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

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

4 files changed, 20 insertions, 4 deletions

diff --git a/Lib/os.py b/Lib/os.py
index 9720479066..28979bf8bd 100644
--- a/Lib/os.py
+++ b/Lib/os.py
@@ -779,11 +779,13 @@ 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,
diff --git a/Lib/platform.py b/Lib/platform.py
index fb65b4ba34..75bc5f3b97 100755
--- a/Lib/platform.py
+++ b/Lib/platform.py
@@ -357,7 +357,7 @@ def dist(distname='',version='',id='',
                               supported_dists=supported_dists,
                               full_distribution_name=0)
 
-def popen(cmd, mode='r', bufsize=None):
+def popen(cmd, mode='r', bufsize=-1):
 
     """ Portable popen() interface.
     """
diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index 4a8adcfa7d..383085ec72 100644
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -743,8 +743,6 @@ class Popen(object):
             if errread != -1:
                 errread = msvcrt.open_osfhandle(errread.Detach(), 0)
 
-        if bufsize == 0:
-            bufsize = 1  # Nearly unbuffered (XXX for now)
         if p2cwrite != -1:
             self.stdin = io.open(p2cwrite, 'wb', bufsize)
             if self.universal_newlines:
diff --git a/Lib/test/test_subprocess.py b/Lib/test/test_subprocess.py
index 2dde245817..6523c8a97e 100644
--- a/Lib/test/test_subprocess.py
+++ b/Lib/test/test_subprocess.py
@@ -1315,6 +1315,22 @@ class POSIXProcessTestCase(BaseTestCase):
                          " non-zero with this error:\n%s" %
                          stderr.decode('utf-8'))
 
+    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()
+
 
 @unittest.skipUnless(mswindows, "Windows specific tests")
 class Win32ProcessTestCase(BaseTestCase):