Fix unexpected 'decode' AttributeError when MySQLdb module is mapped by PyMySQL (#336)

* Skip if MySQLdb module is mapped by PyMySQL

In some use cases, `pymysql.install_as_MySQLdb()` will be used to initialize a 'fake' MySQLdb module,
which is actually mapped to pymysql module.
Then the sw_mysqlclient plugin will change the data type of `connection.db` from bytes into str, 
cause the `AttributeError: 'str' object has no attribute 'decode'` exception in sw_pymysql.py line 45, 
which regard it as bytes.

Author: ZEALi

CHANGELOG.md

@@ -13,6 +13,7 @@
   - Fix unexpected 'No active span' IllegalStateError (#311)
   - **Tentative**: Set upper bound <=5.9.5 for psutil package due to test failure. (#326)
   - Remove `DeprecationWarning` from `pkg_resources` by replace it with `importlib_metadata` (#329)
+  - Fix unexpected 'decode' AttributeError when MySQLdb module is mapped by PyMySQL (#336)
 
 ### 1.0.1
 

skywalking/plugins/sw_mysqlclient.py

@@ -32,6 +32,9 @@ def install():
     import wrapt
     import MySQLdb
 
+    if hasattr(MySQLdb, 'install_as_MySQLdb'):
+        raise ImportError('SKIP MySQLdb module mapped by PyMySQL', name='MySQLdb', path='')
+
     _connect = MySQLdb.connect
 
     def _sw_connect(*args, **kwargs):