gh-90522: add test

Subprocess opening /dev/stdin fails if it is a socketpair.

Author: tifv

Lib/test/test_asyncio/test_subprocess.py

@@ -401,6 +401,26 @@ async def empty_error():
         self.assertEqual(output, None)
         self.assertEqual(exitcode, 0)
 
+    @unittest.skipIf(sys.platform != 'linux', "Don't have /dev/stdin")
+    def test_devstdin_input(self):
+
+        async def devstdin_input(message):
+            code = 'file = open("/dev/stdin"); data = file.read(); print(len(data))'
+            proc = await asyncio.create_subprocess_exec(
+                sys.executable, '-c', code,
+                stdin=asyncio.subprocess.PIPE,
+                stdout=asyncio.subprocess.PIPE,
+                stderr=asyncio.subprocess.PIPE,
+                close_fds=False,
+            )
+            stdout, stderr = await proc.communicate(message)
+            exitcode = await proc.wait()
+            return (stdout, exitcode)
+
+        output, exitcode = self.loop.run_until_complete(devstdin_input(b'abc'))
+        self.assertEqual(output.rstrip(), b'3')
+        self.assertEqual(exitcode, 0)
+
     def test_cancel_process_wait(self):
         # Issue #23140: cancel Process.wait()