Lazy initialize UUID for Background context

[ibuildthecloud]

Fixes #782

This will lazy initialize the UUID using sync/once. Given that in the happy path of already initialized value, sync/once does a atomic.LoadUint32() instead of a full Lock(), I can't imagine this will have any real performance impact.

I don't really know under what context this is used so not totally sure if a tests should be added for this or if in general it should be obvious if this code works because of other tests.

[aaronlehmann]

This looks okay to me. I agree with you that it's better not to do UUID generation in init, and I doubt performance is the primary consideration here if we're using strings as keys for this lookup. But I'm curious to hear what @stevvooe thinks.

[noxiouz]

I think there's a chance to return an empty Id, as Once does not prevent other goroutines to return valur

[aaronlehmann]

That's an interesting point, but I think the code is actually correct. once.Do will block in other threads on the slow path mutex. The mutex unlock involves a memory barrier, so when Do returns in the other goroutines, the write to ic.id will be visible on those threads.

[noxiouz]

Oh, my bad. It prevents. Shame on me :(

[stevvooe]

@ibuildthecloud This looks fine. Move the once declaration to the struct and we should be good to merge.

[tiborvass]

Removed my approval, I also need this:

diff --git a/context/logger.go b/context/logger.go
index b0f0c50..78e4212 100644
--- a/context/logger.go
+++ b/context/logger.go
@@ -3,8 +3,6 @@ package context
 import (
    "fmt"

-   "github.com/docker/distribution/uuid"
-
    "github.com/Sirupsen/logrus"
 )

@@ -101,8 +99,3 @@ func getLogrusLogger(ctx Context, keys ...interface{}) *logrus.Entry {

    return logger.WithFields(fields)
 }
-
-func init() {
-   // inject a logger into the uuid library.
-   uuid.Loggerf = GetLogger(Background()).Warnf
-}

[tiborvass]

Ping @stevvooe @ibuildthecloud

[stevvooe]

Closing for #792.