Fixed global variable buffer read overflow [#CLICKHOUSE-4191]

libs/libcommon/include/common/DateLUTImpl.h

@@ -247,6 +247,8 @@ public:
 
         time_t res = t - lut[index].date;
 
+        /// NOTE We doesn't support cases when time change result in switching to previous day.
+        /// Data is cleaned to avoid these cases, so no underflow occurs here.
         if (res >= lut[index].time_at_offset_change)
             res += lut[index].amount_of_offset_change;
 

libs/libcommon/src/DateLUTImpl.cpp

@@ -116,6 +116,15 @@ DateLUTImpl::DateLUTImpl(const std::string & time_zone_)
 
 /*                std::cerr << lut[i - 1].year << "-" << int(lut[i - 1].month) << "-" << int(lut[i - 1].day_of_month)
                     << " offset was changed at " << lut[i - 1].time_at_offset_change << " for " << lut[i - 1].amount_of_offset_change << " seconds.\n";*/
+
+                /** We doesn't support cases when time change results in switching to previous day.
+                  * As an example, it was a case in Moscow at years 1981..1983 on October 1:
+                  *  clock was adjusted one hour backwards exactly at midnight (that was lead to extra hour 23 of Sep 30th).
+                  * We must clean data (and we will make it slightly incorrect) to avoid these cases.
+                  * (In previous example, it will lead to extra hour 0 of Sep 30 instead.)
+                  */
+                if (static_cast<int>(lut[i - 1].time_at_offset_change) + static_cast<int>(lut[i - 1].amount_of_offset_change) < 0)
+                    lut[i - 1].time_at_offset_change = -lut[i - 1].amount_of_offset_change;
             }
         }