bpo-32236: open() emits RuntimeWarning if buffering=1 for binary mode (GH-4842)

If buffering=1 is specified for open() in binary mode, it is silently
treated as buffering=-1 (i.e., the default buffer size).
Coupled with the fact that line buffering is always supported in Python 2,
such behavior caused several issues (e.g., bpo-10344, bpo-21332).

Warn that line buffering is not supported if open() is called with
binary mode and buffering=1.

1 files changed, 9 insertions, 0 deletions

diff --git a/Modules/_io/_iomodule.c b/Modules/_io/_iomodule.c
index eedca01..965c484 100644
--- a/Modules/_io/_iomodule.c
+++ b/Modules/_io/_iomodule.c
@@ -363,6 +363,15 @@ _io_open_impl(PyObject *module, PyObject *file, const char *mode,
         goto error;
     }
 
+    if (binary && buffering == 1) {
+        if (PyErr_WarnEx(PyExc_RuntimeWarning,
+                         "line buffering (buffering=1) isn't supported in "
+                         "binary mode, the default buffer size will be used",
+                         1) < 0) {
+           goto error;
+        }
+    }
+
     /* Create the Raw file stream */
     {
         PyObject *RawIO_class = (PyObject *)&PyFileIO_Type;