fix: use SLB-Forwarded headers when proxying webforms

apps/cms/gatsby-node.mjs

@@ -38,12 +38,21 @@ export const createPages = async ({ actions }) => {
   });
 
   // Proxy Drupal webforms.
+  const netlifyUrl = new URL(
+    process.env.NETLIFY_URL || 'http://127.0.0.1:8000',
+  );
   Object.values(Locale).forEach((locale) => {
-    actions.createRedirect({
-      fromPath: `/${locale}/form/*`,
-      toPath: `${process.env.GATSBY_DRUPAL_URL}/${locale}/form/:splat`,
-      statusCode: 200,
-    });
+    global.netlifyTomlParts.push(`
+      [[redirects]]
+        from = "/${locale}/form/*"
+        to = "${process.env.GATSBY_DRUPAL_URL}/${locale}/form/:splat"
+        status = 200
+        force = true
+        [redirects.headers]
+          SLB-Forwarded-Proto = "${netlifyUrl.protocol.slice(0, -1)}"
+          SLB-Forwarded-Host = "${netlifyUrl.host}"
+          SLB-Forwarded-Port = "${netlifyUrl.port}"
+    `);
   });
 
   // Additionally proxy themes and modules as they can have additional

apps/website/.gitignore

@@ -8,3 +8,6 @@ persisted-store
 
 # Deno comes with "netlify dev". We don't need its lock file.
 /deno.lock
+
+# Created by gatsby-node.mjs
+netlify.toml

apps/website/gatsby-node.mjs

@@ -5,8 +5,16 @@ import {
   Locale,
   NotFoundPageQuery,
 } from '@custom/schema';
+import fs from 'fs';
 import { resolve } from 'path';
 
+if (!('netlifyTomlParts' in global)) {
+  Object.defineProperty(global, 'netlifyTomlParts', {
+    value: [],
+    writable: true,
+  });
+}
+
 /**
  * @type {import('gatsby').GatsbyNode['onCreateWebpackConfig']}
  */
@@ -109,9 +117,50 @@ export const createPages = async ({ actions }) => {
   // Any unhandled requests are handed to strangler, which will try to pass
   // them to all registered legacy systems and return 404 if none of them
   // respond.
-  actions.createRedirect({
-    fromPath: '/*',
-    toPath: `/.netlify/functions/strangler`,
-    statusCode: 200,
-  });
+  // We put it into netlify.toml because it should be really the last one.
+  // See https://docs.netlify.com/routing/redirects/#rule-processing-order
+  // @ts-expect-error Not typed.
+  global.netlifyTomlParts.push(`
+    [[redirects]]
+      from = "/*"
+      to = "/.netlify/functions/strangler"
+      status = 200
+  `);
+};
+
+/**
+ *
+ * @type {import('gatsby').GatsbyNode['onPostBuild']}
+ */
+export const onPostBuild = async () => {
+  const netlifyTomlBase = fs
+    .readFileSync(resolve('./netlify-base.toml'), 'utf8')
+    .trim();
+  /** @type {Array<string>} */
+  const netlifyTomlParts =
+    // @ts-expect-error Not typed.
+    global.netlifyTomlParts;
+  fs.writeFileSync(
+    resolve('./netlify.toml'),
+    [netlifyTomlBase]
+      .concat(netlifyTomlParts.map(removeIndentation))
+      .join('\n\n'),
+  );
 };
+
+/**
+ * @param {string} str
+ * @returns {string}
+ */
+function removeIndentation(str) {
+  const lines = str.split('\n');
+  const minIndent = Math.min(
+    ...lines
+      .filter((line) => line.trim().length > 0)
+      .map((line) => (line.match(/^\s*/) || [''])[0].length),
+  );
+  return lines
+    .map((line) => line.slice(minIndent))
+    .join('\n')
+    .trim();
+}