Added migration to transform all post content urls from absolute to relative (#12658)

refs https://github.com/TryGhost/Team/issues/467
refs https://github.com/TryGhost/Team/issues/221

- we added robust url transformation in Ghost 3.0 which ensures newly created or edited content is stored with relative URLs in the database
- this migration loops over all posts (and posts_meta) in the database performing the absolute->relative transformation to normalise data for old posts

core/server/data/migrations/versions/4.0/18-transform-posts-absolute-to-relative.js

@@ -0,0 +1,77 @@
+const logging = require('../../../../../shared/logging');
+const urlUtils = require('../../../../../shared/url-utils');
+const {createIrreversibleMigration} = require('../../utils');
+
+module.exports = createIrreversibleMigration(async (knex) => {
+    logging.info('Transforming all internal urls in posts from absolute to relative');
+
+    await knex.transaction(async (trx) => {
+        // get list of posts ids, use .forUpdate to lock rows until the transaction is finished
+        const postIdRows = await knex('posts')
+            .transacting(trx)
+            .forUpdate()
+            .select('id');
+
+        // transform each post individually to avoid dumping all posts into memory and
+        // pushing all queries into the query builder buffer in parallel
+        // https://stackoverflow.com/questions/54105280/how-to-loop-through-multi-line-sql-query-and-use-them-in-knex-transactions
+
+        for (const postIdRow of postIdRows) {
+            const {id} = postIdRow;
+            const [post] = await knex('posts')
+                .transacting(trx)
+                .where({id})
+                .select([
+                    'mobiledoc',
+                    'custom_excerpt',
+                    'codeinjection_head',
+                    'codeinjection_foot',
+                    'feature_image',
+                    'canonical_url'
+                ]);
+
+            /* eslint-disable camelcase */
+            const mobiledoc = urlUtils.mobiledocAbsoluteToRelative(post.mobiledoc);
+            const custom_excerpt = urlUtils.htmlAbsoluteToRelative(post.custom_excerpt);
+            const codeinjection_head = urlUtils.htmlAbsoluteToRelative(post.codeinjection_head);
+            const codeinjection_foot = urlUtils.htmlAbsoluteToRelative(post.codeinjection_foot);
+            const feature_image = urlUtils.absoluteToRelative(post.feature_image);
+            const canonical_url = urlUtils.absoluteToRelative(post.canonical_url, {ignoreProtocol: false});
+
+            await knex('posts')
+                .transacting(trx)
+                .where({id})
+                .update({
+                    mobiledoc,
+                    custom_excerpt,
+                    codeinjection_head,
+                    codeinjection_foot,
+                    feature_image,
+                    canonical_url
+                });
+
+            const [postMeta] = await knex('posts_meta')
+                .transacting(trx)
+                .where({post_id: id})
+                .select([
+                    'og_image',
+                    'twitter_image'
+                ]);
+
+            if (postMeta) {
+                const og_image = urlUtils.absoluteToRelative(postMeta.og_image);
+                const twitter_image = urlUtils.absoluteToRelative(postMeta.twitter_image);
+
+                await knex('posts_meta')
+                    .transacting(trx)
+                    .where({post_id: id})
+                    .update({
+                        og_image,
+                        twitter_image
+                    });
+            }
+        }
+
+        return 'transaction complete';
+    });
+});