Fix multi-frame Zstandard response decoding

changelog/3008.bugfix.rst

@@ -0,0 +1 @@
+Fixed response decoding with Zstandard when compressed data is made of several frames.

src/urllib3/response.py

@@ -169,10 +169,15 @@ def __init__(self) -> None:
         def decompress(self, data: bytes) -> bytes:
             if not data:
                 return b""
-            return self._obj.decompress(data)  # type: ignore[no-any-return]
+            data_parts = [self._obj.decompress(data)]
+            while self._obj.eof and self._obj.unused_data:
+                unused_data = self._obj.unused_data
+                self._obj = zstd.ZstdDecompressor().decompressobj()
+                data_parts.append(self._obj.decompress(unused_data))
+            return b"".join(data_parts)
 
         def flush(self) -> bytes:
-            ret = self._obj.flush()
+            ret = self._obj.flush()  # note: this is a no-op
             if not self._obj.eof:
                 raise DecodeError("Zstandard data is incomplete")
             return ret  # type: ignore[no-any-return]

test/test_response.py

@@ -332,6 +332,25 @@ def test_decode_zstd(self) -> None:
         r = HTTPResponse(fp, headers={"content-encoding": "zstd"})
         assert r.data == b"foo"
 
+    @onlyZstd()
+    def test_decode_multiframe_zstd(self) -> None:
+        data = (
+            # Zstandard frame
+            zstd.compress(b"foo")
+            # skippable frame (must be ignored)
+            + bytes.fromhex(
+                "50 2A 4D 18"  # Magic_Number (little-endian)
+                "07 00 00 00"  # Frame_Size (little-endian)
+                "00 00 00 00 00 00 00"  # User_Data
+            )
+            # Zstandard frame
+            + zstd.compress(b"bar")
+        )
+
+        fp = BytesIO(data)
+        r = HTTPResponse(fp, headers={"content-encoding": "zstd"})
+        assert r.data == b"foobar"
+
     @onlyZstd()
     def test_chunked_decoding_zstd(self) -> None:
         data = zstd.compress(b"foobarbaz")