MDEV-29149 Assertion `!is_valid_datetime() || fraction_remainder(((item->decimals) < (6) ? (item->decimals) : (6))) == 0' failed in Datetime_truncation_not_needed::Datetime_truncation_not_needed

TIME-alike string and numeric arguments to TIMEDIFF()
can get additional fractional seconds during the supported
TIME range adjustment in get_time().

For example, during TIMEDIFF('839:00:00','00:00:00') evaluation
in Item_func_timediff::get_date(), the call for args[0]->get_time()
returns MYSQL_TIME '838:59:59.999999'.

Item_func_timediff::get_date() did not handle these extra digits
and returned a MYSQL_TIME result with fractional digits outside
of Item_func_timediff::decimals. This mismatch could further be
caught by a DBUG_ASSERT() in various other pieces of the code,
leading to a crash.

Fix:

In case if get_time() returned MYSQL_TIMESTAMP_TIME,
let's truncate all extra digits using my_time_trunc(&l_time,decimals).
This guarantees that the rest of the code returns a MYSQL_TIME
with second_part not conflicting with Item_func_timediff::decimals.

mysql-test/main/func_time.result

@@ -6373,3 +6373,57 @@ NULL
 SELECT FROM_UNIXTIME(LEAST(3696610869, NULL));
 FROM_UNIXTIME(LEAST(3696610869, NULL))
 NULL
+#
+# Start of 10.5 tests
+#
+#
+# MDEV-29149 Assertion `!is_valid_datetime() || fraction_remainder(((item->decimals) < (6) ? (item->decimals) : (6))) == 0' failed in Datetime_truncation_not_needed::Datetime_truncation_not_needed
+#
+SET @@timestamp= UNIX_TIMESTAMP('2022-07-21 23:00:00');
+SELECT DATE_SUB('2022-07-21 00:00:00', INTERVAL 800 HOUR) AS expected_result;
+expected_result
+2022-06-17 16:00:00
+SELECT
+IF(1,TIMEDIFF('38:59:59','839:00:00'),CAST('2022-12-12' AS DATE)) AS c1,
+IF(1,TIMEDIFF('-839:00:00','-38:59:59'),CAST('2022-12-12' AS DATE)) AS c2;
+c1	c2
+2022-06-17 16:00:00	2022-06-17 16:00:00
+Warnings:
+Warning	1292	Truncated incorrect time value: '839:00:00'
+Warning	1292	Truncated incorrect time value: '-839:00:00'
+SELECT
+IF(1,TIMEDIFF(385959,8390000),CAST('2022-12-12' AS DATE)) AS c1,
+IF(1,TIMEDIFF(-8390000,-385959),CAST('2022-12-12' AS DATE)) AS c2;
+c1	c2
+2022-06-17 16:00:00	2022-06-17 16:00:00
+Warnings:
+Warning	1292	Truncated incorrect time value: '8390000'
+Warning	1292	Truncated incorrect time value: '-8390000'
+SELECT
+TIMEDIFF('38:59:59','839:00:00') AS c1,
+CAST(TIMEDIFF('38:59:59','839:00:00') AS TIME(6)) AS c2,
+TIMEDIFF('839:00:00','38:59:59') AS c3,
+CAST(TIMEDIFF('839:00:00','38:59:59') AS TIME(6)) AS c4;
+c1	c2	c3	c4
+-800:00:00	-800:00:00.000000	800:00:00	800:00:00.000000
+Warnings:
+Warning	1292	Truncated incorrect time value: '839:00:00'
+Warning	1292	Truncated incorrect time value: '839:00:00'
+Warning	1292	Truncated incorrect time value: '839:00:00'
+Warning	1292	Truncated incorrect time value: '839:00:00'
+SELECT
+TIMEDIFF(385959,8390000) AS c1,
+CAST(TIMEDIFF(385959,8390000) AS TIME(6)) AS c2,
+TIMEDIFF(8390000,385959) AS c3,
+CAST(TIMEDIFF(8390000,385959) AS TIME(6)) AS c4;
+c1	c2	c3	c4
+-800:00:00	-800:00:00.000000	800:00:00	800:00:00.000000
+Warnings:
+Warning	1292	Truncated incorrect time value: '8390000'
+Warning	1292	Truncated incorrect time value: '8390000'
+Warning	1292	Truncated incorrect time value: '8390000'
+Warning	1292	Truncated incorrect time value: '8390000'
+SET @@timestamp= DEFAULT;
+#
+# End of 10.5 tests
+#

