bpo-44687: Ensure BufferedReader objects with unread buffers can peek…

… even when the underlying file is closed (GH-28457)

(cherry picked from commit a450398)

Co-authored-by: AngstyDuck <solsticedante@gmail.com>

Misc/NEWS.d/next/C API/2021-09-19-17-18-25.bpo-44687.3fqDRC.rst

@@ -0,0 +1 @@
+:meth:`BufferedReader.peek` no longer raises :exc:`ValueError` when the entire file has already been buffered.

Modules/_io/bufferedio.c

@@ -341,11 +341,10 @@ _enter_buffered_busy(buffered *self)
      : buffered_closed(self)))
 
 #define CHECK_CLOSED(self, error_msg) \
-    if (IS_CLOSED(self)) { \
+    if (IS_CLOSED(self) & (Py_SAFE_DOWNCAST(READAHEAD(self), Py_off_t, Py_ssize_t) == 0)) { \
         PyErr_SetString(PyExc_ValueError, error_msg); \
         return NULL; \
-    }
-
+    } \
 
 #define VALID_READ_BUFFER(self) \
     (self->readable && self->read_end != -1)
@@ -530,6 +529,9 @@ buffered_close(buffered *self, PyObject *args)
         Py_CLEAR(res);
     }
 
+    self->read_end = 0;
+    self->pos = 0;
+
 end:
     LEAVE_BUFFERED(self)
     return res;