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

[miss-islington]

https://github.com/python/cpython/commit/57100c86baa8451a568348646834380cd425b858
commit: 57100c86baa8451a568348646834380cd425b858
branch: 3.10
author: Miss Islington (bot) <31488909+miss-islington at users.noreply.github.com>
committer: miss-islington <31488909+miss-islington at users.noreply.github.com>
date: 2021-11-26T16:42:00-08:00
summary:

[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 7edb6270a78c695e4c2ae2432797dc18105374fc)


Co-authored-by: Ma Lin <animalize at users.noreply.github.com>

Automerge-Triggered-By: GH:gpshead

files:
A Misc/NEWS.d/next/Library/2021-11-16-18-13-49.bpo-41735.D72UY1.rst
M Modules/zlibmodule.c

diff --git a/Misc/NEWS.d/next/Library/2021-11-16-18-13-49.bpo-41735.D72UY1.rst b/Misc/NEWS.d/next/Library/2021-11-16-18-13-49.bpo-41735.D72UY1.rst
new file mode 100644
index 0000000000000..101da0e9ce648
--- /dev/null
+++ b/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``.
diff --git a/Modules/zlibmodule.c b/Modules/zlibmodule.c
index 68de701efa45f..a6940f2fd43a3 100644
--- a/Modules/zlibmodule.c
+++ b/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;