# HG changeset patch
# Parent ca54c27a9045c655ef1ad9ce248f02eb00137476

diff -r ca54c27a9045 Modules/_io/fileio.c
--- a/Modules/_io/fileio.c	Tue Aug 21 14:54:22 2012 +0100
+++ b/Modules/_io/fileio.c	Wed Aug 22 13:36:00 2012 +0100
@@ -604,22 +604,16 @@
 #ifdef HAVE_FSTAT
     struct stat st;
     Py_off_t pos, end;
-#endif
     PyObject *result;
     Py_ssize_t total = 0;
     int n;
-    size_t newsize;
+    size_t size;
 
     if (self->fd < 0)
         return err_closed();
     if (!_PyVerify_fd(self->fd))
         return PyErr_SetFromErrno(PyExc_IOError);
 
-    result = PyBytes_FromStringAndSize(NULL, SMALLCHUNK);
-    if (result == NULL)
-        return NULL;
-
-#ifdef HAVE_FSTAT
 #if defined(MS_WIN64) || defined(MS_WINDOWS)
     pos = _lseeki64(self->fd, 0L, SEEK_CUR);
 #else
@@ -629,36 +623,49 @@
         end = st.st_size;
     else
         end = (Py_off_t)-1;
-#endif
+    if (pos < 0 || end <= 0)
+#endif /* HAVE_FSTAT */
+    {
+        /* We can't calculate the buffer size needed, so fall back to
+           reading in chunks. */
+        _Py_IDENTIFIER(readall);
+        return _PyObject_CallMethodId((PyObject*)&PyRawIOBase_Type,
+                                      &PyId_readall, "O", self);
+    }
+
+#ifdef HAVE_FSTAT
+    size = new_buffersize(self, total, pos, end);
+    result = PyBytes_FromStringAndSize(NULL, size);
+    if (result == NULL)
+        return NULL;
+
     while (1) {
-#ifdef HAVE_FSTAT
-        newsize = new_buffersize(self, total, pos, end);
-#else
-        newsize = new_buffersize(self, total);
-#endif
-        if (newsize > PY_SSIZE_T_MAX || newsize <= 0) {
-            PyErr_SetString(PyExc_OverflowError,
-                "unbounded read returned more bytes "
-                "than a Python string can hold ");
-            Py_DECREF(result);
-            return NULL;
-        }
+        if (total >= (Py_ssize_t)size) {
+            size = new_buffersize(self, total, pos, end);
+            if (size > PY_SSIZE_T_MAX || size <= 0) {
+                PyErr_SetString(PyExc_OverflowError,
+                                "unbounded read returned more bytes "
+                                "than a Python string can hold ");
+                Py_DECREF(result);
+                return NULL;
+            }
 
-        if (PyBytes_GET_SIZE(result) < (Py_ssize_t)newsize) {
-            if (_PyBytes_Resize(&result, newsize) < 0) {
-                if (total == 0) {
-                    Py_DECREF(result);
-                    return NULL;
+            if (PyBytes_GET_SIZE(result) < (Py_ssize_t)size) {
+                if (_PyBytes_Resize(&result, size) < 0) {
+                    if (total == 0) {
+                        Py_DECREF(result);
+                        return NULL;
+                    }
+                    PyErr_Clear();
+                    break;
                 }
-                PyErr_Clear();
-                break;
             }
         }
         Py_BEGIN_ALLOW_THREADS
         errno = 0;
         n = read(self->fd,
                  PyBytes_AS_STRING(result) + total,
-                 newsize - total);
+                 size - total);
         Py_END_ALLOW_THREADS
         if (n == 0)
             break;
@@ -681,9 +688,7 @@
             return NULL;
         }
         total += n;
-#ifdef HAVE_FSTAT
         pos += n;
-#endif
     }
 
     if (PyBytes_GET_SIZE(result) > total) {
@@ -694,6 +699,7 @@
         }
     }
     return result;
+#endif /* HAVE_FSTAT */
 }
 
 static PyObject *