fix setting pipe buffer size

test.py

@@ -4,10 +4,14 @@
 import io
 import os
 import platform
+import sys
 import time
+from fcntl import fcntl
 from tempfile import TemporaryFile
 from unittest import mock
 
+import pytest
+
 import wurlitzer
 from wurlitzer import (
     PIPE,
@@ -158,3 +162,18 @@ def test_pipe_max_size():
         assert 65535 <= max_pipe_size <= 1024 * 1024
     else:
         assert max_pipe_size is None
+
+
+@pytest.mark.skipif(
+    wurlitzer._get_max_pipe_size() is None, reason="requires _get_max_pipe_size"
+)
+def test_bufsize():
+    default_bufsize = wurlitzer._get_max_pipe_size()
+    with wurlitzer.pipes() as (stdout, stderr):
+        assert fcntl(sys.__stdout__, wurlitzer.F_GETPIPE_SZ) == default_bufsize
+        assert fcntl(sys.__stderr__, wurlitzer.F_GETPIPE_SZ) == default_bufsize
+
+    bufsize = 32768  # seems to only accept powers of two?
+    with wurlitzer.pipes(bufsize=bufsize) as (stdout, stderr):
+        assert fcntl(sys.__stdout__, wurlitzer.F_GETPIPE_SZ) == bufsize
+        assert fcntl(sys.__stderr__, wurlitzer.F_GETPIPE_SZ) == bufsize

wurlitzer.py

@@ -32,8 +32,9 @@
 try:
     from fcntl import F_SETPIPE_SZ
 except ImportError:
-    # ref: linux fcntl.h
+    # ref: linux uapi/linux/fcntl.h
     F_SETPIPE_SZ = 1024 + 7
+    F_GETPIPE_SZ = 1024 + 8
 
 libc = ctypes.CDLL(None)
 
@@ -200,17 +201,16 @@ def _setup_pipe(self, name):
         self._save_fds[name] = save_fd
 
         pipe_out, pipe_in = os.pipe()
-        dup2(pipe_in, real_fd)
-        os.close(pipe_in)
-        self._real_fds[name] = real_fd
-
         # set max pipe buffer size (linux only)
         if self._bufsize:
             try:
                 fcntl(pipe_in, F_SETPIPE_SZ, self._bufsize)
             except OSError:
-                # ignore failure to set pipe size
-                pass
+                warnings.warn("Failed to set pipe buffer size", RuntimeWarning)
+
+        dup2(pipe_in, real_fd)
+        os.close(pipe_in)
+        self._real_fds[name] = real_fd
 
         # make pipe_out non-blocking
         flags = fcntl(pipe_out, F_GETFL)