Default provider path is hard-coded rather than using homebrew directory on OS X

[ismarc]

Summary

When a different install location for homebrew is used besides the default (such as /Users/<user>/brew), brew installed providers are not found unless the full path to the installation location is provided (summon -p /Users/<user>/brew/lib/summon/summon-conjur vs summon -p summon-conjur). This makes switching between different providers annoying and non-obvious.

Steps to Reproduce

Steps to reproduce the behavior:

1. Extract https://github.com/Homebrew/brew/tarball/master in a different location than default
2. Add extracted location to path
3. install summon and summon-conjur using brew
4. Attempt to run summon with the summon-conjur provider with summon -p summon-conjur --yaml 'FOO: !var path/to/var'

Expected Results

summon is able to find the summon-conjur provider

Actual Results (including error logs, if applicable)

stat /usr/local/lib/summon/summon-conjur: no such file or directory

Reproducible

• Always
• Sometimes
• Non-Reproducible

Version/Tag number

0.8.4

Environment setup

OS X Catalina 10.15.7 with highly restrictive permissions on the system.

Additional Information

/usr/local/lib/summon/ is hardcoded as the default provider path (which is the default homebrew install location). Homebrew does not use environment variables to determine where packages/it is installed, instead it determines the location based on where the brew binary is located. Because of this, I would suggest this be resolved by having an environment like SUMMON_DEFAULT_PROVIDER_DIR environment variable or similar so that it can be defined and then the short provider names used.