From 517416c4d95eb31d40ac3fa81921ac6873f3a72b Mon Sep 17 00:00:00 2001
From: Simon-Pierre Gingras <simon.pierre.gingras@gmail.com>
Date: Fri, 19 May 2017 15:59:25 -0700
Subject: [PATCH] feat(s3) HeadObject: honor If-Modified-Since header

---
 moto/core/utils.py       |  8 +++++++-
 moto/s3/responses.py     | 12 +++++++++++-
 tests/test_s3/test_s3.py | 28 ++++++++++++++++++++++++++++
 3 files changed, 46 insertions(+), 2 deletions(-)

diff --git a/moto/core/utils.py b/moto/core/utils.py
index 7d4a9d412..9ee0c1814 100644
--- a/moto/core/utils.py
+++ b/moto/core/utils.py
@@ -174,11 +174,17 @@ def iso_8601_datetime_without_milliseconds(datetime):
     return datetime.strftime("%Y-%m-%dT%H:%M:%S") + 'Z'
 
 
+RFC1123 = '%a, %d %b %Y %H:%M:%S GMT'
+
+
 def rfc_1123_datetime(datetime):
-    RFC1123 = '%a, %d %b %Y %H:%M:%S GMT'
     return datetime.strftime(RFC1123)
 
 
+def str_to_rfc_1123_datetime(str):
+    return datetime.datetime.strptime(str, RFC1123)
+
+
 def unix_time(dt=None):
     dt = dt or datetime.datetime.utcnow()
     epoch = datetime.datetime.utcfromtimestamp(0)
diff --git a/moto/s3/responses.py b/moto/s3/responses.py
index fd33c5ead..43e27a815 100644
--- a/moto/s3/responses.py
+++ b/moto/s3/responses.py
@@ -3,6 +3,7 @@ from __future__ import unicode_literals
 import re
 
 import six
+from moto.core.utils import str_to_rfc_1123_datetime
 from six.moves.urllib.parse import parse_qs, urlparse
 
 import xmltodict
@@ -595,12 +596,21 @@ class ResponseObject(_TemplateEnvironmentMixin):
     def _key_response_head(self, bucket_name, query, key_name, headers):
         response_headers = {}
         version_id = query.get('versionId', [None])[0]
+
+        if_modified_since = headers.get('if-modified-since', None)
+        if if_modified_since:
+            if_modified_since = str_to_rfc_1123_datetime(if_modified_since)
+
         key = self.backend.get_key(
             bucket_name, key_name, version_id=version_id)
         if key:
             response_headers.update(key.metadata)
             response_headers.update(key.response_dict)
-            return 200, response_headers, ""
+
+            if if_modified_since and key.last_modified < if_modified_since:
+                return 304, response_headers, 'Not Modified'
+            else:
+                return 200, response_headers, ""
         else:
             return 404, response_headers, ""
 
diff --git a/tests/test_s3/test_s3.py b/tests/test_s3/test_s3.py
index de9c6a7de..6af653f9e 100644
--- a/tests/test_s3/test_s3.py
+++ b/tests/test_s3/test_s3.py
@@ -1,6 +1,8 @@
 # -*- coding: utf-8 -*-
 
 from __future__ import unicode_literals
+
+import datetime
 from six.moves.urllib.request import urlopen
 from six.moves.urllib.error import HTTPError
 from functools import wraps
@@ -10,6 +12,7 @@ import json
 import boto
 import boto3
 from botocore.client import ClientError
+import botocore.exceptions
 from boto.exception import S3CreateError, S3ResponseError
 from boto.s3.connection import S3Connection
 from boto.s3.key import Key
@@ -1266,6 +1269,31 @@ def test_boto3_head_object_with_versioning():
     old_head_object['ContentLength'].should.equal(len(old_content))
 
 
+@mock_s3
+def test_boto3_head_object_if_modified_since():
+    s3 = boto3.client('s3', region_name='us-east-1')
+    bucket_name = "blah"
+    s3.create_bucket(Bucket=bucket_name)
+
+    key = 'hello.txt'
+
+    with freeze_time(datetime.datetime.now() - datetime.timedelta(hours=3)):
+        s3.put_object(
+            Bucket=bucket_name,
+            Key=key,
+            Body='test'
+        )
+
+    with assert_raises(botocore.exceptions.ClientError) as err:
+        s3.head_object(
+            Bucket=bucket_name,
+            Key=key,
+            IfModifiedSince=datetime.datetime.now() - datetime.timedelta(hours=2)
+        )
+    e = err.exception
+    e.response['Error'].should.equal({'Code': '304', 'Message': 'Not Modified'})
+
+
 @mock_s3
 @reduced_min_part_size
 def test_boto3_multipart_etag():