worker/uniter: simplify EnsureTools and make it concurrent-safe

worker/uniter: simplify EnsureTools and make it concurrent-safe

If someone breaks the symlinks, the user "gets everything they deserve".
Avoiding the link reading logic makes it trivial for EnsureTools
to be idempotent, even when run concurrently.

Also fix the agent name to match our upstart convention
which prefixes the tools directory with the agent kind.

https://code.launchpad.net/~rogpeppe/juju-core/uniter-ensuretools-simplify/+merge/118748

(do not edit description out of merge proposal)

[rog, 2012-08-08 13:25:24]

Please take a look.

[rog, 2012-08-08 13:41:52]

Please take a look.

[fwereade, 2012-08-08 13:47:38]

LGTM

https://codereview.appspot.com/6461061/diff/4001/worker/uniter/tools.go
File worker/uniter/tools.go (right):

https://codereview.appspot.com/6461061/diff/4001/worker/uniter/tools.go#newco...
worker/uniter/tools.go:25: // TODO(rog) use os.IsExist when fix is released.
Maybe a bug link, for when someone who doesn't know that whereof you speak comes
across this?

https://codereview.appspot.com/6461061/diff/4001/worker/uniter/tools.go#newco...
worker/uniter/tools.go:34: return "unit-" + unitFsName(unitName)
Ah, cool, hadn't picked up on that. Thanks.

[rog, 2012-08-08 13:55:30]

Please take a look.

https://codereview.appspot.com/6461061/diff/4001/worker/uniter/tools.go
File worker/uniter/tools.go (right):

https://codereview.appspot.com/6461061/diff/4001/worker/uniter/tools.go#newco...
worker/uniter/tools.go:25: // TODO(rog) use os.IsExist when fix is released.
On 2012/08/08 13:47:38, fwereade wrote:
> Maybe a bug link, for when someone who doesn't know that whereof you speak
comes
> across this?

Aood point. the fix is already in tip, BTW, and will probably be released with
Go 1.0.3. Actually there's no bug, but I can reference the CL that fixed this
issue.

[niemeyer, 2012-08-08 13:58:41]

LGTM

[rog, 2012-08-08 14:01:19]

*** Submitted:

worker/uniter: simplify EnsureTools and make it concurrent-safe

If someone breaks the symlinks, the user "gets everything they deserve".
Avoiding the link reading logic makes it trivial for EnsureTools
to be idempotent, even when run concurrently.

Also fix the agent name to match our upstart convention
which prefixes the tools directory with the agent kind.

R=fwereade, niemeyer
CC=
https://codereview.appspot.com/6461061