Reject trailing Brotli data to prevent infinite drain loops

HISTORY.rst

@@ -1,6 +1,12 @@
 Changelog
 =========
 
+Unreleased
+----------
+
+- Fixed infinite loops in streaming consumers by rejecting trailing data
+  after a complete Brotli stream.
+
 1.2.0.2 (2026-08-21)
 --------------------
 

src/brotlicffi/_api.py

@@ -492,6 +492,10 @@ def _decompress(self, data, output_buffer_limit):
                     b"Decompression error: %s" % ffi.string(error_message)
                 )
 
+            # Reject bytes remaining after the Brotli stream finishes.
+            if rc == lib.BROTLI_DECODER_RESULT_SUCCESS and available_in[0]:
+                raise error("Decompression error: trailing data after stream.")
+
             # Next, copy the result out.
             chunk = ffi.buffer(out_buffer, buffer_size - available_out[0])[:]
             chunks.append(chunk)

test/test_simple_decompression.py

@@ -108,6 +108,19 @@ def test_high_expansion_prefix_without_output_buffer_limit():
     assert result == uncompressed
 
 
+@pytest.mark.parametrize('output_buffer_limit', [None, 50])
+def test_decompressobj_rejects_trailing_data(output_buffer_limit):
+    o = brotlicffi.Decompressor()
+    data = brotlicffi.compress(b'A' * 100) + b'tail'
+    if output_buffer_limit is not None:
+        assert o.decompress(data, output_buffer_limit=50) == b'A' * 50
+        assert not o.can_accept_more_data()
+        data = b''
+
+    with pytest.raises(brotlicffi.error, match='trailing data'):
+        o.decompress(data, output_buffer_limit=output_buffer_limit)
+
+
 def test_drip_feed(simple_compressed_file):
     """
     Sending in the data one byte at a time still works.