[3.10] bpo-41735: Fix thread lock in zlib.Decompress.flush() may go wrong (GH-29587) (GH-29811)

* Fix thread lock in zlib.Decompress.flush() may go wrong

Getting `.unconsumed_tail` before acquiring the thread lock may mix up decompress state.
(cherry picked from commit 7edb627)


Co-authored-by: Ma Lin <[email protected]>

Automerge-Triggered-By: GH:gpshead

Author: miss-islington

Misc/NEWS.d/next/Library/2021-11-16-18-13-49.bpo-41735.D72UY1.rst

@@ -0,0 +1 @@
+Fix thread lock in ``zlib.Decompress.flush()`` method before ``PyObject_GetBuffer``.

Modules/zlibmodule.c

@@ -1266,12 +1266,13 @@ zlib_Decompress_flush_impl(compobject *self, PyTypeObject *cls,
         return NULL;
     }
 
+    ENTER_ZLIB(self);
+
     if (PyObject_GetBuffer(self->unconsumed_tail, &data, PyBUF_SIMPLE) == -1) {
+        LEAVE_ZLIB(self);
         return NULL;
     }
 
-    ENTER_ZLIB(self);
-
     self->zst.next_in = data.buf;
     ibuflen = data.len;