From 137791e9606b87ef0de31f5cdac0ef016d799e20 Mon Sep 17 00:00:00 2001
From: "Riccardo M. Cefala" <riccardo.cefala@mendix.com>
Date: Tue, 14 Jun 2016 17:58:11 +0200
Subject: [PATCH] add ECS describe_container_instances

---
 moto/ecs/models.py               | 30 ++++++++++++++++++++++--
 moto/ecs/responses.py            |  9 ++++++++
 tests/test_ecs/test_ecs_boto3.py | 39 ++++++++++++++++++++++++++++++++
 3 files changed, 76 insertions(+), 2 deletions(-)

diff --git a/moto/ecs/models.py b/moto/ecs/models.py
index 45ced304e..30d7c21e3 100644
--- a/moto/ecs/models.py
+++ b/moto/ecs/models.py
@@ -108,6 +108,19 @@ class ContainerInstance(BaseObject):
             return response_object
 
 
+class Failure(BaseObject):
+    def __init__(self, reason, arn):
+        self.reason = reason
+        self.arn = arn
+
+    @property
+    def response_object(self):
+        response_object = self.gen_response_object()
+        response_object['reason'] = self.reason
+        response_object['arn'] = self.arn
+        return response_object
+
+
 class EC2ContainerServiceBackend(BaseBackend):
     def __init__(self):
         self.clusters = {}
@@ -252,8 +265,21 @@ class EC2ContainerServiceBackend(BaseBackend):
         cluster_name = cluster_str.split('/')[-1]
         return sorted(self.container_instances[cluster_name].keys())
 
-    def describe_container_instances(self, cluster_str, list_container_instances_str):
-        pass
+    def describe_container_instances(self, cluster_str, list_container_instance_ids):
+        cluster_name = cluster_str.split('/')[-1]
+        if cluster_name not in self.clusters:
+            raise Exception("{0} is not a cluster".format(cluster_name))
+        failures = []
+        container_instance_objects = []
+        for container_instance_id in list_container_instance_ids:
+            container_instance_arn = 'arn:aws:ecs:us-east-1:012345678910:container-instance/{0}'.format(container_instance_id)
+            container_instance = self.container_instances[cluster_name].get(container_instance_arn, None)
+            if container_instance is not None:
+                container_instance_objects.append(container_instance)
+            else:
+                failures.append(Failure(reason='MISSING', arn=container_instance_arn))
+
+        return container_instance_objects, failures
 
     def deregister_container_instance(self, cluster_str, container_instance_str):
         pass
diff --git a/moto/ecs/responses.py b/moto/ecs/responses.py
index 0adf7813c..b9770a435 100644
--- a/moto/ecs/responses.py
+++ b/moto/ecs/responses.py
@@ -130,3 +130,12 @@ class EC2ContainerServiceResponse(BaseResponse):
         return json.dumps({
             'containerInstanceArns': container_instance_arns
         })
+
+    def describe_container_instances(self):
+        cluster_str = self._get_param('cluster')
+        list_container_instance_arns = self._get_param('containerInstances')
+        container_instances, failures = self.ecs_backend.describe_container_instances(cluster_str, list_container_instance_arns)
+        return json.dumps({
+                'failures': [ci.response_object for ci in failures],
+                'containerInstances': [ci.response_object for ci in container_instances]
+        })
diff --git a/tests/test_ecs/test_ecs_boto3.py b/tests/test_ecs/test_ecs_boto3.py
index d671fde7d..9ca367e0a 100644
--- a/tests/test_ecs/test_ecs_boto3.py
+++ b/tests/test_ecs/test_ecs_boto3.py
@@ -414,3 +414,42 @@ def test_list_container_instances():
     len(response['containerInstanceArns']).should.equal(instance_to_create)
     for arn in test_instance_arns:
         response['containerInstanceArns'].should.contain(arn)
+
+
+@mock_ec2
+@mock_ecs
+def test_describe_container_instances():
+    ecs_client = boto3.client('ecs', region_name='us-east-1')
+    ec2 = boto3.resource('ec2', region_name='us-east-1')
+
+    test_cluster_name = 'test_ecs_cluster'
+    _ = ecs_client.create_cluster(
+        clusterName=test_cluster_name
+    )
+
+    instance_to_create = 3
+    test_instance_arns = []
+    for i in range(0, instance_to_create):
+        test_instance = ec2.create_instances(
+            ImageId="ami-1234abcd",
+            MinCount=1,
+            MaxCount=1,
+        )[0]
+
+        instance_id_document = json.dumps(
+            ec2_utils.generate_instance_identity_document(test_instance)
+        )
+
+        response = ecs_client.register_container_instance(
+            cluster=test_cluster_name,
+            instanceIdentityDocument=instance_id_document)
+
+        test_instance_arns.append(response['containerInstance']['containerInstanceArn'])
+
+    test_instance_ids = map((lambda x: x.split('/')[1]), test_instance_arns)
+    response = ecs_client.describe_container_instances(cluster=test_cluster_name, containerInstances=test_instance_ids)
+    len(response['failures']).should.equal(0)
+    len(response['containerInstances']).should.equal(instance_to_create)
+    response_arns = [ci['containerInstanceArn'] for ci in response['containerInstances']]
+    for arn in test_instance_arns:
+        response_arns.should.contain(arn)