Issue #11459: A `bufsize` value of 0 in subprocess.Popen() really creates

unbuffered pipes, such that select() works properly on them.

Doc/library/platform.rst

@@ -205,7 +205,7 @@ Windows Platform
 Win95/98 specific
 ^^^^^^^^^^^^^^^^^
 
-.. function:: popen(cmd, mode='r', bufsize=None)
+.. function:: popen(cmd, mode='r', bufsize=-1)
 
    Portable :func:`popen` interface.  Find a working popen implementation
    preferring :func:`win32pipe.popen`.  On Windows NT, :func:`win32pipe.popen`

Lib/os.py

@@ -629,11 +629,13 @@ if not _exists("urandom"):
         return bs
 
 # Supply os.popen()
-def popen(cmd, mode="r", buffering=None):
+def popen(cmd, mode="r", buffering=-1):
     if not isinstance(cmd, str):
         raise TypeError("invalid cmd type (%s, expected string)" % type(cmd))
     if mode not in ("r", "w"):
         raise ValueError("invalid mode %r" % mode)
+    if buffering == 0 or buffering == None:
+        raise ValueError("popen() does not support unbuffered streams")
     import subprocess, io
     if mode == "r":
         proc = subprocess.Popen(cmd,

Lib/platform.py

@@ -398,7 +398,7 @@ class _popen:
     # Alias
     __del__ = close
 
-def popen(cmd, mode='r', bufsize=None):
+def popen(cmd, mode='r', bufsize=-1):
 
     """ Portable popen() interface.
     """

Lib/subprocess.py

@@ -653,8 +653,6 @@ class Popen(object):
             if errread is not None:
                 errread = msvcrt.open_osfhandle(errread.Detach(), 0)
 
-        if bufsize == 0:
-            bufsize = 1  # Nearly unbuffered (XXX for now)
         if p2cwrite is not None:
             self.stdin = io.open(p2cwrite, 'wb', bufsize)
             if self.universal_newlines:

Lib/test/test_subprocess.py

@@ -864,6 +864,21 @@ class ProcessTestCase(BaseTestCase):
             def prepare():
                 raise ValueError("surrogate:\uDCff")
 
+        def test_select_unbuffered(self):
+            # Issue #11459: bufsize=0 should really set the pipes as
+            # unbuffered (and therefore let select() work properly).
+            select = support.import_module("select")
+            p = subprocess.Popen([sys.executable, "-c",
+                                  'import sys;'
+                                  'sys.stdout.write("apple")'],
+                                 stdout=subprocess.PIPE,
+                                 bufsize=0)
+            f = p.stdout
+            try:
+                self.assertEqual(f.read(4), b"appl")
+                self.assertIn(f, select.select([f], [], [], 0.0)[0])
+            finally:
+                p.wait()
 
     #
     # Windows tests

Misc/NEWS

@@ -40,6 +40,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #11459: A ``bufsize`` value of 0 in subprocess.Popen() really creates
+  unbuffered pipes, such that select() works properly on them.
+
 - Issue #5421: Fix misleading error message when one of socket.sendto()'s
   arguments has the wrong type.  Patch by Nikita Vetoshkin.