don't explode if uwsgi isn't started in master mode (#446)

closes #446

Author: beniwohli

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Unreleased
 [Check the diff](https://github.com/elastic/apm-agent-python/compare/v4.2.1...master)
  * fixed an issue with Celery and the prefork worker pool (#444)
+ * fixed an issue when running uwsgi without a master process (#446)
 
 ## v4.2.1 
 [Check the diff](https://github.com/elastic/apm-agent-python/compare/v4.2.0...v4.2.1)

elasticapm/utils/compat.py

@@ -170,7 +170,17 @@ def multidict_to_dict(d):
 
 
 try:
-    from uwsgidecorators import postfork
+    import uwsgi
+
+    # check if a master is running before importing postfork
+    if uwsgi.masterpid() != 0:
+        from uwsgidecorators import postfork
+    else:
+
+        def postfork(f):
+            return f
+
+
 except ImportError:
 
     def postfork(f):