Fix data race in error path

[larsve]

Fixes a data race in ToMiddleware() by creating a new instance of the err variable in the returned function.

[aldas]

@larsve could you provide testcase showing this race. I struggling to understand how this can be racy. Is it because that error instance is created at extractors, err := CreateExtractors(config.TokenLookup)?

[larsve]

Yes, I hit that issue when GO's race detector broke my integration test, complaining about:

WARNING: DATA RACE
Write at 0x00c0002aa760 by goroutine 105:
github.com/labstack/echo-jwt/v4.Config.ToMiddleware.func2.1()
/go/pkg/mod/github.com/labstack/echo-jwt/v4@v4.0.0/jwt.go:217 +0x264

And se far as I could see, it complained because the returned function is reusing the err instance that is created in the ToMiddleware() func. With this fix, the race detector isn't triggered anymore.

[larsve]

As I understand the issue, it's when there are concurrent calls to the returned function, since every call to the returned function is reusing the same err variable.

[larsve]

Added a small testcase now, if I revert my fix, I get this when running go test -race ./...:

Write at 0x00c00021a990 by goroutine 49:                                                                                                
  github.com/labstack/echo-jwt/v4.Config.ToMiddleware.func2.1()
      /home/lars-erik/proj/echo-jwt/jwt.go:228 +0x264
  github.com/labstack/echo-jwt/v4.TestToMiddlewareRace.func4()
      /home/lars-erik/proj/echo-jwt/jwt_test.go:776 +0xda
                                                                    
Previous write at 0x00c00021a990 by goroutine 58:
  github.com/labstack/echo-jwt/v4.Config.ToMiddleware.func2.1()
      /home/lars-erik/proj/echo-jwt/jwt.go:228 +0x264
  github.com/labstack/echo-jwt/v4.TestToMiddlewareRace.func4()   
      /home/lars-erik/proj/echo-jwt/jwt_test.go:776 +0xda

[aldas]

LGTM

[aldas]

I'll release patch release asap

[aldas]

Thank you @larsve and v4.0.1 is out