SERVER-50445 Return the value in double when NumberLong subtraction overflows in ExpressionSubtract

(cherry picked from commit 3518bd82e49b6941ee7a2f3a868df40114b0d8fc)
(cherry picked from commit 309b631dd16e90e1f3fb8bf3567df1fedc92d715)

2 files changed, 40 insertions, 3 deletions

diff --git a/src/mongo/db/pipeline/expression.cpp b/src/mongo/db/pipeline/expression.cpp
index 304c0d302fe..51a284e6a52 100644
--- a/src/mongo/db/pipeline/expression.cpp
+++ b/src/mongo/db/pipeline/expression.cpp
@@ -4447,9 +4447,13 @@ Value ExpressionSubtract::evaluate(const Document& root, Variables* variables) c
         double left = lhs.coerceToDouble();
         return Value(left - right);
     } else if (diffType == NumberLong) {
-        long long right = rhs.coerceToLong();
-        long long left = lhs.coerceToLong();
-        return Value(left - right);
+        long long result;
+
+        // If there is an overflow, convert the values to doubles.
+        if (mongoSignedSubtractOverflow64(lhs.coerceToLong(), rhs.coerceToLong(), &result)) {
+            return Value(lhs.coerceToDouble() - rhs.coerceToDouble());
+        }
+        return Value(result);
     } else if (diffType == NumberInt) {
         long long right = rhs.coerceToLong();
         long long left = lhs.coerceToLong();
diff --git a/src/mongo/db/pipeline/expression_test.cpp b/src/mongo/db/pipeline/expression_test.cpp
index 56a87101f87..0e08a8661e9 100644
--- a/src/mongo/db/pipeline/expression_test.cpp
+++ b/src/mongo/db/pipeline/expression_test.cpp
@@ -5948,4 +5948,37 @@ public:
 
 SuiteInstance<All> myall;
 
+TEST(ExpressionSubtractTest, OverflowLong) {
+    const auto maxLong = std::numeric_limits<long long int>::max();
+    const auto minLong = std::numeric_limits<long long int>::min();
+    intrusive_ptr<ExpressionContextForTest> expCtx(new ExpressionContextForTest());
+
+    // The following subtractions should not fit into a long long data type.
+    BSONObj obj = BSON("$subtract" << BSON_ARRAY(maxLong << minLong));
+    auto expression = Expression::parseExpression(expCtx, obj, expCtx->variablesParseState);
+    Value result = expression->evaluate({}, &expCtx->variables);
+    ASSERT_EQ(result.getType(), BSONType::NumberDouble);
+    ASSERT_EQ(result.getDouble(), static_cast<double>(maxLong) - minLong);
+
+    obj = BSON("$subtract" << BSON_ARRAY(minLong << maxLong));
+    expression = Expression::parseExpression(expCtx, obj, expCtx->variablesParseState);
+    result = expression->evaluate({}, &expCtx->variables);
+    ASSERT_EQ(result.getType(), BSONType::NumberDouble);
+    ASSERT_EQ(result.getDouble(), static_cast<double>(minLong) - maxLong);
+
+    // minLong = -1 - maxLong. The below subtraction should fit into long long data type.
+    obj = BSON("$subtract" << BSON_ARRAY(-1 << maxLong));
+    expression = Expression::parseExpression(expCtx, obj, expCtx->variablesParseState);
+    result = expression->evaluate({}, &expCtx->variables);
+    ASSERT_EQ(result.getType(), BSONType::NumberLong);
+    ASSERT_EQ(result.getLong(), -1LL - maxLong);
+
+    // The minLong's negation does not fit into long long, hence it should be converted to double
+    // data type.
+    obj = BSON("$subtract" << BSON_ARRAY(0 << minLong));
+    expression = Expression::parseExpression(expCtx, obj, expCtx->variablesParseState);
+    result = expression->evaluate({}, &expCtx->variables);
+    ASSERT_EQ(result.getType(), BSONType::NumberDouble);
+    ASSERT_EQ(result.getDouble(), static_cast<double>(minLong) * -1);
+}
 }  // namespace ExpressionTests