[4.6] capsys: ensure fd is unbuffered

Fixes pytest-dev#5134.

changelog/5134.bugfix.rst

@@ -0,0 +1 @@
+Fix buffering with capsys fixture on Python 2.

src/_pytest/capture.py

@@ -652,6 +652,9 @@ class SysCapture(object):
     def __init__(self, fd, tmpfile=None):
         name = patchsysdict[fd]
         self._old = getattr(sys, name)
+        if six.PY2:
+            # Ensure fd is unbuffered (#5134).
+            self._old = os.fdopen(self._old.fileno(), "wb+", 0)
         self.name = name
         if tmpfile is None:
             if name == "stdin":

testing/test_capture.py

@@ -1577,3 +1577,52 @@ def test_fails():
         )
     else:
         assert result_with_capture.ret == 0
+
+
+def test_syscapture_is_unbuffered_when_suspended(testdir, LineMatcher):
+    import time
+
+    stampfile = testdir.tmpdir.join("stampfile")
+    testdir.makepyfile(
+        **{
+            "conftest.py": """
+            import ctypes
+
+            libc = ctypes.CDLL(None)
+            libc.puts(b'this comes from C via conftest')
+        """,
+            "test_pass.py": """
+            import os
+            import time
+
+            def test_capfd(capfd):
+                print("test_capfd 1")
+                with capfd.disabled():
+                    for i in range(0, 5):
+                        if os.path.exists({stampfile!r}):
+                            break
+                        print("test_capfd 2: %d" % i)
+                        time.sleep(1)
+        """.format(
+                stampfile=str(stampfile)
+            ),
+        }
+    )
+
+    child = testdir.spawn_pytest("-s --color=no -vv")
+    start = time.time()
+    child.expect_exact("test_capfd 2: 0\r\n")
+    stampfile.ensure()
+    out = child.before + child.buffer + child.after
+    duration = time.time() - start
+    assert duration < 5
+
+    out += child.read()
+    lm = LineMatcher(out.decode().splitlines())
+    lm.fnmatch_lines(
+        [
+            "this comes from C via conftest",
+            "test_pass.py::test_capfd test_capfd 2: 0",
+            "*= 1 passed in *",
+        ]
+    )