Tidb: global system variable can't take effect right away for new connection

[july2993]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.
   set global variable -> return success -> check the value in new connect but get the old value

I have also tried another variable tidb_disable_txn_auto_retry It also can't take effect right away.
mainly test code :

 func setGloableVar(dsn string, increment int, offset int) error {
      var err error
      /*
          err = checkRetry(dsn, "on")
          if err != nil {
              return errors.Trace(err)
          }
          err = checkRetry(dsn, "off")
          if err != nil {
              return errors.Trace(err)
          }

          db, err := sql.Open("mysql", dsn)
          if err != nil {
              return errors.Trace(err)
          }
      */
      db, err := sql.Open("mysql", dsn)
      if err != nil {
          return errors.Trace(err)
      }
      defer db.Close()

      _, err = db.Exec(fmt.Sprintf("SET @@GLOBAL.auto_increment_increment = %d;", increment))
      if err != nil {
          return errors.Trace(err)
      }

      _, err = db.Exec(fmt.Sprintf("SET @@GLOBAL.auto_increment_offset = %d;", offset))
      if err != nil {
          return errors.Trace(err)
      }

      // time.Sleep(time.Second * 3)
      // check value for new connection
      db.Close()
      db2, err := sql.Open("mysql", dsn)
      if err != nil {
          return errors.Trace(err)
      }

      var v int
      row := db2.QueryRow("SELECT @@auto_increment_increment;")
      err = row.Scan(&v)
      if err != nil {
          return errors.Trace(err)
      }

      if v != increment {
          return errors.Errorf("increment get: %d after set as: %d", v, increment)
      }

      row = db2.QueryRow("SELECT @@auto_increment_offset;")
      err = row.Scan(&v)
      if err != nil {
          return errors.Trace(err)
      }

      if v != offset {
          return errors.Errorf("offset get: %d after set as: %d", v, offset)
      }

      return nil
  }

when offset = 1
run the func to set as 2, it will failed:
from tidb log we can see:

[2020/01/19 14:22:09.991 +08:00] [INFO] [session.go:1903] [GENERAL_LOG] [conn=210] [[email protected]] [schemaVersion=22] [txnStartTS=0] [current_db=] [sql="use `test`"]
[2020/01/19 14:22:09.991 +08:00] [INFO] [server.go:415] ["new connection"] [conn=210] [remoteAddr=127.0.0.1:51485]
[2020/01/19 14:22:09.991 +08:00] [INFO] [session.go:1903] [GENERAL_LOG] [conn=210] [[email protected]] [schemaVersion=22] [txnStartTS=0] [current_db=test] [sql="SET @@GLOBAL.auto_increment_increment = 2;"]
[2020/01/19 14:22:09.992 +08:00] [INFO] [session.go:1903] [GENERAL_LOG] [conn=210] [[email protected]] [schemaVersion=22] [txnStartTS=0] [current_db=test] [sql="SET @@GLOBAL.auto_increment_offset = 2;"]
[2020/01/19 14:22:09.994 +08:00] [INFO] [server.go:418] ["connection closed"] [conn=210]
[2020/01/19 14:22:09.994 +08:00] [INFO] [session.go:1903] [GENERAL_LOG] [conn=211] [[email protected]] [schemaVersion=22] [txnStartTS=0] [current_db=] [sql="use `test`"]
[2020/01/19 14:22:09.994 +08:00] [INFO] [server.go:415] ["new connection"] [conn=211] [remoteAddr=127.0.0.1:51486]
[2020/01/19 14:22:09.995 +08:00] [INFO] [session.go:1903] [GENERAL_LOG] [conn=211] [[email protected]] [schemaVersion=22] [txnStartTS=0] [current_db=test] [sql="SELECT @@auto_increment_increment"]
[2020/01/19 14:22:09.995 +08:00] [INFO] [session.go:1903] [GENERAL_LOG] [conn=211] [[email protected]] [schemaVersion=22] [txnStartTS=0] [current_db=test] [sql="SELECT @@auto_increment_offset"]
[2020/01/19 14:22:09.995 +08:00] [INFO] [server.go:418] ["connection closed"] [conn=211]

1. What did you expect to see?
   global system variable can't take effect right away for the new connection

2. What did you see instead?
   It needs a few seconds to take effect

3. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?
   v3.0.9

[AilinKid]

This says it will also take 2-3 seconds for the new session to have access to the changed other global variables, not just auto_increment_offset. confused~

[AilinKid]

Problem located:

[AilinKid]

From above: the new session after the global variable set will still use old gvc (global variable cache) to initiate it sysvar as long as the interval between these two action less than 2 seconds.

TiDB gvc is used to cache global variables in case of hot access, updated per 2 seconds.

[AilinKid]

Issue close suggested.

[july2993]

Issue close suggested.

we should emphasize this 'feature' at the document?

[july2993]

can we improve it by evicting the cache right away so at least for the same tidb instance there's no this issue

[AilinKid]

Issue close suggested.

we should emphasize this 'feature' at the document?

Make sense~ I will add it in.