fix: cache_url caps the remote body at MAX_CACHE_URL_BYTES

T2-15: cache_url buffered the whole remote body with response.read(), no cap
(a 200 MB body sat in memory before discord rejected it). refuse a
Content-Length over the cap, and stream with iter_chunked, raising
DPYCacheError once the running total passes it. the cap (exported module
constant) is 500 MiB, discord's largest upload tier, so nothing discord
would accept is refused.

Signed-off-by: disqualifier <dev@disqualifier.me>

src/dpy_cache/__init__.py

@@ -6,6 +6,7 @@ from importlib.metadata import PackageNotFoundError, version
 
 from .dpy_cache import (
     MAX_ATTACHMENTS_PER_MESSAGE,
+    MAX_CACHE_URL_BYTES,
     CacheResult,
     DPYCache,
     DPYCacheError,
@@ -25,5 +26,6 @@ __all__ = [
     "CacheResult",
     "FileContent",
     "MAX_ATTACHMENTS_PER_MESSAGE",
+    "MAX_CACHE_URL_BYTES",
     "__version__",
 ]

src/dpy_cache/dpy_cache.py

@@ -41,10 +41,11 @@ error contract (fail-loud)
 --------------------------
 nothing is swallowed. lib-specific invariants raise :class:`DPYCacheError` (attachment-count
 mismatch after a send, a duplicate filename in a list input to ``cache()``, ``cache_url``
-non-200, a ``lookup`` jump-url pointing at a different channel than the injected one, a
+non-200 or a body over ``MAX_CACHE_URL_BYTES``, a ``lookup`` jump-url pointing at a
-``discord.File`` source that can't be safely rebuilt for a repeat send). unnamed ``bytes`` in
+different channel than the injected one, a ``discord.File`` source that can't be safely
-a list input raises ``ValueError``. raw discord errors (``Forbidden`` / ``HTTPException`` /
+rebuilt for a repeat send). unnamed ``bytes`` in a list input raises ``ValueError``. raw
-``NotFound``) propagate UNWRAPPED so callers can still branch on discord's types.
+discord errors (``Forbidden`` / ``HTTPException`` / ``NotFound``) propagate UNWRAPPED so
+callers can still branch on discord's types.
 """
 from __future__ import annotations
 
@@ -60,6 +61,9 @@ import discord
 log = logging.getLogger(__name__)
 
 MAX_ATTACHMENTS_PER_MESSAGE = 10
+# cache_url's body cap: discord's largest upload tier (500 MB, nitro), so nothing discord would
+# accept is refused; it bounds memory, it is not the bot's real (per-guild) upload limit
+MAX_CACHE_URL_BYTES = 500 * 1024 * 1024
 
 FileContent = Union[bytes, bytearray, str, "os.PathLike[str]", discord.File]
 
@@ -178,12 +182,21 @@ class DPYCache:
         content: "str | None" = None,
     ) -> FileRef:
         """fetch a remote url then cache its bytes as ``name``; raises DPYCacheError on
-        non-200"""
+        non-200, or when the body (declared or streamed) exceeds ``MAX_CACHE_URL_BYTES``"""
         async with aiohttp.ClientSession() as session:
             async with session.get(url) as response:
                 if response.status != 200:
                     raise DPYCacheError(f"cache_url: GET {url} returned status {response.status}")
-                data = await response.read()
+                if (response.content_length or 0) > MAX_CACHE_URL_BYTES:
+                    raise DPYCacheError(
+                        f"cache_url: GET {url} is {response.content_length} bytes, over the "
+                        f"{MAX_CACHE_URL_BYTES}-byte cap"
+                    )
+                data = bytearray()
+                async for chunk in response.content.iter_chunked(1 << 16):
+                    data += chunk
+                    if len(data) > MAX_CACHE_URL_BYTES:
+                        raise DPYCacheError(f"cache_url: GET {url} body exceeds the {MAX_CACHE_URL_BYTES}-byte cap")
         return await self.cache_one(name, data, content=content)
 
     async def resolve(self, ref: FileRef) -> str: