bpo-42369: Fix thread safety of zipfile._SharedFile.tell (GH-26974)

The `_SharedFile` tracks its own virtual position into the file as
`self._pos` and updates it after reading or seeking. `tell()` should
return this position instead of calling into the underlying file object,
since if multiple `_SharedFile` instances are being used concurrently on
the same file, another one may have moved the real file position.
Additionally, calling into the underlying `tell` may expose thread
safety issues in the underlying file object because it was called
without taking the lock.

1 files changed, 3 insertions, 1 deletions

diff --git a/Lib/zipfile.py b/Lib/zipfile.py
index 41bf49a..385adc8 100644
--- a/Lib/zipfile.py
+++ b/Lib/zipfile.py
@@ -747,7 +747,9 @@ class _SharedFile:
         self._lock = lock
         self._writing = writing
         self.seekable = file.seekable
-        self.tell = file.tell
+
+    def tell(self):
+        return self._pos
 
     def seek(self, offset, whence=0):
         with self._lock: