Clarify whether nested output directories are permitted

[jmillikin-stripe]

The comments for Command.output_files state that an output file cannot be the parent of another output file, but there is not a corresponding comment for directories.

Please add a comment to clarify which, if any, of these output layouts are valid:

Output directory is child of another output directory:

output_directories: "upper"
output_directories: "upper/lower"

Output file is child of output directory:

output_directories: "upper"
output_files: "upper/lower"

Output directory is child of output file (possible if file is a symlink)

output_files: "upper"
output_directories: "upper/lower"

[ola-rozenfeld]

Anything not explicitly disallowed is allowed:

Output directory/file is child of another output directory: allowed.
Output directory is child of output file: not possible and will not work, because then the output file is not actually file, which will cause a FAILED_PRECONDITION error. If we expect an output to be a symlink to a directory, it should be specified as an output directory, not an output file.

[ola-rozenfeld]

... and perhaps it's worth it to clarify this point in the proto :-)
Will do.

[jmillikin-stripe]

Output directory is child of output file: not possible and will not work, because then the output file is not actually file, which will cause a FAILED_PRECONDITION error. If we expect an output to be a symlink to a directory, it should be specified as an output directory, not an output file.

Does this mean that execution servers that preserve symlinks need to test the type of the symlink target? I was previously assuming symlinks were uploaded as-is. If the server is expected to validate the target also, can you add a comment for that?

[ola-rozenfeld]

If the server is expected to validate the target also, can you add a comment for that?

Done. (#74)
Also, in general I would like to remove the explicit type specification of outputs by the client in V3 (#75).