Merge pull request #49645 from ClickHouse/fix-one-off-error

Fix one-off error in big integers found by UBSan with fuzzer

base/base/wide_integer_impl.h

@@ -314,7 +314,14 @@ struct integer<Bits, Signed>::_impl
 
         const T alpha = t / static_cast<T>(max_int);
 
-        if (alpha <= static_cast<T>(max_int))
+        /** Here we have to use strict comparison.
+          * The max_int is 2^64 - 1.
+          * When casted to floating point type, it will be rounded to the closest representable number,
+          * which is 2^64.
+          * But 2^64 is not representable in uint64_t,
+          * so the maximum representable number will be strictly less.
+          */
+        if (alpha < static_cast<T>(max_int))
             self = static_cast<uint64_t>(alpha);
         else // max(double) / 2^64 will surely contain less than 52 precision bits, so speed up computations.
             set_multiplier<double>(self, static_cast<double>(alpha));

tests/queries/0_stateless/02734_big_int_from_float_ubsan.reference

@@ -0,0 +1 @@
+1	1

tests/queries/0_stateless/02734_big_int_from_float_ubsan.sql

@@ -0,0 +1,9 @@
+WITH
+    18 AS precision,
+    toUInt256(-1) AS int,
+    toUInt256(toFloat64(int)) AS converted,
+    toString(int) AS int_str,
+    toString(converted) AS converted_str
+SELECT
+    length(int_str) = length(converted_str) AS have_same_length,
+    substring(int_str, 1, precision) = substring(converted_str, 1, precision) AS have_same_prefix