Fix thread locks in zlib module may go wrong in rare case (#22130)

Setting `next_in` before acquiring the thread lock may mix up compress/decompress state in other threads.

Author: Ma Lin

Misc/NEWS.d/next/Library/2020-09-07-21-40-07.bpo-41735.NKqGKy.rst

@@ -0,0 +1 @@
+Fix thread locks in zlib module may go wrong in rare case. Patch by Ma Lin.

Modules/zlibmodule.c

@@ -667,11 +667,11 @@ zlib_Compress_compress_impl(compobject *self, Py_buffer *data)
     Py_ssize_t ibuflen, obuflen = DEF_BUF_SIZE;
     int err;
 
+    ENTER_ZLIB(self);
+
     self->zst.next_in = data->buf;
     ibuflen = data->len;
 
-    ENTER_ZLIB(self);
-
     do {
         arrange_input_buffer(&self->zst, &ibuflen);
 
@@ -785,15 +785,15 @@ zlib_Decompress_decompress_impl(compobject *self, Py_buffer *data,
     else
         hard_limit = max_length;
 
+    ENTER_ZLIB(self);
+
     self->zst.next_in = data->buf;
     ibuflen = data->len;
 
     /* limit amount of data allocated to max_length */
     if (max_length && obuflen > max_length)
         obuflen = max_length;
 
-    ENTER_ZLIB(self);
-
     do {
         arrange_input_buffer(&self->zst, &ibuflen);