add etag to static responses

datasette/utils/asgi.py

@@ -1,3 +1,4 @@
+import hashlib
 import json
 from datasette.utils import MultiParams
 from mimetypes import guess_type
@@ -12,6 +13,8 @@
 # Thanks, Starlette:
 # https://github.com/encode/starlette/blob/519f575/starlette/responses.py#L17
 
+etag_cache = {}
+
 
 class Base400(Exception):
     status = 400
@@ -285,6 +288,7 @@ 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:
@@ -305,8 +309,27 @@ async def asgi_send_file(
             )
 
 
+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
+
+
 def asgi_static(root_path, chunk_size=4096, headers=None, content_type=None):
     root_path = Path(root_path)
+    headers = headers or {}
 
     async def inner_static(request, send):
         path = request.scope["url_route"]["kwargs"]["path"]
@@ -325,7 +348,15 @@ async def inner_static(request, send):
             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