More accurate cursor unwrapping

If the cursor()/chunked_cursor() methods of a connection had already
been monkey patched by some other code before wrap_cursor() was called,
unwrap_cursor() would undo the previous monkey patch as well as the one
performed by wrap_cursor().

This can occur when testing if multiple databases are defined but only
some of them are allowed to be used by the tests. [1]  Django's
SimpleTestCase wraps the connections for any disallowed databases to
raise an exception if they are accessed. [2]  Without this commit,
unwrap_cursor() was undoing Django's monkey patch, resulting in an
exception when Django tried to undo its monkey patch which was no longer
there.

Update unwrap_cursor() to preserve a previous monkey patch if present.

[1] https://docs.djangoproject.com/en/stable/topics/testing/tools/#django.test.SimpleTestCase.databases
[2] https://github.com/django/django/blob/ce586ed6931092d3a5f06df9031cdeb891793ddb/django/test/testcases.py#L350

Author: living180

debug_toolbar/panels/sql/tracking.py

@@ -60,10 +60,22 @@ def chunked_cursor(*args, **kwargs):
 
 def unwrap_cursor(connection):
     if hasattr(connection, "_djdt_cursor"):
+        # Sometimes the cursor()/chunked_cursor() methods of the DatabaseWrapper
+        # instance are already monkey patched before wrap_cursor() is called.  (In
+        # particular, Django's SimpleTestCase monkey patches those methods for any
+        # disallowed databases to raise an exception if they are accessed.)  Thus only
+        # delete our monkey patch if the method we saved is the same as the class
+        # method.  Otherwise, restore the prior monkey patch from our saved method.
+        if connection._djdt_cursor == connection.__class__.cursor:
+            del connection.cursor
+        else:
+            connection.cursor = connection._djdt_cursor
         del connection._djdt_cursor
+        if connection._djdt_chunked_cursor == connection.__class__.chunked_cursor:
+            del connection.chunked_cursor
+        else:
+            connection.chunked_cursor = connection._djdt_chunked_cursor
         del connection._djdt_chunked_cursor
-        del connection.cursor
-        del connection.chunked_cursor
 
 
 class BaseCursorWrapper: