Moved autosetting of host_cache_size and back_log to proper place

- Clean up formulas and comments for host_cache_size and back_log
- Added test of autoset (for host_cache_size)
- Marked open_files_limit as auto_set

mysql-test/r/mysqld--help.result

@@ -569,9 +569,10 @@ The following options may be given as the first argument:
  --open-files-limit=# 
  If this is not 0, then mysqld will use this value to
  reserve file descriptors to use with setrlimit(). If this
- value is 0 then mysqld will reserve max_connections*5 or
- max_connections + table_cache*2 (whichever is larger)
- number of file descriptors
+ value is 0 or autoset then mysqld will reserve
+ max_connections*5 or max_connections + table_cache*2
+ (whichever is larger) number of file descriptors
+ (Automatically configured unless set explicitly)
  --optimizer-prune-level=# 
  Controls the heuristic(s) applied during query
  optimization to prune less-promising partial plans from

mysql-test/suite/sys_vars/r/host_cache_size_auto.result

+select @@global.host_cache_size;
+@@global.host_cache_size
+653

mysql-test/suite/sys_vars/r/sysvars_server_notembedded.result

@@ -5216,7 +5216,7 @@ COMMAND_LINE_ARGUMENT	OPTIONAL
 VARIABLE_NAME	OPEN_FILES_LIMIT
 VARIABLE_SCOPE	GLOBAL
 VARIABLE_TYPE	BIGINT UNSIGNED
-VARIABLE_COMMENT	If this is not 0, then mysqld will use this value to reserve file descriptors to use with setrlimit(). If this value is 0 then mysqld will reserve max_connections*5 or max_connections + table_cache*2 (whichever is larger) number of file descriptors
+VARIABLE_COMMENT	If this is not 0, then mysqld will use this value to reserve file descriptors to use with setrlimit(). If this value is 0 or autoset then mysqld will reserve max_connections*5 or max_connections + table_cache*2 (whichever is larger) number of file descriptors
 NUMERIC_MIN_VALUE	0
 NUMERIC_MAX_VALUE	4294967295
 NUMERIC_BLOCK_SIZE	1

mysql-test/suite/sys_vars/t/host_cache_size_auto-master.opt

+--max_connections=1000 --open-files-limit=1000 --autoset-host-cache-size

mysql-test/suite/sys_vars/t/host_cache_size_auto.test

+select @@global.host_cache_size;

sql/mysqld.cc

@@ -4400,21 +4400,6 @@ static int init_common_variables()
     SYSVAR_AUTOSIZE(threadpool_size, my_getncpus());
 #endif
 
-  /* Fix host_cache_size. */
-  if (IS_SYSVAR_AUTOSIZE(&host_cache_size))
-  {
-    if (max_connections <= 628 - 128)
-      SYSVAR_AUTOSIZE(host_cache_size, 128 + max_connections);
-    else if (max_connections <= ((ulong)(2000 - 628)) * 20 + 500)
-      SYSVAR_AUTOSIZE(host_cache_size, 628 + ((max_connections - 500) / 20));
-    else
-      SYSVAR_AUTOSIZE(host_cache_size, 2000);
-  }
-
-  /* Fix back_log (back_log == 0 added for MySQL compatibility) */
-  if (back_log == 0 || IS_SYSVAR_AUTOSIZE(&back_log))
-    SYSVAR_AUTOSIZE(back_log, MY_MIN(900, (50 + max_connections / 5)));
-
   /* connections and databases needs lots of files */
   {
     uint files, wanted_files, max_open_files;
@@ -4439,7 +4424,7 @@ static int init_common_variables()
 
     if (files < wanted_files)
     {
-      if (!open_files_limit)
+      if (!open_files_limit || IS_SYSVAR_AUTOSIZE(&open_files_limit))
       {
         /*
           If we have requested too much file handles than we bring
@@ -4468,6 +4453,36 @@ static int init_common_variables()
     }
     SYSVAR_AUTOSIZE(open_files_limit, files);
   }
+
+  /*
+    Max_connections is now set.
+    Now we can fix other variables depending on this variable.
+  */
+
+  /* Fix host_cache_size */
+  if (IS_SYSVAR_AUTOSIZE(&host_cache_size))
+  {
+    /*
+      The default value is 128.
+      The autoset value is 128, plus 1 for a value of max_connections
+      up to 500, plus 1 for every increment of 20 over 500 in the
+      max_connections value, capped at 2000.
+    */
+    uint size= (HOST_CACHE_SIZE + MY_MIN(max_connections, 500) +
+                MY_MAX(((long) max_connections)-500,0)/20);
+    SYSVAR_AUTOSIZE(host_cache_size, size);
+  }
+
+  /* Fix back_log (back_log == 0 added for MySQL compatibility) */
+  if (back_log == 0 || IS_SYSVAR_AUTOSIZE(&back_log))
+  {
+    /*
+      The default value is 150.
+      The autoset value is 50 + max_connections / 5 capped at 900
+    */
+    SYSVAR_AUTOSIZE(back_log, MY_MIN(900, (50 + max_connections / 5)));
+  }
+
   unireg_init(opt_specialflag); /* Set up extern variabels */
   if (!(my_default_lc_messages=
         my_locale_by_name(lc_messages)))

sql/sys_vars.cc

@@ -2311,10 +2311,10 @@ export sys_var *Sys_old_passwords_ptr= &Sys_old_passwords; // for sql_acl.cc
 static Sys_var_ulong Sys_open_files_limit(
        "open_files_limit",
        "If this is not 0, then mysqld will use this value to reserve file "
-       "descriptors to use with setrlimit(). If this value is 0 then mysqld "
-       "will reserve max_connections*5 or max_connections + table_cache*2 "
-       "(whichever is larger) number of file descriptors",
-       READ_ONLY GLOBAL_VAR(open_files_limit), CMD_LINE(REQUIRED_ARG),
+       "descriptors to use with setrlimit(). If this value is 0 or autoset "
+       "then mysqld will reserve max_connections*5 or max_connections + "
+       "table_cache*2 (whichever is larger) number of file descriptors",
+       AUTO_SET READ_ONLY GLOBAL_VAR(open_files_limit), CMD_LINE(REQUIRED_ARG),
        VALID_RANGE(0, OS_FILE_LIMIT), DEFAULT(0), BLOCK_SIZE(1));
 
 /// @todo change to enum