Reject infinity in decode_double to match encode_double

encode_double already rejects both NaN and infinity, but decode_double
only rejected NaN. A malicious or buggy server could send infinity bytes
that would be silently accepted. This adds the matching isinf() check.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antoineleclair

src/dqlitewire/types.py

@@ -74,6 +74,8 @@ def decode_double(data: bytes) -> float:
     result: float = struct.unpack("<d", data[:8])[0]
     if math.isnan(result):
         raise DecodeError("Received NaN double value, not supported by dqlite wire protocol")
+    if math.isinf(result):
+        raise DecodeError("Received infinity double value, not supported by dqlite wire protocol")
     return result
 
 

tests/test_types.py

@@ -124,6 +124,22 @@ def test_decode_nan_rejected(self) -> None:
         with pytest.raises(DecodeError, match="NaN"):
             decode_double(nan_bytes)
 
+    def test_decode_positive_infinity_rejected(self) -> None:
+        """Decoding +inf should raise DecodeError, matching encode_double's rejection."""
+        import struct
+
+        inf_bytes = struct.pack("<d", float("inf"))
+        with pytest.raises(DecodeError, match="[Ii]nfinity"):
+            decode_double(inf_bytes)
+
+    def test_decode_negative_infinity_rejected(self) -> None:
+        """Decoding -inf should raise DecodeError, matching encode_double's rejection."""
+        import struct
+
+        inf_bytes = struct.pack("<d", float("-inf"))
+        with pytest.raises(DecodeError, match="[Ii]nfinity"):
+            decode_double(inf_bytes)
+
 
 class TestPadding:
     def test_pad_to_word_aligned(self) -> None: