Calls to os.Exit or log.Fatal and friends made in function with defer statements do not execute those statements if control reaches the os.Exit or log.Fatal and friends.

If the os.Exit function happens in a goroutine, all the goroutines (including the main one) will terminate immediately without executing any deferred statement. Internally, log.Fatal and friends (log.Fatalln and log.Fatalf) internally uses os.Exit(1), which results in the immediate termination of the program without executing the deferred statements.

It is recommended to handle such cases gracefully so that deferred statements are executed to necessary operations such as closing open file descriptors, cleanup, etc.

Bad practice

defer os.Remove(filename)
if bad {
    log.Fatalf("something bad happened")
}

Recommended

defer os.Remove(filename)
if bad {
    log.Printf("something bad happened")
    return // exits with "exit status 0"
}

retcode := 0
defer func() { os.Exit(retcode) }
defer os.Remove(filename)
if bad {
    log.Printf("something bad happened")
    retcode = 1
    return // exits with "exit status 1" because of topmost `defer` call
}

References

• os.Exit
• log.Fatal