trial should handle concurrent usage in the same directory gracefully

[exarkun]

Currently a second trial process started in the same directory as a first will destroy the first's _trial_temp directory causing most or all of the remaining tests run by the first process to fail. trial should handle this case without breaking things in this manner.

[exarkun]

(In [24287]) Branching to 'concurrent-trial_temp-2338'

[exarkun]

okay, maybe that's done.

build results:  http://buildbot.twistedmatrix.com/boxes-supported?branch=/branches/concurrent-trial_temp-2338

[therve]

Looks good. One minor comment:

+ testdir = base + '-' + str(counter)

"%s-%s" would make that more readable.

Thanks!

[exarkun]

(In [24320]) Use interpolation instead of concatenation to create the temp dir name

refs #2338

[exarkun]

(In [24321]) avoid the repeated stringification

refs #2338

[exarkun]

(In [24325]) Merge concurrent-trial_temp-2338

Author: exarkun Reviewer: therve Fixes: #2338

Add a lock to the _trial_temp directory so that different test processes don't try to use the same directory at the same time. If no temporary directory is specified, a different default will silently be selected if the preferred default is found to be in use. If a temporary directory is specified and found to be in use, an error will be reported.