FIX plugin sw_pymysql error when connection haven't db and monkey compatibility (#113)

Author: dafu-wu

skywalking/agent/__init__.py

@@ -73,6 +73,15 @@ def __fini():
 
 
 def start():
+    flag = False
+    try:
+        from gevent import monkey
+        flag = monkey.is_module_patched("socket")
+    except ModuleNotFoundError:
+        logger.debug("it was found that no gevent was used, if you don't use, please ignore.")
+    if flag:
+        import grpc.experimental.gevent as grpc_gevent
+        grpc_gevent.init_gevent()
     global __started
     if __started:
         raise RuntimeError('the agent can only be started once')

skywalking/plugins/sw_pymysql.py

@@ -36,7 +36,7 @@ def _sw_execute(this: Cursor, query, args=None):
             res = _execute(this, query, args)
 
             span.tag(Tag(key=tags.DbType, val="mysql"))
-            span.tag(Tag(key=tags.DbInstance, val=this.connection.db.decode("utf-8")))
+            span.tag(Tag(key=tags.DbInstance, val=(this.connection.db or b'').decode("utf-8")))
             span.tag(Tag(key=tags.DbStatement, val=query))
 
             if config.mysql_trace_sql_parameters and args: