fix(functions): fixes an issue where the functions could not be served (

#3863)

* fix(functions): fixes an issue where the functions could not be served

* chore: solve in a more elegant fashion 💅

* chore: tests were writing on the real fs avoid that

* chore: update test to only stub the mkdir function

Co-authored-by: Erez Rokah <erezrokah@users.noreply.github.com>
Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

src/lib/functions/registry.js

@@ -1,5 +1,6 @@
 // @ts-check
 const { mkdir } = require('fs').promises
+const { isAbsolute, join } = require('path')
 const { env } = require('process')
 
 const terminalLink = require('terminal-link')
@@ -146,7 +147,10 @@ class FunctionsRegistry {
     log(`${NETLIFYDEVLOG} ${chalk.green('Loaded')} function ${terminalLink(chalk.yellow(name), func.url)}.`)
   }
 
-  async scan(directories) {
+  async scan(relativeDirs) {
+    const directories = relativeDirs.filter(Boolean).map((dir) => (isAbsolute(dir) ? dir : join(this.projectRoot, dir)))
+
+    // check after filtering to filter out [undefined] for example
     if (directories.length === 0) {
       return
     }

src/lib/functions/server.test.js

@@ -1,3 +1,4 @@
+const fs = require('fs')
 const { platform } = require('os')
 const { join } = require('path')
 
@@ -8,12 +9,15 @@ const mockRequire = require('mock-require')
 const sinon = require('sinon')
 const request = require('supertest')
 
-const { FunctionsRegistry } = require('./registry')
-const { createHandler } = require('./server')
-
 const projectRoot = platform() === 'win32' ? 'C:\\my-functions' : `/my-functions`
 const functionsPath = `functions`
 
+// mock mkdir for the functions folder
+sinon.stub(fs.promises, 'mkdir').withArgs(join(projectRoot, functionsPath)).returns(Promise.resolve())
+
+const { FunctionsRegistry } = require('./registry')
+const { createHandler } = require('./server')
+
 /** @type { express.Express} */
 let app