driver can drop server-side query timeout error

[acharis]

If a query takes too long in the sense of exceeding max_execution_time then the driver should report an error. I can reproduce lacking this error.

package main

import (
 "database/sql"
 "fmt"
 "time"
 _ "github.com/go-sql-driver/mysql"
)

func main() {
 db, err := sql.Open("mysql", "username:password@tcp(1.2.3.4:3306)/dbname")
 if err != nil {
    fmt.Println(err)
    return
 }

  defer db.Close()
  start := time.Now()
  rows, err := db.Query("SELECT count(*) FROM foo")
  if err != nil {
   fmt.Println(err.Error())
   return
  }

  t := time.Now()
  elapsed := t.Sub(start)
  fmt.Println(elapsed)

  var count int64
  for rows.Next() {
    err := rows.Scan(&count)
    if err != nil {
      fmt.Println(err)
      return
    }
    fmt.Printf("count: %d\n", count)
  }
}

I expect an error after db.Query

Built against go-sql-driver/mysql commit 681ffa8

go version
go version go1.12.14 linux/amd64

mysql version: 5.7.25-28-log Percona Server (GPL), Release 28, Revision c335905

uname -a
Linux pause-pod 5.4.20-hs768.el6.x86_64 #1 SMP Tue Mar 17 16:38:55 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

[acharis]

I have also reproduced this against:

go version
go version go1.13.6 linux/amd64

[methane]

Have you checked rows.Err()?

[acharis]

checking that now, via:

package main

import (
 "database/sql"
 "fmt"
 "time"
 _ "github.com/go-sql-driver/mysql"
)

func main() {
 db, err := sql.Open("mysql", "vt_hsmysql_read:T}MLBFCNroXKFzR;oPZUJ2Qve@tcp(172.18.153.145:3306)/VitessTest")
 if err != nil {
    fmt.Println(err)
    return
 }

  defer db.Close()
  start := time.Now()
  rows, err := db.Query("SELECT count(*) FROM foo")
  if err != nil {
   fmt.Println(err.Error())
   return
  }

  t := time.Now()
  elapsed := t.Sub(start)
  fmt.Println(elapsed)

   err = rows.Err()
   if err != nil {
           fmt.Println(err)
           return
   }

  var count int64
  for rows.Next() {
    err := rows.Scan(&count)
    if err != nil {
      fmt.Println(err)
      return
    }
    fmt.Printf("count: %d\n", count)
  }
}

i still don't see any error.

[acharis]

i've also now reproduced this in v1.2.0, v1.3.0, v1.4.0 as well as the original v1.5.0
(all those reproductions against golang 1.13.6)

[acharis]

is that only populated upon call to Next ?

[acharis]

ah yeah, so we've got to call next then check for error

[acharis]

x-ref: github/gh-ost#822

[acharis]

i'm closing this, b/c it seems like the user (here the gh-ost application) isn't checking for the error in the way that go-sql-driver expects, but i'll note that it's surprising to me that we'd need to call Next to be able to see this error. that feels unergonomic to me. i'd expect the error in the rows, err := db.Query line to have this timeout error.

[methane]

but i'll note that it's surprising to me that we'd need to call Next to be able to see this error. that feels unergonomic to me.

This is low level library. It is not designed for ergonomic.
This library return error when server returned error. If you get error from rows.Err() instead of db.Query(), your server sent the error while sending result rows instead of result headers.

You can use some wrapper library if you want "ergonomic" design.
For example, see sqlx.Select.