fix(devframe): let a devframe at base / fall through to the host app

docs/content/1.guide/1.tutorial-server-data-inspector.md

@@ -176,12 +176,14 @@ export function App() {
 To try what we've built, let Vite serve the UI and hand RPC traffic to devframe:
 
 ```ts [app/vite.config.ts]
+import { fileURLToPath } from 'node:url'
 import { devframeViteBridge } from '@devframes/vite/single'
 import react from '@vitejs/plugin-react'
 import { defineConfig } from 'vite'
 import dataInspectorFrame from '../src/node/data-inspector.ts'
 
 export default defineConfig({
+  root: fileURLToPath(new URL('.', import.meta.url)), // serve app/, wherever you run vite from
   base: './', // relative asset URLs, so the built UI works under any mount path
   build: { outDir: '../dist/client', emptyOutDir: true },
   plugins: [

packages/devframe/src/adapters/__tests__/initiate.test.ts

@@ -211,6 +211,48 @@ describe('adapters/handler', () => {
     }
   })
 
+  it('nodeMiddleware at base `/` hands unmatched paths back to the host', async () => {
+    const host = '127.0.0.1'
+    const port = await getPort({ port: 18135, host })
+
+    let devtoolsRef!: ReturnType<typeof initDevframe>
+    const server = createServer((req, res) => {
+      devtoolsRef.nodeMiddleware(req, res, () => {
+        res.statusCode = 200
+        res.setHeader('content-type', 'text/html')
+        res.end(`host app ${req.url}`)
+      })
+    })
+    // No distDir: the host app owns the pages, the devframe only its routes.
+    devtoolsRef = initDevframe(defineTestDef('handler-root'), { base: '/', auth: false, distDir: false, server })
+    await new Promise<void>(resolve => server.listen(port, host, resolve))
+
+    try {
+      await devtoolsRef.ready
+
+      // Every path sits inside base `/`, but the devframe has no route for
+      // these, so next() ran and the host app answered.
+      for (const path of ['/', '/main.tsx', '/nested/page?x=1']) {
+        const res = await fetch(`http://${host}:${port}${path}`)
+        expect(res.status).toBe(200)
+        expect(await res.text()).toBe(`host app ${path}`)
+      }
+
+      // The devframe's own routes still answer.
+      const meta = await (await fetch(`http://${host}:${port}/__connection.json`)).json()
+      expect(meta).toEqual({ backend: 'websocket', websocket: { path: '__ws' }, sse: { path: '__sse' } })
+
+      // Without a `next`, a miss is still the devframe's own 404.
+      const bare = await devtoolsRef.handler(new Request(`http://${host}:${port}/main.tsx`))
+      expect(bare.status).toBe(404)
+    }
+    finally {
+      await devtoolsRef.close()
+      server.close()
+      server.closeAllConnections()
+    }
+  })
+
   it('shared-server tier: a TLS host server advertises wss://', async () => {
     const host = '127.0.0.1'
     const server = createSecureServer({ allowHTTP1: true })

packages/devframe/src/adapters/initiate.ts

@@ -175,7 +175,9 @@ export interface DevframeInstance {
    * Connect/Express-style middleware over the same surface, for
    * `viteServer.middlewares.use(handler.nodeMiddleware)` or any other
    * node middleware stack. Mount it un-prefixed: paths outside the base
-   * call `next()` so the rest of the stack keeps working.
+   * call `next()` so the rest of the stack keeps working. At base `/`, a
+   * path no devframe route answers calls `next()` too, so the host app
+   * keeps its own pages.
