Add support for a custom _redirects file

Since _redirects always takes precedence over netlify.toml,
allowing a custom _redirects file to be prepended to the
published one is the only way to force a redirect of a path that
matches content handled by next-on-netlify. This can be
useful when using netlify to redirect from one domain to another.

Author: etrepum

Diff for: README.md

@@ -118,11 +118,12 @@ From now on, whenever you want to preview your application locally, just run:
 
 #### Custom Netlify Redirects
 
-You can define custom redirects in the `netlify.toml` file.
-Routes defined by `next-on-netlify` take precedence over routes
-defined in `netlify.toml`.
+You can define custom redirects in a `_redirects` and/or in your `netlify.toml` file.
+The precedence of these rules are:
 
-In the past, it was possible to define custom redirects in a `_redirects` file. This is not possible anymore. Let me know if you have a need for this feature and we can add it back.
+- `_redirects`
+- `next-on-netlify` redirects
+- `netlify.toml`
 
 [Read more about Netlify redirects here](https://docs.netlify.com/routing/redirects/).  
 

Diff for: lib/config.js

@@ -23,6 +23,8 @@ const NEXT_DIST_DIR           = getNextDistDir({ nextConfigPath: NEXT_CONFIG_PAT
 // This is the Netlify Function template that wraps all SSR pages
 const FUNCTION_TEMPLATE_PATH  = join(__dirname, "netlifyFunctionTemplate.js")
 
+// This is the file where custom redirects can be configured
+const CUSTOM_REDIRECTS_PATH          = join(".", "_redirects")
 
 module.exports = {
   NETLIFY_PUBLISH_PATH,
@@ -31,4 +33,5 @@ module.exports = {
   NEXT_CONFIG_PATH,
   NEXT_DIST_DIR,
   FUNCTION_TEMPLATE_PATH,
+  CUSTOM_REDIRECTS_PATH,
 }

Diff for: lib/setupRedirects.js

@@ -1,9 +1,11 @@
 const path                        = require('path')
 const { join }                    = path
-const { writeFileSync }           = require('fs-extra')
+const { existsSync, readFileSync,
+        writeFileSync }           = require('fs-extra')
 const { default: isDynamicRoute } = require("@sls-next/lambda-at-edge/dist/lib/isDynamicRoute")
 const { getSortedRoutes }         = require("@sls-next/lambda-at-edge/dist/lib/sortedRoutes")
-const { NETLIFY_PUBLISH_PATH }    = require('./config')
+const { NETLIFY_PUBLISH_PATH,
+        CUSTOM_REDIRECTS_PATH }   = require('./config')
 const allNextJsPages              = require('./allNextJsPages')
 const getNetlifyRoute             = require('./getNetlifyRoute')
 const getNetlifyFunctionName      = require('./getNetlifyFunctionName')
@@ -30,6 +32,12 @@ const setupRedirects = () => {
 
   // Generate redirects as array
   const redirects = []
+  if(existsSync(CUSTOM_REDIRECTS_PATH)) {
+    console.log("  ", "# Prepending custom redirects")
+    redirects.push(readFileSync(CUSTOM_REDIRECTS_PATH))
+  }
+  redirects.push("# Next-on-Netlify Redirects")
+
   sortedPages.forEach(page => {
     // Generate redirect for each page route
     page.routesAsArray.forEach(route => {
@@ -64,7 +72,6 @@ const setupRedirects = () => {
   // Write redirects to _redirects file
   writeFileSync(
     join(NETLIFY_PUBLISH_PATH, "_redirects"),
-    "# Next-on-Netlify Redirects" + "\n" +
     redirects.join("\n")
   )
 }

Diff for: tests/customRedirects.test.js

@@ -0,0 +1,84 @@
+// Test next-on-netlify when a custom distDir is set in next.config.js
+const { parse, join } = require('path')
+const { copySync, emptyDirSync, readFileSync, writeFileSync } = require('fs-extra')
+const npmRunBuild = require("./helpers/npmRunBuild")
+
+// The name of this test file (without extension)
+const FILENAME = parse(__filename).name
+
+// The directory which will be used for testing.
+// We simulate a NextJS app within that directory, with pages, and a
+// package.json file.
+const PROJECT_PATH = join(__dirname, "builds", FILENAME)
+
+// The directory that contains the fixtures, such as NextJS pages,
+// NextJS config, and package.json
+const FIXTURE_PATH = join(__dirname, "fixtures")
+
+// Capture the output of `npm run build` to verify successful build
+let BUILD_OUTPUT
+
+beforeAll(
+  async () => {
+    // Clear project directory
+    emptyDirSync(PROJECT_PATH)
+    emptyDirSync(join(PROJECT_PATH, "pages"))
+
+    // Copy NextJS pages and config
+    copySync(
+      join(FIXTURE_PATH, "pages-with-static-props-index"),
+      join(PROJECT_PATH, "pages")
+    )
+    copySync(
+      join(FIXTURE_PATH, "next.config.js"),
+      join(PROJECT_PATH, "next.config.js")
+    )
+
+    // Copy package.json
+    copySync(
+      join(FIXTURE_PATH, "package.json"),
+      join(PROJECT_PATH, "package.json")
+    )
+
+    // Create a _redirects file
+    writeFileSync(
+      join(PROJECT_PATH, "_redirects"),
+      "# Custom Redirect Rules\n" +
+      "https://old.example.com/* https://new.example.com/:splat 301!\n"
+    )
+
+    // Invoke `npm run build`: Build Next and run next-on-netlify
+    const { stdout } = await npmRunBuild({ directory: PROJECT_PATH })
+    BUILD_OUTPUT = stdout
+  },
+  // time out after 180 seconds
+  180 * 1000
+)
+
+describe('Next', () => {
+  test('builds successfully', () => {
+    // NextJS output
+    expect(BUILD_OUTPUT).toMatch("Creating an optimized production build...")
+    expect(BUILD_OUTPUT).toMatch("Automatically optimizing pages...")
+    expect(BUILD_OUTPUT).toMatch("First Load JS shared by all")
+
+    // Next on Netlify output
+    expect(BUILD_OUTPUT).toMatch("Next on Netlify")
+    expect(BUILD_OUTPUT).toMatch("Success! All done!")
+  })
+})
+
+describe('Routing',() => {
+  test('includes custom redirect rules', async () => {
+    // Read _redirects file
+    const contents = readFileSync(join(PROJECT_PATH, "out_publish", "_redirects"))
+
+    const redirects = contents.toString().split(/\n/)
+    expect(redirects[0]).toEqual("# Custom Redirect Rules")
+    expect(redirects[1]).toEqual("https://old.example.com/* https://new.example.com/:splat 301!")
+
+    // Check that other routes are present
+    expect(redirects).toContain("/  /index.html  200")
+    expect(redirects).toContain("/index  /index.html  200")
+  })
+})