Complete correctness for system variables

## What

Add restriction for system variable `thread_pool_size`.

## Reference

1. [MySQL global variable: thread_pool_size](https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_version_compile_os)
2. [TiDB guide to cover more restrictions on system variable  ](pingcap#7195)

executor/set_test.go

@@ -502,6 +502,19 @@ func (s *testSuite2) TestValidateSetVar(c *C) {
 	_, err = tk.Exec("set @@global.max_connections='hello'")
 	c.Assert(terror.ErrorEqual(err, variable.ErrWrongTypeForVar), IsTrue)
 
+	tk.MustExec("set @@global.thread_pool_size=65")
+	tk.MustQuery("show warnings").Check(testutil.RowsWithSep("|", "Warning|1292|Truncated incorrect thread_pool_size value: '65'"))
+	result = tk.MustQuery("select @@global.thread_pool_size;")
+	result.Check(testkit.Rows("64"))
+
+	tk.MustExec("set @@global.thread_pool_size=-1")
+	tk.MustQuery("show warnings").Check(testutil.RowsWithSep("|", "Warning|1292|Truncated incorrect thread_pool_size value: '-1'"))
+	result = tk.MustQuery("select @@global.thread_pool_size;")
+	result.Check(testkit.Rows("1"))
+
+	_, err = tk.Exec("set @@global.thread_pool_size='hello'")
+	c.Assert(terror.ErrorEqual(err, variable.ErrWrongTypeForVar), IsTrue)
+
 	tk.MustExec("set @@global.max_allowed_packet=-1")
 	tk.MustQuery("show warnings").Check(testutil.RowsWithSep("|", "Warning|1292|Truncated incorrect max_allowed_packet value: '-1'"))
 	result = tk.MustQuery("select @@global.max_allowed_packet;")

sessionctx/variable/sysvar.go

@@ -638,6 +638,7 @@ var defaultSysVars = []*SysVar{
 	{ScopeSession, WarningCount, "0"},
 	{ScopeSession, ErrorCount, "0"},
 	{ScopeGlobal | ScopeSession, "information_schema_stats_expiry", "86400"},
+	{ScopeGlobal, "thread_pool_size", "16"},
 	/* TiDB specific variables */
 	{ScopeSession, TiDBSnapshot, ""},
 	{ScopeSession, TiDBOptAggPushDown, BoolToIntStr(DefOptAggPushDown)},
@@ -971,6 +972,8 @@ const (
 	CollationServer = "collation_server"
 	// NetWriteTimeout is the name of 'net_write_timeout' variable.
 	NetWriteTimeout = "net_write_timeout"
+	// ThreadPoolSize is the name of 'thread_pool_size' variable.
+	ThreadPoolSize = "thread_pool_size"
 )
 
 // GlobalVarAccessor is the interface for accessing global scope system and status variables.

sessionctx/variable/varsutil.go

@@ -425,6 +425,8 @@ func ValidateSetSystemVar(vars *SessionVars, name string, value string) (string,
 		return value, ErrWrongValueForVar.GenWithStackByArgs(name, value)
 	case MaxExecutionTime:
 		return checkUInt64SystemVar(name, value, 0, math.MaxUint64, vars)
+	case ThreadPoolSize:
+		return checkUInt64SystemVar(name, value, 1, 64, vars)
 	case TiDBEnableTablePartition:
 		switch {
 		case strings.EqualFold(value, "ON") || value == "1":