    */
   nodeMiddleware: (req: IncomingMessage, res: ServerResponse, next?: (err?: unknown) => void) => void
   /**

packages/devframe/src/node/instance-shell.ts

@@ -17,7 +17,7 @@ import process from 'node:process'
 import { Server as TlsServer } from 'node:tls'
 import { validateOriginCandidate } from 'devframe/utils/origin'
 import { joinURL, withLeadingSlash, withoutLeadingSlash, withoutTrailingSlash } from 'devframe/utils/url'
-import { defineHandler, H3 as H3App, toNodeHandler } from 'h3'
+import { defineHandler, fromNodeHandler, H3 as H3App, toNodeHandler } from 'h3'
 import { DEVFRAME_SSE_ROUTE, DEVFRAME_WS_ROUTE } from '../constants'
 import { createInteractiveAuth } from '../recipes/interactive-auth'
 import { diagnostics } from './diagnostics'
@@ -625,6 +625,35 @@ export function createInstanceShell<TContext extends DevframeNodeContext>(
     maybeRegister()
   }
 
+  /**
+   * The host stack's `next` for each in-flight `nodeMiddleware` request at
+   * base `/`, read by the fall-through middleware mounted at the end of
+   * `init()`.
+   */
+  const hostFallthrough = new WeakMap<object, () => void>()
+
+  /**
+   * A devframe at `/` shares the origin root with its host, so it can't claim
+   * every path the way it owns a `/__<id>/` base. A `nodeMiddleware` request
+   * that no route answers goes to the host's `next` instead of h3's 404, so
+   * the host app (e.g. Vite with the bridge) keeps serving its own pages.
+   */
+  function mountHostFallthrough(): void {
+    if (base !== '/')
+      return
+    app.use((event, next) => {
+      if (event.context.matchedRoute)
+        return next()
+      const nodeReq = event.runtime?.node?.req
+      const hostNext = nodeReq && hostFallthrough.get(nodeReq)
+      if (!hostNext)
+        return next()
+      hostFallthrough.delete(nodeReq)
+      // Resolves once the host stack has sent its own response.
+      return fromNodeHandler((_req, _res, _next) => hostNext())(event)
+    })
+  }
+
   let started: StartedServer | undefined
   let transport: WsRpcTransport | undefined
   let dispose: (() => Promise<void>) | undefined
@@ -835,6 +864,9 @@ export function createInstanceShell<TContext extends DevframeNodeContext>(
 
     await options.mount?.(ctx, meta, api)
 
+    // Last in the chain, after every route the instance mounts.
+    mountHostFallthrough()
+
     // A pinned origin means the registry record needn't wait for a first
     // request.
     maybeRegister()
@@ -911,6 +943,8 @@ export function createInstanceShell<TContext extends DevframeNodeContext>(
           const { toNodeHandler } = await import('h3/node')
           nodeHandler = toNodeHandler(app)
         }
+        if (next && base === '/')
+          hostFallthrough.set(req, () => next())
         return nodeHandler(req, res)
       })
       .catch((err: unknown) => {

packages/vite/test/single.test.ts

@@ -196,6 +196,31 @@ describe('devframeViteBridge (bridge mode mcp)', () => {
       rpc.$close?.()
     }
   })
+
+  it('at base `/`, leaves the host app\'s own pages to Vite', async () => {
+    const host = '127.0.0.1'
+    const vitePort = await getPort({ port: 19717, host })
+    bridge = devframeViteBridge(defineTestDef(), { host, base: '/', auth: false })
+
+    vite = fakeViteServer()
+    await vite.listen(vitePort, host)
+    await bridge.configureServer(vite)
+    // Vite's own middlewares (transform, static, index.html) run after a
+    // plugin's `configureServer` middlewares.
+    vite.middlewares.use((req, res) => {
+      res.setHeader('content-type', 'text/html')
+      res.end(`vite ${req.url}`)
+    })
+
+    for (const path of ['/', '/main.tsx']) {
+      const res = await fetch(`http://${host}:${vitePort}${path}`)
+      expect(res.status).toBe(200)
+      expect(await res.text()).toBe(`vite ${path}`)
+    }
+
+    const meta = await (await fetch(`http://${host}:${vitePort}/__connection.json`)).json()
+    expect(meta.websocket).toEqual({ path: '__ws' })
+  })
 })
 
 describe('devframeViteBridge (auth default)', () => {