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

kevinmehall · miss-islington · commit 8f93941d654f · 2022-03-20T14:26:50.000Z
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. (cherry picked from commit e730ae7) Co-authored-by: Kevin Mehall <km@kevinmehall.net>
diff --git a/Lib/zipfile.py b/Lib/zipfile.py
@@ -721,7 +721,9 @@ def __init__(self, file, pos, close, lock, writing):
         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:
diff --git a/Misc/NEWS.d/next/Library/2022-03-19-19-56-04.bpo-42369.Ok828t.rst b/Misc/NEWS.d/next/Library/2022-03-19-19-56-04.bpo-42369.Ok828t.rst
@@ -0,0 +1 @@
+Fix thread safety of :meth:`zipfile._SharedFile.tell` to avoid a "zipfile.BadZipFile: Bad CRC-32 for file" exception when reading a :class:`ZipFile` from multiple threads.

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1 @@`
	`1`	+Fix thread safety of :meth:`zipfile._SharedFile.tell` to avoid a "zipfile.BadZipFile: Bad CRC-32 for file" exception when reading a :class:`ZipFile` from multiple threads.