Issue #21332: Ensure that ``bufsize=1`` in subprocess.Popen() selects line buffering, rather than block buffering.

2 files changed, 35 insertions, 1 deletions

diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index bb7d0dc..3ce0ed1 100644
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -841,7 +841,8 @@ class Popen(object):
         if p2cwrite != -1:
             self.stdin = io.open(p2cwrite, 'wb', bufsize)
             if universal_newlines:
-                self.stdin = io.TextIOWrapper(self.stdin, write_through=True)
+                self.stdin = io.TextIOWrapper(self.stdin, write_through=True,
+                                              line_buffering=(bufsize == 1))
         if c2pread != -1:
             self.stdout = io.open(c2pread, 'rb', bufsize)
             if universal_newlines:
diff --git a/Lib/test/test_subprocess.py b/Lib/test/test_subprocess.py
index d0ab718..9616da0 100644
--- a/Lib/test/test_subprocess.py
+++ b/Lib/test/test_subprocess.py
@@ -1008,6 +1008,39 @@ class ProcessTestCase(BaseTestCase):
         p = subprocess.Popen([sys.executable, "-c", "pass"], bufsize=None)
         self.assertEqual(p.wait(), 0)
 
+    def _test_bufsize_equal_one(self, line, expected, universal_newlines):
+        # subprocess may deadlock with bufsize=1, see issue #21332
+        with subprocess.Popen([sys.executable, "-c", "import sys;"
+                               "sys.stdout.write(sys.stdin.readline());"
+                               "sys.stdout.flush()"],
+                              stdin=subprocess.PIPE,
+                              stdout=subprocess.PIPE,
+                              stderr=subprocess.DEVNULL,
+                              bufsize=1,
+                              universal_newlines=universal_newlines) as p:
+            p.stdin.write(line) # expect that it flushes the line in text mode
+            os.close(p.stdin.fileno()) # close it without flushing the buffer
+            read_line = p.stdout.readline()
+            try:
+                p.stdin.close()
+            except OSError:
+                pass
+            p.stdin = None
+        self.assertEqual(p.returncode, 0)
+        self.assertEqual(read_line, expected)
+
+    def test_bufsize_equal_one_text_mode(self):
+        # line is flushed in text mode with bufsize=1.
+        # we should get the full line in return
+        line = "line\n"
+        self._test_bufsize_equal_one(line, line, universal_newlines=True)
+
+    def test_bufsize_equal_one_binary_mode(self):
+        # line is not flushed in binary mode with bufsize=1.
+        # we should get empty response
+        line = b'line' + os.linesep.encode() # assume ascii-based locale
+        self._test_bufsize_equal_one(line, b'', universal_newlines=False)
+
     def test_leaking_fds_on_error(self):
         # see bug #5179: Popen leaks file descriptors to PIPEs if
         # the child fails to execute; this will eventually exhaust