Fix Notion migrations not working in when served by Netlify. (#8960)

Found why the redirections are not working in prod. In local, NextJS
handles the redirections (logical), but in prod, we let Netlify do it
(logical, as it's serving static content).

But, the semantics in between the two are not the same: 

- NextJS sets up the redirection "normally" 
- Netlify instead, will flat out ignore the redirection if the page
exists.

So the fix was to patch the code that generates the `_redirects` file
that Netlify needs, to add the `301!` in `/source https://where 301!`
required for it to stop ignoring those.

I really didn't see coming the different semantics here :/

---------

Co-authored-by: jhchabran <[email protected]>

data/notion_migration.yaml

@@ -2,7 +2,7 @@
 # - 'source': URL path to redirect, as currently visible on handbook.sourcegraph.com
 # - 'destination': New URL top direct to. If a public 'sourcegraph.notion.site' page is available, prefer that instead of the internal Notion URL.
 redirections:
-  - source: /departments/engineering/teams/devinfra
+  - source: /departments/engineering/teams/devinfra/
     destination: https://sourcegraph.notion.site/Developer-Infrastructure-Team-a46433b93bb2445abc1966c93a570a26
   # Core Services
   - source: /departments/engineering/teams/core-services/

src/scripts/generate-redirects.mjs

@@ -7,7 +7,9 @@ import fs from 'fs/promises'
 
 import redirects from './redirects.mjs'
 
-const redirectLines = (await redirects()).map(({ source, destination }) => `${source} ${destination}`).join('\n')
+const redirectLines = (await redirects())
+    .map(entry => `${entry.source} ${entry.destination}${entry.force ? ' 301!' : ''}`)
+    .join('\n')
 const lines = `\n# Generated by generate-redirects.mjs from Git history\n${redirectLines}\n`
 
 console.log('Redirects:\n', lines)

src/scripts/redirects.mjs

@@ -31,10 +31,21 @@ export function cleanupRedirects(movedPages) {
 /**
  * Reads Notion specific redirections from data/notion_migration.yaml.
  *
- * @returns {redirections: {source: string, destination: string}[]}
+ * @returns {redirections: {source: string, destination: string, force: true}[]}
  */
 async function readNotionMigrationRedirects() {
-    return load(await readFile('data/notion_migration.yaml', 'utf8'))
+    const data = load(await readFile('data/notion_migration.yaml', 'utf8'))
+
+    // While NextJS is okay if content exists when setting up a redirection,
+    // Netlify doesn't: it will skip the redirection entirely if it finds
+    // existing content.
+    // To go around that, we add a new field 'force' that the script that
+    // generate the final _redirects file used by Netlify uses to append
+    // 301! on that entry, effectively forcing the redirection.
+    for (const entry of data.redirections) {
+        entry.force = true
+    }
+    return data
 }
 
 /**
@@ -47,12 +58,10 @@ export default async function redirects() {
     const notionRedirections = await readNotionMigrationRedirects()
     return cleanupRedirects([
         ...movedPages,
-        ...notionRedirections.redirections,
-
         // Add custom redirects
         {
             source: '/careers',
             destination: 'https://about.sourcegraph.com/jobs',
         },
-    ])
+    ]).concat(notionRedirections.redirections) // we skip the cleanup because notion redirects are always going outside.
 }