Rare race condition in makedirs with parallel processes

[timj]

The cbook.makedirs code does not catch all the problems with multi-threading. We are seeing a very rare failure in the LSST test suite when multiple tests are using matplotlib in parallel. I think it is because the try/catch in makedirs is assuming that only the last part of the path could have the race and only does a single catch around the entire for loop. If both processes are racing to create path A/B/C and both notice that B is missing then the second one to attempt to create B will abort when it finds that B now exists. It should just continue on with trying to make C. I think the try/catch should be in the inner part of the loop around the os.makedirs call.

This is the error we get:

import matplotlib.colorbar
  File "/home/vagrant/stack/Linux64/anaconda/2.2.0/lib/python2.7/site-packages/matplotlib/colorbar.py", line 36, in <module>
    import matplotlib.contour as contour
  File "/home/vagrant/stack/Linux64/anaconda/2.2.0/lib/python2.7/site-packages/matplotlib/contour.py", line 27, in <module>
    import matplotlib.texmanager as texmanager
  File "/home/vagrant/stack/Linux64/anaconda/2.2.0/lib/python2.7/site-packages/matplotlib/texmanager.py", line 89, in <module>
    class TexManager:
  File "/home/vagrant/stack/Linux64/anaconda/2.2.0/lib/python2.7/site-packages/matplotlib/texmanager.py", line 124, in TexManager
    mkdirs(texcache)
  File "/home/vagrant/stack/Linux64/anaconda/2.2.0/lib/python2.7/site-packages/matplotlib/cbook.py", line 1003, in mkdirs
    os.makedirs(thispart, mode)
  File "/home/vagrant/stack/Linux64/anaconda/2.2.0/lib/python2.7/os.py", line 157, in makedirs
    mkdir(name, mode)
OSError: [Errno 17] File exists: '/tmp/matplotlib-vagrant'

with v1.4.3.

Originally filed as an addendum of #2046. @tacaswell requested I file this separately.

See the PR in #5224

[timj]

I think this ticket can be closed given that #5224 has been merged.