fix(firestore): disallow float in BSONDecimal128 and validate input strings

Disallow passing float to BSONDecimal128 to prevent binary floating-point
precision loss and edge cases, aligning with PyMongo's Decimal128
behavior. Validate string and numeric inputs via decimal.Decimal in
__init__ to reject invalid numeric strings.

Author: ohmayr

packages/google-cloud-firestore/google/cloud/firestore_v1/bson.py

@@ -413,12 +413,12 @@ class BSONDecimal128(_BSONType):
     """Represents a BSON 128-bit Decimal container for Firestore.
 
     Args:
-        value (Union[str, int, float, decimal.Decimal, BSONDecimal128]):
-            The decimal value as a string, integer, float, decimal.Decimal,
+        value (Union[str, int, decimal.Decimal, BSONDecimal128]):
+            The decimal value as a string, integer, decimal.Decimal,
             or BSONDecimal128 instance.
 
     Raises:
-        TypeError: If value is a boolean or unsupported type.
+        TypeError: If value is a boolean or an unsupported type.
         ValueError: If value cannot be parsed as a valid decimal number.
 
     Example:
@@ -433,18 +433,26 @@ class BSONDecimal128(_BSONType):
 
     def __init__(
         self,
-        value: Union[str, int, float, decimal.Decimal, "BSONDecimal128"],
+        value: Union[str, int, decimal.Decimal, "BSONDecimal128"],
     ):
         if isinstance(value, BSONDecimal128):
             self._value: str = value._value
-        elif isinstance(value, (str, int, float, decimal.Decimal)) and not isinstance(
+        elif isinstance(value, (str, int, decimal.Decimal)) and not isinstance(
             value, bool
         ):
+            try:
+                # Validate the value parses as a valid decimal number
+                decimal.Decimal(value)
+            except decimal.InvalidOperation as exc:
+                raise ValueError(f"Cannot convert {value!r} to Decimal: {exc}") from exc
             self._value = str(value)
-        else:
+        elif isinstance(value, float):
             raise TypeError(
-                "BSONDecimal128 value must be a Decimal, str, int, or float."
+                "BSONDecimal128 does not accept float values due to potential precision loss. "
+                "Convert the float to a str or decimal.Decimal first."
             )
+        else:
+            raise TypeError("BSONDecimal128 value must be a Decimal, str, or int.")
 
     @property
     def value(self) -> str:

packages/google-cloud-firestore/tests/unit/v1/test_bson.py

@@ -496,14 +496,11 @@ def test_bson_decimal128_valid():
     dec2 = BSONDecimal128(42)
     assert dec2.value == "42"
 
-    dec3 = BSONDecimal128(1.5)
-    assert dec3.value == "1.5"
+    dec3 = BSONDecimal128(decimal.Decimal("99.99"))
+    assert dec3.value == "99.99"
 
-    dec4 = BSONDecimal128(decimal.Decimal("99.99"))
-    assert dec4.value == "99.99"
-
-    dec5 = BSONDecimal128(dec1)
-    assert dec5.value == "123.45"
+    dec4 = BSONDecimal128(dec1)
+    assert dec4.value == "123.45"
 
 
 def test_bson_decimal128_float_and_int():
@@ -523,9 +520,6 @@ def test_bson_decimal128_float_and_int():
         "-NaN",
         "sNaN",
         "-sNaN",
-        float("inf"),
-        float("-inf"),
-        float("nan"),
     ],
 )
 def test_bson_decimal128_special_values(special_val):
@@ -545,9 +539,14 @@ def test_bson_decimal128_special_values(special_val):
 @pytest.mark.parametrize(
     "val_input, exc_type, match_msg",
     [
-        (True, TypeError, "value must be a Decimal, str, int, or float"),
-        (False, TypeError, "value must be a Decimal, str, int, or float"),
-        ([1, 2], TypeError, "value must be a Decimal, str, int, or float"),
+        (True, TypeError, "value must be a Decimal, str, or int"),
+        (False, TypeError, "value must be a Decimal, str, or int"),
+        ([1, 2], TypeError, "value must be a Decimal, str, or int"),
+        (1.5, TypeError, "does not accept float values"),
+        (float("inf"), TypeError, "does not accept float values"),
+        (float("nan"), TypeError, "does not accept float values"),
+        ("not-a-number", ValueError, "Cannot convert 'not-a-number' to Decimal"),
+        ("12.34.56", ValueError, "Cannot convert '12.34.56' to Decimal"),
     ],
 )
 def test_bson_decimal128_invalid_inputs(val_input, exc_type, match_msg):