gh-110184: Fix subprocess test_pipesize_default() (#110465)

For proc.stdin, get the size of the read end of the test pipe.

Use subprocess context manager ("with proc:").

Lib/test/test_subprocess.py

@@ -749,31 +749,36 @@ class ProcessTestCase(BaseTestCase):
     @unittest.skipUnless(fcntl and hasattr(fcntl, 'F_GETPIPE_SZ'),
                          'fcntl.F_GETPIPE_SZ required for test.')
     def test_pipesize_default(self):
-        p = subprocess.Popen(
+        proc = subprocess.Popen(
             [sys.executable, "-c",
              'import sys; sys.stdin.read(); sys.stdout.write("out"); '
              'sys.stderr.write("error!")'],
             stdin=subprocess.PIPE, stdout=subprocess.PIPE,
             stderr=subprocess.PIPE, pipesize=-1)
-        try:
-            fp_r, fp_w = os.pipe()
+
+        with proc:
             try:
-                default_pipesize = fcntl.fcntl(fp_w, fcntl.F_GETPIPE_SZ)
-                for fifo in [p.stdin, p.stdout, p.stderr]:
-                    self.assertEqual(
-                        fcntl.fcntl(fifo.fileno(), fcntl.F_GETPIPE_SZ),
-                        default_pipesize)
+                fp_r, fp_w = os.pipe()
+                try:
+                    default_read_pipesize = fcntl.fcntl(fp_r, fcntl.F_GETPIPE_SZ)
+                    default_write_pipesize = fcntl.fcntl(fp_w, fcntl.F_GETPIPE_SZ)
+                finally:
+                    os.close(fp_r)
+                    os.close(fp_w)
+
+                self.assertEqual(
+                    fcntl.fcntl(proc.stdin.fileno(), fcntl.F_GETPIPE_SZ),
+                    default_read_pipesize)
+                self.assertEqual(
+                    fcntl.fcntl(proc.stdout.fileno(), fcntl.F_GETPIPE_SZ),
+                    default_write_pipesize)
+                self.assertEqual(
+                    fcntl.fcntl(proc.stderr.fileno(), fcntl.F_GETPIPE_SZ),
+                    default_write_pipesize)
+                # On other platforms we cannot test the pipe size (yet). But above
+                # code using pipesize=-1 should not crash.
             finally:
-                os.close(fp_r)
-                os.close(fp_w)
-            # On other platforms we cannot test the pipe size (yet). But above
-            # code using pipesize=-1 should not crash.
-            p.stdin.close()
-            p.stdout.close()
-            p.stderr.close()
-        finally:
-            p.kill()
-            p.wait()
+                proc.kill()
 
     def test_env(self):
         newenv = os.environ.copy()