fix conn data race

[liangjunmo]

• [x] Do only one thing
• [x] Non breaking API changes
• [x] Tested

What did this pull request do?

Fix conn DARA RACE issue. There are some related issues like #96.

User Case Description

I use this package for sharding bill table. It runs normally on test environment, but occur DATA RACE issue on online environment.

I write unit test for this issue:

• sharding_test.go

  func TestDataRace(t *testing.T) {
  ctx, cancel := context.WithCancel(context.Background())
  ch := make(chan error)
  
  for i := 0; i < 2; i++ {
      go func() {
          for {
              select {
              case <-ctx.Done():
                  return
              default:
                  err := db.Model(&Order{}).Where("user_id", 100).Find(&[]Order{}).Error
                  if err != nil {
                      ch <- err
                      return
                  }
              }
          }
      }()
  }
  
  select {
  case <-time.After(time.Millisecond * 50):
      cancel()
  case err := <-ch:
      cancel()
      t.Fatal(err)
  }
  }

Run this unit test on main branch with go test --race -count=1 -v --run=TestDataRace, it will output:

=== RUN   TestDataRace
==================
WARNING: DATA RACE
Write at 0x00c0000b3048 by goroutine 15:
  command-line-arguments.(*Sharding).switchConn()
      /Users/liangjunmo/workspace/gorm-sharding/sharding.go:268 +0x168
  command-line-arguments.(*Sharding).switchConn-fm()
      <autogenerated>:1 +0x44
  gorm.io/gorm.(*processor).Execute()
      /Users/liangjunmo/.gvm/pkgsets/go1.19.9/global/pkg/mod/gorm.io/gorm@v1.25.1/callbacks.go:130 +0xbe1
  gorm.io/gorm.(*DB).Find()
      /Users/liangjunmo/.gvm/pkgsets/go1.19.9/global/pkg/mod/gorm.io/gorm@v1.25.1/finisher_api.go:172 +0x238
  command-line-arguments.TestDataRace.func1()
      /Users/liangjunmo/workspace/gorm-sharding/sharding_test.go:413 +0x20d

Previous write at 0x00c0000b3048 by goroutine 14:
  command-line-arguments.(*Sharding).switchConn()
      /Users/liangjunmo/workspace/gorm-sharding/sharding.go:268 +0x168
  command-line-arguments.(*Sharding).switchConn-fm()
      <autogenerated>:1 +0x44
  gorm.io/gorm.(*processor).Execute()
      /Users/liangjunmo/.gvm/pkgsets/go1.19.9/global/pkg/mod/gorm.io/gorm@v1.25.1/callbacks.go:130 +0xbe1
  gorm.io/gorm.(*DB).Find()
      /Users/liangjunmo/.gvm/pkgsets/go1.19.9/global/pkg/mod/gorm.io/gorm@v1.25.1/finisher_api.go:172 +0x238
  command-line-arguments.TestDataRace.func1()
      /Users/liangjunmo/workspace/gorm-sharding/sharding_test.go:413 +0x20d

Goroutine 15 (running) created at:
  command-line-arguments.TestDataRace()
      /Users/liangjunmo/workspace/gorm-sharding/sharding_test.go:407 +0x87
  testing.tRunner()
      /Users/liangjunmo/.gvm/gos/go1.19.9/src/testing/testing.go:1446 +0x216
  testing.(*T).Run.func1()
      /Users/liangjunmo/.gvm/gos/go1.19.9/src/testing/testing.go:1493 +0x47

Goroutine 14 (running) created at:
  command-line-arguments.TestDataRace()
      /Users/liangjunmo/workspace/gorm-sharding/sharding_test.go:407 +0x87
  testing.tRunner()
      /Users/liangjunmo/.gvm/gos/go1.19.9/src/testing/testing.go:1446 +0x216
  testing.(*T).Run.func1()
      /Users/liangjunmo/.gvm/gos/go1.19.9/src/testing/testing.go:1493 +0x47
==================
    testing.go:1319: race detected during execution of test
--- FAIL: TestDataRace (0.05s)
=== CONT
    testing.go:1319: race detected during execution of test
FAIL
FAIL    command-line-arguments  0.867s
FAIL