Report why the Node SSR server failed, and serve without SSR on the expected port

.changeset/loud-nodes-render.md

@@ -0,0 +1,6 @@
+---
+"@self/app": patch
+"gradio": patch
+---
+
+fix:Fix SSR mode failing on every render (`Cannot find module 'postcss'`) in installed builds, and serve without SSR on the expected port when the Node server can't start

gradio/blocks.py

@@ -71,6 +71,7 @@
     Error,
     InvalidApiNameError,
     InvalidComponentError,
+    ServerFailedToStartError,
     ShareCertificateWriteError,
 )
 from gradio.helpers import create_tracker, skip, special_args
@@ -2860,6 +2861,9 @@ def reverse(text):
                 resolved_num_workers = int(env_val)
 
         self._node_is_proxy = False
+        # Set when SSR was requested but Node couldn't serve, and we fell back to
+        # serving the app from Python without SSR.
+        self._ssr_degraded = False
         static_worker_ports: list[int] = []
         # Stashed kwargs for the deferred production Node proxy start.
         # When set, the user-facing Node front proxy is started after
@@ -3063,20 +3067,41 @@ def reverse(text):
                     self.local_api_url = f"{self.local_url.rstrip('/')}{API_PREFIX}/"
                     if self.mcp_server_obj:
                         self.mcp_server_obj._local_url = self.local_url
-                elif not quiet:
-                    warnings.warn(
-                        "Failed to start Node front proxy for SSR; Gradio is "
-                        f"reachable directly on the internal Python port "
-                        f":{self.server_port}. Check the Node installation "
-                        "or set GRADIO_NODE_PATH."
+                else:
+                    # Python is listening on an internal port that only Node was
+                    # ever going to route to, so with Node gone the app is
+                    # unreachable at the address the user (or the Space's health
+                    # check) is watching. Move Python onto the user-facing port and
+                    # serve without SSR: worse than SSR, far better than dead.
+                    self._ssr_degraded = self._serve_without_node_proxy(
+                        user_port=_pending_node_proxy_kwargs["server_port"],
+                        ssl_keyfile=ssl_keyfile,
+                        ssl_certfile=ssl_certfile,
+                        ssl_keyfile_password=ssl_keyfile_password,
                     )
+                    if not quiet:
+                        if self._ssr_degraded:
+                            warnings.warn(
+                                "Failed to start Node front proxy for SSR; serving "
+                                "without SSR on "
+                                f"{self.protocol}://{self.server_name}:{self.server_port}. "
+                                "See the Node server output above."
+                            )
+                        else:
+                            warnings.warn(
+                                "Failed to start Node front proxy for SSR, and the "
+                                f"user-facing port could not be taken over; Gradio is "
+                                f"reachable only on the internal Python port "
+                                f":{self.server_port}. See the Node server output "
+                                "above, or set ssr_mode=False."
+                            )
 
             if not self.is_colab and not quiet:
                 if self._node_is_proxy and self.node_port is not None:
                     print(
                         f"* Running on local URL:  {self.protocol}://{self.server_name}:{self.node_port}, with SSR ⚡ (Node proxy -> Python :{self.server_port})"
                     )
-                elif self.ssr_mode:
+                elif self.ssr_mode and not self._ssr_degraded:
                     print(
                         f"* Running on local URL:  {self.protocol}://{self.server_name}:{self.server_port}, with SSR ⚡ (dev mode)"
                     )
@@ -3351,6 +3376,60 @@ def integrate(
             data = {"integration": analytics_integration}
             analytics.integration_analytics(data)
 
+    def _serve_without_node_proxy(
+        self,
+        user_port: int,
+        ssl_keyfile: str | None = None,
+        ssl_certfile: str | None = None,
+        ssl_keyfile_password: str | None = None,
+    ) -> bool:
+        """Moves the running Python server from its internal port onto the
+        user-facing port, for when the Node front proxy failed to start.
+
+        In proxy mode Python deliberately binds an internal port and lets Node own
+        the user-facing one. If Node never comes up, that leaves the app answering
+        on a port nobody is asking about — on Spaces the container is judged by the
+        user-facing port, so the app is reported as crashed even though Python is
+        healthy. Rebinding there serves the app client-side rendered instead.
+
+        Returns whether the move happened; the existing server keeps running if the
+        user-facing port could not be bound.
+        """
+        from gradio import http_server
+
+        old_server = self.server
+        try:
+            server_name, server_port, local_url, server = http_server.start_server(
+                app=self.app,
+                server_name=self.server_name,
+                server_port=user_port,
+                ssl_keyfile=ssl_keyfile,
+                ssl_certfile=ssl_certfile,
+                ssl_keyfile_password=ssl_keyfile_password,
+            )
+        except (OSError, ServerFailedToStartError):
+            return False
+
+        # Only let go of the internal port once the new one is actually serving, so
+        # a failure here leaves the app reachable where it already was.
+        if old_server is not None:
+            try:
+                old_server.close()
+            except Exception:
+                pass
+
+        self.server_name = server_name
+        self.server_port = server_port
+        self.local_url = local_url
+        self.local_api_url = f"{local_url.rstrip('/')}{API_PREFIX}/"
+        self.server = server
+        self.protocol = (
+            "https" if local_url.startswith("https") or self.is_colab else "http"
+        )
+        if self.mcp_server_obj:
+            self.mcp_server_obj._local_url = local_url
+        return True
+
     def close(self, verbose: bool = True) -> None:
         """
         Closes the Interface that was launched and frees the port.

