bpo-41586: Attempt to make the pipesize tests more robust. (GH-22839)

Several buildbots are failing on these, likely due to an inability to
set the pipe size to the desired test value.

Lib/test/test_fcntl.py

@@ -190,15 +190,22 @@ class TestFcntl(unittest.TestCase):
         res = fcntl.fcntl(self.f.fileno(), fcntl.F_GETPATH, bytes(len(expected)))
         self.assertEqual(expected, res)
 
-    @unittest.skipIf(not (hasattr(fcntl, "F_SETPIPE_SZ") and hasattr(fcntl, "F_GETPIPE_SZ")),
-                     "F_SETPIPE_SZ and F_GETPIPE_SZ are not available on all unix platforms.")
+    @unittest.skipUnless(
+        hasattr(fcntl, "F_SETPIPE_SZ") and hasattr(fcntl, "F_GETPIPE_SZ"),
+        "F_SETPIPE_SZ and F_GETPIPE_SZ are not available on all platforms.")
     def test_fcntl_f_pipesize(self):
         test_pipe_r, test_pipe_w = os.pipe()
+        try:
             # Get the default pipesize with F_GETPIPE_SZ
             pipesize_default = fcntl.fcntl(test_pipe_w, fcntl.F_GETPIPE_SZ)
-        # Multiply the default with 2 to get a new value.
-        fcntl.fcntl(test_pipe_w, fcntl.F_SETPIPE_SZ, pipesize_default * 2)
-        self.assertEqual(fcntl.fcntl(test_pipe_w, fcntl.F_GETPIPE_SZ), pipesize_default * 2)
+            pipesize = pipesize_default // 2  # A new value to detect change.
+            if pipesize < 512:  # the POSIX minimum
+                raise unittest.SkitTest(
+                    'default pipesize too small to perform test.')
+            fcntl.fcntl(test_pipe_w, fcntl.F_SETPIPE_SZ, pipesize)
+            self.assertEqual(fcntl.fcntl(test_pipe_w, fcntl.F_GETPIPE_SZ),
+                             pipesize)
+        finally:
             os.close(test_pipe_r)
             os.close(test_pipe_w)
 

Lib/test/test_subprocess.py

@@ -666,44 +666,65 @@ class ProcessTestCase(BaseTestCase):
         p.wait()
         self.assertEqual(p.stdin, None)
 
+    @unittest.skipUnless(fcntl and hasattr(fcntl, 'F_GETPIPE_SZ'),
+                         'fcntl.F_GETPIPE_SZ required for test.')
     def test_pipesizes(self):
-        # stdin redirection
-        pipesize = 16 * 1024
-        p = 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=pipesize)
-        # We only assert pipe size has changed on platforms that support it.
-        if sys.platform != "win32" and hasattr(fcntl, "F_GETPIPE_SZ"):
+        test_pipe_r, test_pipe_w = os.pipe()
+        try:
+            # Get the default pipesize with F_GETPIPE_SZ
+            pipesize_default = fcntl.fcntl(test_pipe_w, fcntl.F_GETPIPE_SZ)
+        finally:
+            os.close(test_pipe_r)
+            os.close(test_pipe_w)
+        pipesize = pipesize_default // 2
+        if pipesize < 512:  # the POSIX minimum
+            raise unittest.SkitTest(
+                'default pipesize too small to perform test.')
+        p = 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=pipesize)
+        try:
             for fifo in [p.stdin, p.stdout, p.stderr]:
-                self.assertEqual(fcntl.fcntl(fifo.fileno(), fcntl.F_GETPIPE_SZ), pipesize)
-        # Windows pipe size can be acquired with the GetNamedPipeInfoFunction
+                self.assertEqual(
+                    fcntl.fcntl(fifo.fileno(), fcntl.F_GETPIPE_SZ),
+                    pipesize)
+            # Windows pipe size can be acquired via GetNamedPipeInfoFunction
             # https://docs.microsoft.com/en-us/windows/win32/api/namedpipeapi/nf-namedpipeapi-getnamedpipeinfo
             # However, this function is not yet in _winapi.
             p.stdin.write(b"pear")
             p.stdin.close()
+        finally:
+            p.kill()
             p.wait()
 
+    @unittest.skipUnless(fcntl and hasattr(fcntl, 'F_GETPIPE_SZ'),
+                         'fcntl.F_GETPIPE_SZ required for test.')
     def test_pipesize_default(self):
-        p = 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)
-        # UNIX tests using fcntl
-        if sys.platform != "win32" and hasattr(fcntl, "F_GETPIPE_SZ"):
+        p = 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()
+            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)
-        # On other platforms we cannot test the pipe size (yet). But above code
-        # using pipesize=-1 should not crash.
+                        fcntl.fcntl(fifo.fileno(), fcntl.F_GETPIPE_SZ),
+                        default_pipesize)
+            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()
+        finally:
+            p.kill()
             p.wait()
 
     def test_env(self):