Improve gzip decision logic.

Previous called get_data which is dangerous for large but
indeterminate response sizes, and the bail chain was more
difficult to reason about than a simple should_gzip true/false.

frontends/etiquette_flask/backend/common.py

@@ -80,29 +80,46 @@ def before_request():
     if site.localhost_only and ip != '127.0.0.1':
         flask.abort(403)
 
-gzip_minimum_size = 500
-gzip_maximum_size = 5 * 2**20
+gzip_minimum_size = 500 * bytestring.BYTE
+gzip_maximum_size = 5 * bytestring.MIBIBYTE
 gzip_level = 3
+def should_gzip(response):
+    if response.direct_passthrough:
+        return False
+
+    if response.status_code < 200:
+        return False
+
+    if response.status_code >= 300:
+        return False
+
+    content_length = response.headers.get('Content-Length', None)
+    if content_length is not None and int(content_length) > gzip_maximum_size:
+        return False
+
+    if content_length is not None and int(content_length) < gzip_minimum_size:
+        return False
+
+    if 'Content-Encoding' in response.headers:
+        return False
+
+    accept_encoding = request.headers.get('Accept-Encoding', '')
+    if 'gzip' not in accept_encoding.lower():
+        return False
+
+    content_type = response.headers.get('Content-Type', '')
+    if not (content_type.startswith('application/json') or content_type.startswith('text/')):
+        return False
+
+    return True
+
 @site.after_request
 def after_request(response):
-    '''
-    Thank you close.io.
-    https://github.com/closeio/Flask-gzip
-    '''
-    accept_encoding = request.headers.get('Accept-Encoding', '')
-
-    bail = False
-    bail = bail or response.status_code < 200
-    bail = bail or response.status_code >= 300
-    bail = bail or response.direct_passthrough
-    bail = bail or int(response.headers.get('Content-Length', 0)) > gzip_maximum_size
-    bail = bail or len(response.get_data()) < gzip_minimum_size
-    bail = bail or 'gzip' not in accept_encoding.lower()
-    bail = bail or 'Content-Encoding' in response.headers
-
-    if bail:
+    if not should_gzip(response):
         return response
 
+    ###
+
     gzip_buffer = io.BytesIO()
     gzip_file = gzip.GzipFile(mode='wb', compresslevel=gzip_level, fileobj=gzip_buffer)
     gzip_file.write(response.get_data())