testing: print mongod log messages

testing: print mongod log messages

If mongodb dies unexpectedly, the test just hangs,
which isn't easy to diagnose. This CL prints the
last few lines of mongod output when that happens.

https://code.launchpad.net/~rogpeppe/juju-core/181-show-mgo-log/+merge/140659

(do not edit description out of merge proposal)

[rog, 2012-12-19 13:06:56]

Please take a look.

[jameinel, 2012-12-20 06:10:44]

I'm happy to say that this does help '-gocheck.vv'. It lets us see that mongo
stopped, and why it stopped.
It seems like the test suite shouldn't hang, but this is certainly much better
than what we had.

LGTM

https://codereview.appspot.com/6966045/diff/2001/testing/mgo.go
File testing/mgo.go (right):

https://codereview.appspot.com/6966045/diff/2001/testing/mgo.go#newcode91
testing/mgo.go:91: }()
This feels big enough to move it into a separate function, rather than a local
func().

[fwereade, 2012-12-21 10:22:25]

LGTM pending explanation (maybe as a comment :))

https://codereview.appspot.com/6966045/diff/2001/testing/mgo.go
File testing/mgo.go (right):

https://codereview.appspot.com/6966045/diff/2001/testing/mgo.go#newcode92
testing/mgo.go:92: mgoExited = exited
I think the above looks pretty comfortable where it is, but I'm not quite sure
what the deal is here -- why can't we just set mgoExited directly above?

[rog, 2012-12-21 12:23:42]

https://codereview.appspot.com/6966045/diff/2001/testing/mgo.go
File testing/mgo.go (right):

https://codereview.appspot.com/6966045/diff/2001/testing/mgo.go#newcode92
testing/mgo.go:92: mgoExited = exited
On 2012/12/21 10:22:25, fwereade wrote:
> I think the above looks pretty comfortable where it is, but I'm not quite sure
> what the deal is here -- why can't we just set mgoExited directly above?

it's defensive programming against any possible race.
because we know that the goroutine is running asynchronously,
it means that even if we call startMgoServer when
the goroutine is still around, there's no possibility
that the old goroutine can be referring to the new
value of mgoExited.

in fact, it's actually not a problem because we always
call destroyMgoServer (which waits on mgoExited) before
calling startMgoServer, but that's less obviously correct.

[rog, 2013-01-14 10:49:23]

*** Submitted:

testing: print mongod log messages

If mongodb dies unexpectedly, the test just hangs,
which isn't easy to diagnose. This CL prints the
last few lines of mongod output when that happens.

R=jameinel, fwereade
CC=
https://codereview.appspot.com/6966045