Write out manifest file so can serve functions from 'gatsby serve'

packages/gatsby/src/commands/serve.ts

@@ -7,6 +7,7 @@ import chalk from "chalk"
 import { match as reachMatch } from "@gatsbyjs/reach-router/lib/utils"
 import onExit from "signal-exit"
 import report from "gatsby-cli/lib/reporter"
+import multer from "multer"
 
 import telemetry from "gatsby-telemetry"
 
@@ -110,6 +111,55 @@ module.exports = async (program: IServeProgram): Promise<void> => {
   router.use(express.static(`public`, { dotfiles: `allow` }))
   const matchPaths = await readMatchPaths(program)
   router.use(matchPathRouter(matchPaths, { root }))
+
+  const compiledFunctionsDir = path.join(
+    program.directory,
+    `.cache`,
+    `functions`
+  )
+
+  const functionsManifest = JSON.parse(
+    fs.readFileSync(path.join(compiledFunctionsDir, `manifest.json`))
+  )
+
+  app.use(
+    `/api/*`,
+    multer().none(),
+    express.urlencoded({ extended: true }),
+    express.text(),
+    express.json(),
+    express.raw(),
+    async (req, res, next) => {
+      const { "0": functionName } = req.params
+
+      if (functionsManifest[functionName]) {
+        const start = Date.now()
+
+        try {
+          const pathToFunction = functionsManifest[functionName]
+          delete require.cache[require.resolve(pathToFunction)]
+          const fn = require(pathToFunction)
+
+          const fnToExecute = (fn && fn.default) || fn
+
+          await Promise.resolve(fnToExecute(req, res))
+        } catch (e) {
+          console.error(e)
+          res.sendStatus(500)
+        }
+
+        const end = Date.now()
+        console.log(
+          `Executed function "/api/${functionName}" in ${end - start}ms`
+        )
+
+        return
+      } else {
+        next()
+      }
+    }
+  )
+
   router.use((req, res, next) => {
     if (req.accepts(`html`)) {
       return res.status(404).sendFile(`404.html`, { root })

packages/gatsby/src/internal-plugins/functions/gatsby-node.ts

@@ -95,6 +95,24 @@ const createWebpackConfig = async ({
       "process.env": `({})`,
     }
   )
+  const compiledFunctionsDir = path.join(
+    siteDirectoryPath,
+    `.cache`,
+    `functions`
+  )
+
+  // Write out manifest for use by `gatsby serve` and plugins
+  const manifest = {}
+  Array.from(knownFunctions).forEach(([, file]) => {
+    const name = path.parse(file).name
+    const compiledPath = path.join(compiledFunctionsDir, name + `.js`)
+    manifest[name] = compiledPath
+  })
+
+  fs.writeFileSync(
+    path.join(compiledFunctionsDir, `manifest.json`),
+    JSON.stringify(manifest, null, 4)
+  )
 
   const entries = {}
   Array.from(knownFunctions).forEach(([, file]) => {
@@ -110,7 +128,7 @@ const createWebpackConfig = async ({
   const config = {
     entry: entries,
     output: {
-      path: path.join(siteDirectoryPath, `.cache`, `functions`),
+      path: compiledFunctionsDir,
       filename: `[name].js`,
       libraryTarget: `commonjs2`,
     },