A nicer interface.

src/allmydata/storage/http_client.py

@@ -4,7 +4,7 @@ HTTP client that talks to the HTTP storage server.
 
 from __future__ import annotations
 
-from typing import Union, Optional, Sequence, Mapping
+from typing import Union, Optional, Sequence, Mapping, BinaryIO
 from base64 import b64encode
 from io import BytesIO
 
@@ -131,25 +131,35 @@ class _LengthLimitedCollector:
         self.f.write(data)
 
 
-def limited_content(response, max_length: int = 30 * 1024 * 1024) -> Deferred:
+def limited_content(response, max_length: int = 30 * 1024 * 1024) -> Deferred[BinaryIO]:
     """
     Like ``treq.content()``, but limit data read from the response to a set
     length.  If the response is longer than the max allowed length, the result
     fails with a ``ValueError``.
+
+    A potentially useful future improvement would be using a temporary file to
+    store the content; since filesystem buffering means that would use memory
+    for small responses and disk for large responses.
     """
     collector = _LengthLimitedCollector(max_length)
     # Make really sure everything gets called in Deferred context, treq might
     # call collector directly...
     d = succeed(None)
     d.addCallback(lambda _: treq.collect(response, collector))
-    d.addCallback(lambda _: collector.f.getvalue())
+
+    def done(_):
+        collector.f.seek(0)
+        return collector.f
+
+    d.addCallback(done)
     return d
 
 
 def _decode_cbor(response, schema: Schema):
     """Given HTTP response, return decoded CBOR body."""
 
-    def got_content(data):
+    def got_content(f: BinaryIO):
+        data = f.read()
         schema.validate_cbor(data)
         return loads(data)
 

src/allmydata/test/test_storage_http.py

@@ -346,7 +346,7 @@ class CustomHTTPServerTests(SyncTestCase):
             )
 
             self.assertEqual(
-                result_of(limited_content(response, at_least_length)),
+                result_of(limited_content(response, at_least_length)).read(),
                 gen_bytes(length),
             )