[Server] Fix stdio server ending by itself on an idle Windows pipe

CHANGELOG.md

@@ -30,6 +30,8 @@ All notable changes to `mcp/sdk` will be documented in this file.
 * [BC Break] Answer `server/discover` with `DiscoverHandler`, which a handler added through `Builder::addRequestHandler()` can replace, and dispatch `RequestEvent`, `ResponseEvent` and `ErrorEvent` for it. A `StatelessProtocol` built without the `Builder` needs `DiscoverHandler` and a `MessageFactory` with `DiscoverRequest` to answer it.
 * Add a `binder` argument to `Builder::setRequestState()` binding the modern era's `requestState` to the request it was minted for, as the 2026-07-28 specification recommends: the binder's value is covered by the state's MAC, and a state echoed on a request that binds to another value is refused like a forged one. `OriginatingRequestBinder` binds the authenticated principal, the method, the tool, prompt or resource, and a digest of the arguments; a token without a subject fails closed, and a `subject` resolver identifies users by another claim. `RequestStateBinderInterface` takes custom bindings. Without a binder, nothing changes.
 * Rotate the `requestState` signing key without failing the calls in flight: `Builder::setRequestState()` and `RequestStateCodec` take a list of keys, the first signing and every key verifying. The wire format is unchanged, and a single key works as before.
+* Fix a stdio server on Windows ending by itself after about 13.5 hours without input, when `fgets()` on the idle pipe reported a false EOF: `StdioTransport` now reads only once `stream_select()` reports input, which also keeps the run loop going while idle. Requires PHP 8.5, as `stream_select()` on Windows reports an idle pipe as readable before that.
+* Fix `StdioTransport` throwing a `ValueError` on an input that cannot be selected, like `php://memory`.
 
 ### Client
 

src/Server/Transport/StdioTransport.php

@@ -162,10 +162,13 @@ public function listen(): int
 
     protected function processInput(): void
     {
+        // fgets() on an idle Windows pipe blocks even in non-blocking mode, then reports a false EOF.
+        if (!$this->waitForInput()) {
+            return;
+        }
+
         $line = fgets($this->input, $this->maxLineBytes);
         if (false === $line) {
-            $this->waitForInput();
-
             return;
         }
 
@@ -202,14 +205,16 @@ protected function processInput(): void
      * Idles until input arrives, for at most 50ms, so the run loop still gets
      * to suspended fibers, open streams and queued messages in between.
      */
-    private function waitForInput(): void
+    private function waitForInput(): bool
     {
         $read = [$this->input];
         $write = $except = null;
 
-        // Streams that cannot be selected, like php://memory, fall back to sleeping.
-        if (false === @stream_select($read, $write, $except, 0, 50000)) {
-            usleep(50000);
+        // A stream that cannot be selected, like php://memory, warns and throws, so it is just read.
+        try {
+            return 0 !== @stream_select($read, $write, $except, 0, 50000);
+        } catch (\ValueError) {
+            return true;
         }
     }
 

tests/Unit/Server/Transport/StdioTransportTest.php

@@ -89,6 +89,51 @@ public function testIdleInputWakesOnArrival(): void
         $this->assertLessThan(45, ($arrivedAt - $answeredAt) / 1e6, 'the idle wait must end when input arrives');
     }
 
+    #[TestDox('an idle pipe does not hold up the run loop')]
+    public function testIdlePipeDoesNotHoldUpTheRunLoop(): void
+    {
+        if (!\function_exists('proc_open')) {
+            $this->markTestSkipped('proc_open() is needed to feed the transport from a real pipe.');
+        }
+
+        if ('Windows' === \PHP_OS_FAMILY && \PHP_VERSION_ID < 80500) {
+            $this->markTestSkipped('Before PHP 8.5, stream_select() on Windows reports an idle pipe as readable.');
+        }
+
+        $process = proc_open([\PHP_BINARY, '-r', 'sleep(3);'], [1 => ['pipe', 'w']], $pipes);
+        $this->assertIsResource($process);
+        stream_set_blocking($pipes[1], false);
+
+        $transport = new StdioTransport(input: $pipes[1], output: $this->stream(''));
+        $start = hrtime(true);
+        (new \ReflectionMethod($transport, 'processInput'))->invoke($transport);
+        $elapsed = (hrtime(true) - $start) / 1e6;
+
+        proc_terminate($process);
+        proc_close($process);
+
+        $this->assertLessThan(1000, $elapsed, 'a tick must not wait for input longer than the idle wait');
+    }
+
+    #[TestDox('an input that cannot be selected is still read')]
+    public function testUnselectableInputIsRead(): void
+    {
+        $input = fopen('php://memory', 'r+');
+        $this->assertNotFalse($input);
+        fwrite($input, '{"valid":1}'."\n");
+        rewind($input);
+
+        $messages = [];
+        $transport = new StdioTransport(input: $input, output: $this->stream(''));
+        $transport->onMessage(static function ($transport, string $payload) use (&$messages): void {
+            $messages[] = $payload;
+        });
+
+        $this->pumpToEof($transport);
+
+        $this->assertSame(['{"valid":1}'], $messages);
+    }
+
     #[TestDox('the line byte cap must be a positive number of bytes')]
     public function testRejectsNonPositiveCap(): void
     {