tests: proof test-stdio.py against buffer fill-up

With the previous code, it could in theory happen that the pipe / PTY buffer of
the child stdout / stderr fills up and the process never finishes.

To prevent that, we read all of the stream before waiting for the end of the
process. To ensure that the stream reaches EOF when the child finishes, we must
close the parent "copy" of the child stdout / stderr.

tests/test-stdio.py

@@ -5,6 +5,7 b' Tests the buffering behavior of stdio st'
 from __future__ import absolute_import
 
 import contextlib
+import errno
 import os
 import subprocess
 import sys
@@ -62,6 +63,23 b' def _ptys():'
         yield rwpair
 
 
+def _readall(fd, buffer_size):
+    buf = []
+    while True:
+        try:
+            s = os.read(fd, buffer_size)
+        except OSError as e:
+            if e.errno == errno.EIO:
+                # If the child-facing PTY got closed, reading from the
+                # parent-facing PTY raises EIO.
+                break
+            raise
+        if not s:
+            break
+        buf.append(s)
+    return b''.join(buf)
+
+
 class TestStdio(unittest.TestCase):
     def _test(self, stream, rwpair_generator, expected_output, python_args=[]):
         assert stream in ('stdout', 'stderr')
@@ -76,9 +94,14 b' class TestStdio(unittest.TestCase):'
                 stdout=child_stream if stream == 'stdout' else None,
                 stderr=child_stream if stream == 'stderr' else None,
             )
+            try:
+                os.close(child_stream)
+                self.assertEqual(
+                    _readall(stream_receiver, 1024), expected_output
+                )
+            finally:
             retcode = proc.wait()
             self.assertEqual(retcode, 0)
-            self.assertEqual(os.read(stream_receiver, 1024), expected_output)
 
     def test_stdout_pipes(self):
         self._test('stdout', _pipes, FULLY_BUFFERED)