[3.6] bpo-32228: Reset raw_pos after unwinding the raw stream (GH-4858) (#5389)

Ensure that ``truncate()`` preserves the file position (as reported by ``tell()``) after writes longer than the buffer size..
(cherry picked from commit 059f58ce938d9c3f0286412a4efb1b9131339421)

1 files changed, 10 insertions, 4 deletions

diff --git a/Modules/_io/bufferedio.c b/Modules/_io/bufferedio.c
index efc7d05..3235581 100644
--- a/Modules/_io/bufferedio.c
+++ b/Modules/_io/bufferedio.c
@@ -1311,7 +1311,6 @@ _io__Buffered_seek_impl(buffered *self, PyObject *targetobj, int whence)
         if (res == NULL)
             goto end;
         Py_CLEAR(res);
-        _bufferedwriter_reset_buf(self);
     }
 
     /* TODO: align on block boundary and read buffer if needed? */
@@ -1878,8 +1877,6 @@ _bufferedwriter_raw_write(buffered *self, char *start, Py_ssize_t len)
     return n;
 }
 
-/* `restore_pos` is 1 if we need to restore the raw stream position at
-   the end, 0 otherwise. */
 static PyObject *
 _bufferedwriter_flush_unlocked(buffered *self)
 {
@@ -1920,9 +1917,18 @@ _bufferedwriter_flush_unlocked(buffered *self)
             goto error;
     }
 
-    _bufferedwriter_reset_buf(self);
 
 end:
+    /* This ensures that after return from this function,
+       VALID_WRITE_BUFFER(self) returns false.
+
+       This is a required condition because when a tell() is called
+       after flushing and if VALID_READ_BUFFER(self) is false, we need
+       VALID_WRITE_BUFFER(self) to be false to have
+       RAW_OFFSET(self) == 0.
+
+       Issue: https://bugs.python.org/issue32228 */
+    _bufferedwriter_reset_buf(self);
     Py_RETURN_NONE;
 
 error: