SERVER-32960 Make $mod always truncate divisor and remainder toward zero

2 files changed, 49 insertions, 2 deletions

diff --git a/src/mongo/db/matcher/expression_parser.cpp b/src/mongo/db/matcher/expression_parser.cpp
index 1077c1ef8ec..cdef3bfd379 100644
--- a/src/mongo/db/matcher/expression_parser.cpp
+++ b/src/mongo/db/matcher/expression_parser.cpp
@@ -512,10 +512,17 @@ StatusWithMatchExpression parseMOD(StringData name,
     if (iter.more())
         return {Status(ErrorCodes::BadValue, "malformed mod, too many elements")};
 
+    auto truncateToInt = [](const BSONElement& element) -> int {
+        if (element.type() == BSONType::NumberDecimal) {
+            return element.numberDecimal().toInt(Decimal128::kRoundTowardZero);
+        }
+        return element.numberInt();
+    };
+
     return {std::make_unique<ModMatchExpression>(
         name,
-        divisor.numberInt(),
-        remainder.numberInt(),
+        truncateToInt(divisor),
+        truncateToInt(remainder),
         doc_validation_error::createAnnotation(
             expCtx, elem.fieldNameStringData().toString(), BSON(name << elem.wrap())))};
 }
diff --git a/src/mongo/db/matcher/expression_parser_leaf_test.cpp b/src/mongo/db/matcher/expression_parser_leaf_test.cpp
index 3c1ac33dc3e..ac71de80045 100644
--- a/src/mongo/db/matcher/expression_parser_leaf_test.cpp
+++ b/src/mongo/db/matcher/expression_parser_leaf_test.cpp
@@ -350,6 +350,46 @@ TEST(MatchExpressionParserLeafTest, SimpleMod1) {
     ASSERT(result.getValue()->matchesBSON(BSON("x" << 8)));
 }
 
+TEST(MatchExpressionParserLeafTest, ModFloatTruncate) {
+    auto assertDivisorAndRemainder =
+        [](const BSONObj& query, int expectedDivisor, int expectedRemainder) {
+            boost::intrusive_ptr<ExpressionContextForTest> expCtx(new ExpressionContextForTest());
+            StatusWithMatchExpression result = MatchExpressionParser::parse(query, expCtx);
+            ASSERT_OK(result.getStatus());
+            auto modExpr = checked_cast<ModMatchExpression*>(result.getValue().get());
+            ASSERT_EQ(modExpr->getDivisor(), expectedDivisor);
+            ASSERT_EQ(modExpr->getRemainder(), expectedRemainder);
+        };
+
+    std::vector<BSONObj> positiveCases = {
+        BSON("x" << BSON("$mod" << BSON_ARRAY(3 << 2))),
+        BSON("x" << BSON("$mod" << BSON_ARRAY(3LL << 2LL))),
+        BSON("x" << BSON("$mod" << BSON_ARRAY(3.2 << 2.2))),
+        BSON("x" << BSON("$mod" << BSON_ARRAY(3.7 << 2.7))),
+        BSON("x" << BSON("$mod" << BSON_ARRAY(Decimal128("3") << Decimal128("2")))),
+        BSON("x" << BSON("$mod" << BSON_ARRAY(Decimal128("3.2") << Decimal128("2.2")))),
+        BSON("x" << BSON("$mod" << BSON_ARRAY(Decimal128("3.7") << Decimal128("2.7")))),
+    };
+
+    for (const auto& testCase : positiveCases) {
+        assertDivisorAndRemainder(testCase, 3, 2);
+    }
+
+    std::vector<BSONObj> negativeCases = {
+        BSON("x" << BSON("$mod" << BSON_ARRAY(-3 << -2))),
+        BSON("x" << BSON("$mod" << BSON_ARRAY(-3LL << -2LL))),
+        BSON("x" << BSON("$mod" << BSON_ARRAY(-3.2 << -2.2))),
+        BSON("x" << BSON("$mod" << BSON_ARRAY(-3.7 << -2.7))),
+        BSON("x" << BSON("$mod" << BSON_ARRAY(Decimal128("-3") << Decimal128("-2")))),
+        BSON("x" << BSON("$mod" << BSON_ARRAY(Decimal128("-3.2") << Decimal128("-2.2")))),
+        BSON("x" << BSON("$mod" << BSON_ARRAY(Decimal128("-3.7") << Decimal128("-2.7")))),
+    };
+
+    for (const auto& testCase : negativeCases) {
+        assertDivisorAndRemainder(testCase, -3, -2);
+    }
+}
+
 TEST(MatchExpressionParserLeafTest, IdCollation) {
     BSONObj query = BSON("$id"
                          << "string");