In version 0.36 and earlier, Fromager supports only a single constraints file. The option --constraints-file is single value. It would be useful to support multiple constraints files. In downstream, we have variant-specific constraints and product-specific constraints for all product variants. For example, we have to constrain some packages for all variants, because latest version of the package does not build for us. Support for multiple constraints files would allow us to move common constraints for all variants into a shared file.

Implementation

Let's start with a simple implementation:

• all constraints file must exist and be readable
• all constraints are combined into a single constraints set
• a package can only be constraint once across all constraints files. The second occurrence of a package raises an error.

The implementation should cover majority of use cases. We can look into more complex ways to deal with multiple constraints if the need arises.

Env vars and URLs

Fromager uses click to parse command line arguments. Click also supports env vars, e.g. --constraints-file can also be set with FROMAGER_CONSTRAINTS_FILE env var. Env vars are a common way to pass settings into containers. Click uses different rules to split multiple values from env vars. Path-like types are split by os.pathsep (: like PATH vars) and other values are split by white space.

A while ago --constraints-file was switched from path-like to str type to support https:// URLs for remote constraints files. We cannot switch --constraints-file to a type=str, multiple=True option. This would break cases like FROMAGER_CONSTRAINTS_FILE="/home/user name/constraints.txt". Click would split the env var into two items /home/user and name/constraints.txt.

I have considered and experimented several approaches how to reconcile local paths, remote URLs, and env vars. My preferred approach is:

• change --constraints-file to multi-value option that accepts only local paths. FROMAGER_CONSTRAINTS_FILE splits on colon ':'.
• introduce second option --constraints-url that accepts multiple remote URLs. FROMAGER_CONSTRAINTS_URL splits on white space ' '.