fix: potential data race on a global variable

[pior]

Running go test -race on some integration tests with Sarama trigger a data race warning:

WARNING: DATA RACE
Read at 0x0001052f7940 by goroutine 11:
  github.com/Shopify/sarama.version()
      /Users/pior/pkg/mod/github.com/!shopify/[email protected]/version.go:8 +0x34
  github.com/Shopify/sarama.(*Broker).Open.func1.1()
      /Users/pior/pkg/mod/github.com/!shopify/[email protected]/broker.go:183 +0x44
  github.com/Shopify/sarama.(*Broker).Open.func1()
      /Users/pior/pkg/mod/github.com/!shopify/[email protected]/broker.go:245 +0x1368
  github.com/Shopify/sarama.withRecover()
      /Users/pior/pkg/mod/github.com/!shopify/[email protected]/utils.go:43 +0x54

Previous write at 0x0001052f7940 by goroutine 12:
  github.com/Shopify/sarama.version()
      /Users/pior/pkg/mod/github.com/!shopify/[email protected]/version.go:16 +0xec
  github.com/Shopify/sarama.(*Broker).Open.func1.1()
      /Users/pior/pkg/mod/github.com/!shopify/[email protected]/broker.go:183 +0x44
  github.com/Shopify/sarama.(*Broker).Open.func1()
      /Users/pior/pkg/mod/github.com/!shopify/[email protected]/broker.go:245 +0x1368
  github.com/Shopify/sarama.withRecover()
      /Users/pior/pkg/mod/github.com/!shopify/[email protected]/utils.go:43 +0x54

The traces from the goroutines

Goroutine 11 (running) created at:
  github.com/Shopify/sarama.(*Broker).Open()
      /Users/pior/pkg/mod/github.com/!shopify/[email protected]/broker.go:172 +0x300
  github.com/Shopify/sarama.(*client).any()
      /Users/pior/pkg/mod/github.com/!shopify/[email protected]/client.go:686 +0xf8
  github.com/Shopify/sarama.(*client).tryRefreshMetadata()
      /Users/pior/pkg/mod/github.com/!shopify/[email protected]/client.go:879 +0xbc
  github.com/Shopify/sarama.(*client).RefreshMetadata()
      /Users/pior/pkg/mod/github.com/!shopify/[email protected]/client.go:489 +0x1d8
  github.com/Shopify/sarama.NewClient()
      /Users/pior/pkg/mod/github.com/!shopify/[email protected]/client.go:170 +0x438
[...]

Goroutine 12 (running) created at:
  github.com/Shopify/sarama.(*Broker).Open()
      /Users/pior/pkg/mod/github.com/!shopify/[email protected]/broker.go:172 +0x300
  github.com/Shopify/sarama.(*client).any()
      /Users/pior/pkg/mod/github.com/!shopify/[email protected]/client.go:686 +0xf8
  github.com/Shopify/sarama.(*client).tryRefreshMetadata()
      /Users/pior/pkg/mod/github.com/!shopify/[email protected]/client.go:879 +0xbc
  github.com/Shopify/sarama.(*client).RefreshMetadata()
      /Users/pior/pkg/mod/github.com/!shopify/[email protected]/client.go:489 +0x1d8
  github.com/Shopify/sarama.NewClient()
      /Users/pior/pkg/mod/github.com/!shopify/[email protected]/client.go:170 +0x438
  github.com/Shopify/sarama.NewConsumerGroup()
      /Users/pior/pkg/mod/github.com/!shopify/[email protected]/consumer_group.go:97 +0x48
[...]

This is the code lazily populating the version from the Go module info:
https://github.com/Shopify/sarama/blob/3f90bc71bae993d5cbb08dfbb06cd10a88969d42/version.go#L7-L20

Solution

I'm proposing to protect this global variable with a sync.Once

[dnwe]

@pior thanks!