Picasso.with() should be synchronized

[Macarse]

I was using Picasso 2.2.0 with OkHttp 1.3.0 and everything was working fine.
I updated OkHttp to 1.5.4 and I started seeing instances of square/okhttp#680

After researching a bit I noticed that I had two Picasso instances in my app generated by a race condition in the method that generates the Picasso singleton. I am using Picasso from the main thread but also from a background service to prefetch images.

Apparently I had this bug before but, as @swankjesse explains in square/okhttp#650, the exception was masked by the fact that the cleanup callable was being run with a Future.

[JakeWharton]

This should use double-checked locking. You're optimizing for the worst case instead of the best case.

[Macarse]

@JakeWharton:

  public static Picasso with(Context context) {
    if (singleton == null) {
      synchronized (Picasso.class) {
        if (singleton == null) {
          singleton = new Builder(context).build();
        }
      }
    }
    return singleton;
  }

[JakeWharton]

Yeah. That looks good! Then we skip the lock and the runtime branching strategy will favor the non-null case over time and the method call becomes almost free.

[Macarse]

@JakeWharton: Good point. I will create a second pull request with the fix.