From 6a467ec50f50a14ad67845b3d46eadab919c8719 Mon Sep 17 00:00:00 2001
From: Ciaran Evans <9111975+ciaranevans@users.noreply.github.com>
Date: Fri, 4 Sep 2020 10:22:21 +0100
Subject: [PATCH] Add validation on execution input

---
 moto/stepfunctions/exceptions.py              |  5 +++
 moto/stepfunctions/models.py                  | 11 +++++
 .../test_stepfunctions/test_stepfunctions.py  | 41 +++++++++++++++++++
 3 files changed, 57 insertions(+)

diff --git a/moto/stepfunctions/exceptions.py b/moto/stepfunctions/exceptions.py
index 6000bab4e..4abb6a8af 100644
--- a/moto/stepfunctions/exceptions.py
+++ b/moto/stepfunctions/exceptions.py
@@ -38,6 +38,11 @@ class InvalidName(AWSError):
     STATUS = 400
 
 
+class InvalidExecutionInput(AWSError):
+    TYPE = "InvalidExecutionInput"
+    STATUS = 400
+
+
 class StateMachineDoesNotExist(AWSError):
     TYPE = "StateMachineDoesNotExist"
     STATUS = 400
diff --git a/moto/stepfunctions/models.py b/moto/stepfunctions/models.py
index 19fb4561d..3184d6456 100644
--- a/moto/stepfunctions/models.py
+++ b/moto/stepfunctions/models.py
@@ -1,3 +1,4 @@
+import json
 import re
 from datetime import datetime
 
@@ -11,6 +12,7 @@ from .exceptions import (
     ExecutionAlreadyExists,
     ExecutionDoesNotExist,
     InvalidArn,
+    InvalidExecutionInput,
     InvalidName,
     StateMachineDoesNotExist,
 )
@@ -209,6 +211,7 @@ class StepFunctionBackend(BaseBackend):
     def start_execution(self, state_machine_arn, name=None, execution_input=None):
         state_machine_name = self.describe_state_machine(state_machine_arn).name
         self._ensure_execution_name_doesnt_exist(name)
+        self._validate_execution_input(execution_input)
         execution = Execution(
             region_name=self.region_name,
             account_id=self._get_account_id(),
@@ -290,6 +293,14 @@ class StepFunctionBackend(BaseBackend):
                     "Execution Already Exists: '" + execution.execution_arn + "'"
                 )
 
+    def _validate_execution_input(self, execution_input):
+        try:
+            json.loads(execution_input)
+        except Exception as ex:
+            raise InvalidExecutionInput(
+                "Invalid State Machine Execution Input: '" + str(ex) + "'"
+            )
+
     def _get_account_id(self):
         return ACCOUNT_ID
 
diff --git a/tests/test_stepfunctions/test_stepfunctions.py b/tests/test_stepfunctions/test_stepfunctions.py
index d94867719..36b08487c 100644
--- a/tests/test_stepfunctions/test_stepfunctions.py
+++ b/tests/test_stepfunctions/test_stepfunctions.py
@@ -425,6 +425,47 @@ def test_state_machine_start_execution_fails_on_duplicate_execution_name():
     )
 
 
+@mock_stepfunctions
+@mock_sts
+def test_state_machine_start_execution_with_custom_input():
+    client = boto3.client("stepfunctions", region_name=region)
+    #
+    sm = client.create_state_machine(
+        name="name", definition=str(simple_definition), roleArn=_get_default_role()
+    )
+    execution_input = json.dumps({"input_key": "input_value"})
+    execution = client.start_execution(
+        stateMachineArn=sm["stateMachineArn"], input=execution_input
+    )
+    #
+    execution["ResponseMetadata"]["HTTPStatusCode"].should.equal(200)
+    uuid_regex = "[a-f0-9]{8}-[a-f0-9]{4}-[a-f0-9]{4}-[a-f0-9]{4}-[a-f0-9]{12}"
+    expected_exec_name = (
+        "arn:aws:states:"
+        + region
+        + ":"
+        + _get_account_id()
+        + ":execution:name:"
+        + uuid_regex
+    )
+    execution["executionArn"].should.match(expected_exec_name)
+    execution["startDate"].should.be.a(datetime)
+
+
+@mock_stepfunctions
+@mock_sts
+def test_state_machine_start_execution_with_invalid_input():
+    client = boto3.client("stepfunctions", region_name=region)
+    #
+    sm = client.create_state_machine(
+        name="name", definition=str(simple_definition), roleArn=_get_default_role()
+    )
+    with assert_raises(ClientError):
+        _ = client.start_execution(stateMachineArn=sm["stateMachineArn"], input="")
+    with assert_raises(ClientError):
+        _ = client.start_execution(stateMachineArn=sm["stateMachineArn"], input="{")
+
+
 @mock_stepfunctions
 @mock_sts
 def test_state_machine_list_executions():