BuildStream misorders elements in subdirectories of symlinks when link_files called in compose elements

Summary

Occasionally in compose elements BuildStream will miss a subdirectory of a symlink when hardlinking files across. This is related to split rules (I think). This causes the build to fail, as it attempts to link a file into a non-existent directory.

Here is the order which some files are passed to safe_link from link_files:

usr/etc                             # directory                                                                  
usr/etc/OpenCL                      # symlink to /usr/lib/GL/OpenCL/                                                               
usr/etc/OpenCL/vendors/mesa.icd     # file (technically in /usr/lib/GL/OpenCL
...
...
usr/lib/GL/OpenCL
usr/lib/GL/OpenCL/vendors

This seems to be an issue in how sorting the files works.

Steps to reproduce

This behaviour happened in these pipelines in freedesktop-sdk.

Beyond rerunning these pipelines I am unsure on how to reproduce.

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.2

• Issue on freedesktop-sdk potentially blocked by this: freedesktop-sdk/freedesktop-sdk#333 (closed)

---