Bug#38020526 Set back_log to max_connections if 0

Description:
  WL#16888 had set the default value of back_log to 10000. It also set
the minimum value of back_log to 1 and removed the section which updates
the value to max_connections if the value was 0 (the previous default).

Issue:
  This introduces an unexpected change in behavior where back_log would
take the value of 1 if the user had explicitly configured it to 0. This
causes performance regressions as the value 1 is too small and leads to
frequent reconnects.

Fix:
  Allow 0 to be the minimum value and update the value of back_log to
max_connections if back_log takes the value 0. Ensure that by default
back_log still takes the value 10000.

Change-Id: Idc849c1bb73cf824f90fb8a5f2811cc8c54d586f

Author: ram1048

sql/mysqld.cc

@@ -6897,6 +6897,9 @@ int init_common_variables() {
       (host_cache_size = 628 + ((max_connections - 500) / 20)) > 2000)
     host_cache_size = 2000;
 
+  /* Fix back_log */
+  if (back_log == 0 && (back_log = max_connections) > 65535) back_log = 65535;
+
   unireg_init(opt_specialflag); /* Set up extern variables */
   while (!(my_default_lc_messages =
                my_locale_by_name(nullptr, lc_messages, strlen(lc_messages)))) {

sql/sys_vars.cc

@@ -1123,7 +1123,7 @@ static Sys_var_ulong Sys_back_log(
     "MySQL can have. This comes into play when the main MySQL thread "
     "gets very many connection requests in a very short time",
     READ_ONLY GLOBAL_VAR(back_log), CMD_LINE(REQUIRED_ARG),
-    VALID_RANGE(1, 65535), DEFAULT(10000), BLOCK_SIZE(1));
+    VALID_RANGE(0, 65535), DEFAULT(10000), BLOCK_SIZE(1));
 
 static Sys_var_charptr Sys_basedir(
     "basedir",