create unique log directory, avoiding race

[dirk-thomas]

Maybe addressing Fixes #129.

The patch ensures that the log directory doesn't exist yet. If it does a serial number is appended until it doesn't.

Without the patch two invocations started in the same second were writing to the same log directory.

[codecov-io]

Codecov Report

Merging #130 into master will increase coverage by 0.19%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #130      +/-   ##
==========================================
+ Coverage   78.37%   78.57%   +0.19%     
==========================================
  Files          51       51              
  Lines        2867     2884      +17     
  Branches      484      482       -2     
==========================================
+ Hits         2247     2266      +19     
+ Misses        594      592       -2     
  Partials       26       26

Impacted Files	Coverage Δ
colcon_core/location.py	100% <100%> (+2.7%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d7bc897...34f33d8. Read the comment docs.

[dirk-thomas]

Confirmed that it fixes #129: #129 (comment)