gradio/node_server.py

@@ -5,8 +5,10 @@
 import socket
 import subprocess
 import sys
+import threading
 import time
 import warnings
+from collections import deque
 from concurrent.futures import TimeoutError
 from contextlib import closing
 from http.client import HTTPConnection
@@ -98,6 +100,10 @@ def start_node_process(
         return None, None
 
     node_process = None
+    # Node's own output is hidden unless debug=True, which means a server that starts
+    # but fails to render leaves no trace. Keep the last of it so a failed startup can
+    # say what actually went wrong instead of guessing at the Node version.
+    node_errors: str = ""
 
     for port in server_ports:
         try:
@@ -139,10 +145,23 @@ def start_node_process(
             node_process = subprocess.Popen(
                 [node_path, "--import", register_file, SSR_APP_PATH],
                 env=env,
-                stdout=subprocess.DEVNULL if not debug else None,
-                stderr=subprocess.DEVNULL if not debug else None,
+                stdout=None if debug else subprocess.DEVNULL,
+                stderr=None if debug else subprocess.PIPE,
             )
 
+            # The pipe has to be drained for the lifetime of the process or Node
+            # blocks once the buffer fills, so a thread keeps reading it and holds
+            # on to only the most recent lines.
+            recent_errors: deque[str] = deque(maxlen=50)
+            stderr_thread = None
+            if node_process.stderr is not None:
+                stderr_thread = threading.Thread(
+                    target=drain_stderr,
+                    args=(node_process.stderr, recent_errors),
+                    daemon=True,
+                )
+                stderr_thread.start()
+
             # Node starts only after the Python backend is already
             # listening (Blocks.launch defers the front-proxy start), so we
             # can verify that Node actually renders a page rather than merely
@@ -162,6 +181,11 @@ def start_node_process(
                 # If verification failed, terminate the process and try the next port
                 node_process.terminate()
                 node_process.wait(timeout=2)
+                # Let the reader finish the lines already in the pipe before we
+                # look at them, otherwise the diagnosis can race the output.
+                if stderr_thread is not None:
+                    stderr_thread.join(timeout=2)
+                node_errors = "".join(recent_errors).strip() or node_errors
                 node_process = None
 
         except OSError:
@@ -179,16 +203,37 @@ def start_node_process(
     print(
         f"Cannot start Node server on any port in the range {server_ports[0]}-{server_ports[-1]}."
     )
-    print(
-        "Please install Node 20 or higher and set the environment variable GRADIO_NODE_PATH to the path of your Node executable."
-    )
+    if node_errors:
+        # Node ran and told us why it couldn't serve requests, so pass that along
+        # rather than pointing at the Node installation, which is evidently fine.
+        # The same failure usually repeats once per probe, so one tail is enough.
+        print("The Node server reported:")
+        print("\n".join(node_errors.splitlines()[-20:]))
+    else:
+        print(
+            "Please install Node 20 or higher and set the environment variable GRADIO_NODE_PATH to the path of your Node executable."
+        )
     print(
         "You can explicitly specify a port by setting the environment variable GRADIO_NODE_PORT."
     )
 
     return None, None
 
 
+def drain_stderr(stream, buffer: deque[str]) -> None:
+    """Reads the Node server's stderr, keeping only the most recent lines."""
+    try:
+        for line in iter(stream.readline, b""):
+            buffer.append(line.decode("utf-8", errors="replace"))
+    except Exception:
+        pass
+    finally:
+        try:
+            stream.close()
+        except Exception:
+            pass
+
+
 def attempt_connection(host: str, port: int) -> bool:
     """Attempts a single connection to the server."""
     try:

js/app/package.json

@@ -4,7 +4,7 @@
 	"private": true,
 	"scripts": {
 		"dev": "vite dev",
-		"build": "vite build && node after_build.js && node cleanup_build.js",
+		"build": "vite build && node after_build.js && node verify_build.js && node cleanup_build.js",
 		"preview": "vite preview",
 		"check": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json",
 		"check:watch": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json --watch",