             const testData = {
-                html: '<img src="/content/images/does-not-exist.jpg" alt="The Ghost Logo" />',
+                html: '<img src="__GHOST_URL__/content/images/does-not-exist.jpg" alt="The Ghost Logo" />',
                 updated_at: 'Wed Jul 27 2016 18:17:22 GMT+0200 (CEST)',
                 id: 1
             };