mysql-test/main/func_time.test

@@ -3213,3 +3213,42 @@ SELECT CONCAT(MAKETIME('01', '01', LEAST( -100, NULL )));
 --echo #
 
 SELECT FROM_UNIXTIME(LEAST(3696610869, NULL));
+
+
+--echo #
+--echo # Start of 10.5 tests
+--echo #
+
+--echo #
+--echo # MDEV-29149 Assertion `!is_valid_datetime() || fraction_remainder(((item->decimals) < (6) ? (item->decimals) : (6))) == 0' failed in Datetime_truncation_not_needed::Datetime_truncation_not_needed
+--echo #
+
+SET @@timestamp= UNIX_TIMESTAMP('2022-07-21 23:00:00');
+
+SELECT DATE_SUB('2022-07-21 00:00:00', INTERVAL 800 HOUR) AS expected_result;
+
+SELECT
+  IF(1,TIMEDIFF('38:59:59','839:00:00'),CAST('2022-12-12' AS DATE)) AS c1,
+  IF(1,TIMEDIFF('-839:00:00','-38:59:59'),CAST('2022-12-12' AS DATE)) AS c2;
+
+SELECT
+  IF(1,TIMEDIFF(385959,8390000),CAST('2022-12-12' AS DATE)) AS c1,
+  IF(1,TIMEDIFF(-8390000,-385959),CAST('2022-12-12' AS DATE)) AS c2;
+
+SELECT
+  TIMEDIFF('38:59:59','839:00:00') AS c1,
+  CAST(TIMEDIFF('38:59:59','839:00:00') AS TIME(6)) AS c2,
+  TIMEDIFF('839:00:00','38:59:59') AS c3,
+  CAST(TIMEDIFF('839:00:00','38:59:59') AS TIME(6)) AS c4;
+
+SELECT
+  TIMEDIFF(385959,8390000) AS c1,
+  CAST(TIMEDIFF(385959,8390000) AS TIME(6)) AS c2,
+  TIMEDIFF(8390000,385959) AS c3,
+  CAST(TIMEDIFF(8390000,385959) AS TIME(6)) AS c4;
+
+SET @@timestamp= DEFAULT;
+
+--echo #
+--echo # End of 10.5 tests
+--echo #

sql/item_timefunc.cc

@@ -2706,6 +2706,24 @@ bool Item_func_timediff::get_date(THD *thd, MYSQL_TIME *ltime, date_mode_t fuzzy
   if (l_time1.neg != l_time2.neg)
     l_sign= -l_sign;
 
+  if (l_time1.time_type == MYSQL_TIMESTAMP_TIME)
+  {
+    /*
+      In case of TIME-alike arguments:
+        TIMEDIFF('38:59:59', '839:00:00')
+      let's truncate extra fractional seconds that might appear if the argument
+      values were out of the supported TIME range. For example, args[n]->get_time()
+      for the string literal '839:00:00' returns TIME'838:59:59.999999'.
+      The fractional part must be truncated according to this->decimals,
+      to avoid returning more fractional seconds than it was detected
+      during this->fix_length_and_dec().
+      Note, the thd rounding mode should not be important here, as we're removing
+      redundant digits from the maximum possible value: '838:59:59.999999'.
+    */
+    my_time_trunc(&l_time1, decimals);
+    my_time_trunc(&l_time2, decimals);
+  }
+
   if (calc_time_diff(&l_time1, &l_time2, l_sign, &l_time3, fuzzydate))
     return (null_value= 1);