Please run your tests with -race

[Tim Penhey]

Thanks Dave for this great write up.

In order to drive the data races to zero, and make a soon to be added CI
test as voting, I propose that as folks are writing tests in packages,
races in at least that file, an possibly that package should be fixed.

Tim

On 21/05/15 12:12, David Cheney wrote:
> Hello,
> 
> TL;DR - juju has lots of problems with data races, please test your
> code with the -race flag to ensure it doesn't get worse while we try
> to fix the problem.
> 
> Longer version:
> 
> In debugging https://bugs.launchpad.net/bugs/1456398 I found that
> there are multiple data races in the Juju code base. It's been long
> suspected that the test's are racy, PatchValue is super easy to
> introduce a race if all the workers started by a test have not exited
> before the suite's tear down function runs.
> 
> However, more serious races have been discovered, such as
> https://bugs.launchpad.net/bugs/1456857 which affects code all the way
> back to 1.22.
> 
> Why is a data race bad ?
> 
> Ok, so you're looking at https://bugs.launchpad.net/bugs/1456857 and
> you're thinking, so maybe the tls code accidentally uses the wrong
> certificate for a little bit, how bad is that?
> 
> The problem is data races affect the integrity of the structures that
> the garbage collector uses. In the example above replacing the
> certificate means one CPU can see the new value, and another
> potentially the old value. When it comes to to run the gc, depending
> on which CPU walks that chain of pointers it may think that the old
> certificate is still live, or the new certificate is unreachable --
> and boom that memory is marked as free and the certificate corrupted.
> 
> The short version is: there are no safe data races, and Juju is not
> reliable until they have all been fixed.
> 
> How to run the race detector ?
> 
> The race detector comes with Go and is available by adding the -race
> flag to invocations of go test, so what was
> 
>     go test github.com/juju/juju/...
> 
> becomes
> 
>     go test -race github.com/juju/juju/...
> 
> The downside of this is the race detector has significant overhead, at
> least 2x, so tests will be even slower.
> 
> Thanks
> 
> Dave
>