Bug#37212019: behavior related to innodb_spin_wait_delay changed in 8.0.30

To keep innodb_spin_wait_delay behavior exactly same, ut::random_from_interval_fast()
is better to be aligned to the previous function ut_rnd_interval() behavior,
even seems a little strange.

ut_rnd_interval(low, high) returned only [low, high - 1], and the current all of users'
innodb_spin_wait_delay settings depend on the behavior.
ut::random_from_interval_fast(low, high) returns [low, high] and the spin_wait_delay
behavior can be longer than the previous 8.0.30.

ut::random_from_interval_fast() is used almost only for innodb_spin_wait_delay currently.
To adjust it is the minimum change and the smallest side-effect.

Change-Id: I98f1f94514fa3a4bfa5484a75b4f395dc944f6cf

Author: Yasufumi Kinoshita

storage/innobase/include/ut0rnd.h

@@ -183,7 +183,17 @@ static inline uint64_t random_from_interval(uint64_t low, uint64_t high) {
 }
 
 static inline uint64_t random_from_interval_fast(uint64_t low, uint64_t high) {
-  return random_from_interval_gen<random_64_fast>(low, high);
+  /* FIXME: To keep backward compatibility with the previous ut_rnd_interval(),
+  high value is not to be returned to keep same behavior for performance tuning
+  parameter.
+  (Bug#37212019: behavior related to innodb_spin_wait_delay changed in 8.0.30)
+  This function is not required accurate randomness, no real problems. */
+
+  if (low == high) {
+    return (low);
+  }
+
+  return random_from_interval_gen<random_64_fast>(low, high - 1);
 }
 
 static inline uint64_t hash_uint64(uint64_t value) {

unittest/gunit/innodb/ut0rnd-t.cc

@@ -369,7 +369,7 @@ static void test_interval_fast_distribution(uint64_t n) {
   const uint64_t max_score = 17000;
 
   for (uint64_t i = 0; i < max_count; i++) {
-    const auto value = ut::random_from_interval_fast(0, n - 1);
+    const auto value = ut::random_from_interval_fast(0, n);
     for (auto *target : target_score) {
       if (value == target[0]) {
         target[1]++;