NIFI-13660 ProcessContext.yieldResources() can be called from Python code

Closes #9182

Signed-off-by: Marton Szasz <szaszm@apache.org>

nifi-docs/src/main/asciidoc/python-developer-guide.adoc

@@ -324,8 +324,10 @@ created in the Processor's Python code. `attributes` and `contents` are both opt
 the FlowFile will still have the usual `filename`, `path` and `uuid` attributes, but no additional ones.
 If `contents` is not provided, a FlowFile with no contents (only attributes) will be created.
 In case there is no useful information to return from the `create` method, `return None` can be used instead of returning an
-empty `FlowFileSourceResult`. When `create` returns with `None`, the processor does not produce any output.
-
+empty `FlowFileSourceResult`. When `create()` returns with `None`, the processor does not produce any output.
+When there is nothing to return, it might be useful to yield the processor's resources and not schedule the processor
+to run for the period of time defined by the processor's Yield Duration. This can be achieved by calling
+`context.yield_resources()` from the processor's `create` method right before returning `None`.
 
 
 [[property-descriptors]]

nifi-framework-bundle/nifi-framework-extensions/nifi-py4j-framework-bundle/nifi-python-extension-api/src/main/python/src/nifiapi/properties.py

@@ -346,6 +346,9 @@ class ProcessContext(PropertyContext):
     def getName(self):
         return self.name
 
+    def yield_resources(self):
+        JvmHolder.java_gateway.get_method(self.java_context, "yield")()
+
 
 class ValidationContext(PropertyContext):
 

nifi-framework-bundle/nifi-framework-extensions/nifi-py4j-framework-bundle/nifi-python-framework/src/main/python/framework/Controller.py

@@ -17,6 +17,7 @@ import logging
 import os
 import sys
 from concurrent.futures import ThreadPoolExecutor, ProcessPoolExecutor
+from py4j import java_gateway
 from py4j.java_gateway import JavaGateway, CallbackServerParameters, GatewayParameters
 
 import ExtensionManager
@@ -122,6 +123,7 @@ if __name__ == "__main__":
     from nifiapi.__jvm__ import JvmHolder
     JvmHolder.jvm = gateway.jvm
     JvmHolder.gateway = gateway
+    JvmHolder.java_gateway = java_gateway
 
     # We need to import PythonProcessorAdapter but cannot import it at the top of the class because we must first initialize the Gateway,
     # since there are statically defined objects in the file that contains PythonProcessorAdapter, and those statically defined objects require the Gateway.