Add ETag header for static responses (#2306)

* add etag to static responses

* fix RuntimeError related to static headers

* Remove unnecessary import

---------

Co-authored-by: Simon Willison <swillison@gmail.com>

datasette/utils/__init__.py

@@ -1,5 +1,6 @@
 import asyncio
 from contextlib import contextmanager
+import aiofiles
 import click
 from collections import OrderedDict, namedtuple, Counter
 import copy
@@ -1418,3 +1419,24 @@ def md5_not_usedforsecurity(s):
     except TypeError:
         # For Python 3.8 which does not support usedforsecurity=False
         return hashlib.md5(s.encode("utf8")).hexdigest()
+
+
+_etag_cache = {}
+
+
+async def calculate_etag(filepath, chunk_size=4096):
+    if filepath in _etag_cache:
+        return _etag_cache[filepath]
+
+    hasher = hashlib.md5()
+    async with aiofiles.open(filepath, "rb") as f:
+        while True:
+            chunk = await f.read(chunk_size)
+            if not chunk:
+                break
+            hasher.update(chunk)
+
+    etag = f'"{hasher.hexdigest()}"'
+    _etag_cache[filepath] = etag
+
+    return etag

datasette/utils/asgi.py

@@ -1,5 +1,6 @@
+import hashlib
 import json
-from datasette.utils import MultiParams
+from datasette.utils import MultiParams, calculate_etag
 from mimetypes import guess_type
 from urllib.parse import parse_qs, urlunparse, parse_qsl
 from pathlib import Path
@@ -285,6 +286,7 @@ async def asgi_send_file(
     headers = headers or {}
     if filename:
         headers["content-disposition"] = f'attachment; filename="{filename}"'
+
     first = True
     headers["content-length"] = str((await aiofiles.os.stat(str(filepath))).st_size)
     async with aiofiles.open(str(filepath), mode="rb") as fp:
@@ -307,9 +309,14 @@ async def asgi_send_file(
 
 def asgi_static(root_path, chunk_size=4096, headers=None, content_type=None):
     root_path = Path(root_path)
+    static_headers = {}
+
+    if headers:
+        static_headers = headers.copy()
 
     async def inner_static(request, send):
         path = request.scope["url_route"]["kwargs"]["path"]
+        headers = static_headers.copy()
         try:
             full_path = (root_path / path).resolve().absolute()
         except FileNotFoundError:
@@ -325,7 +332,15 @@ def asgi_static(root_path, chunk_size=4096, headers=None, content_type=None):
             await asgi_send_html(send, "404: Path not inside root path", 404)
             return
         try:
-            await asgi_send_file(send, full_path, chunk_size=chunk_size)
+            # Calculate ETag for filepath
+            etag = await calculate_etag(full_path, chunk_size=chunk_size)
+            headers["ETag"] = etag
+            if_none_match = request.headers.get("if-none-match")
+            if if_none_match and if_none_match == etag:
+                return await asgi_send(send, "", 304)
+            await asgi_send_file(
+                send, full_path, chunk_size=chunk_size, headers=headers
+            )
         except FileNotFoundError:
             await asgi_send_html(send, "404: File not found", 404)
             return

tests/test_html.py

@@ -78,6 +78,10 @@ async def test_static(ds_client):
     response = await ds_client.get("/-/static/app.css")
     assert response.status_code == 200
     assert "text/css" == response.headers["content-type"]
+    assert "etag" in response.headers
+    etag = response.headers.get("etag")
+    response = await ds_client.get("/-/static/app.css", headers={"if-none-match": etag})
+    assert response.status_code == 304
 
 
 def test_static_mounts():

tests/test_utils.py

@@ -706,3 +706,15 @@ def test_truncate_url(url, length, expected):
 def test_pairs_to_nested_config(pairs, expected):
     actual = utils.pairs_to_nested_config(pairs)
     assert actual == expected
+
+
+@pytest.mark.asyncio
+async def test_calculate_etag(tmp_path):
+    path = tmp_path / "test.txt"
+    path.write_text("hello")
+    etag = '"5d41402abc4b2a76b9719d911017c592"'
+    assert etag == await utils.calculate_etag(path)
+    assert utils._etag_cache[path] == etag
+    utils._etag_cache[path] = "hash"
+    assert "hash" == await utils.calculate_etag(path)
+    utils._etag_cache.clear()