fix(cli) properly look for serf in known paths

[subnetmarco]

Summary

Our distributions put the serf executable in a known path at /usr/local/bin/serf. Adds this path as a known path (like we do for resty) when searching for Serf, if serf_path is not specified in the configuration file.

Full changelog

• Look for serf in known paths, if serf_path is not specified.

Issues resolved

Fix #1953

[thibaultcha]

This shouldn't override all of the other potential locations. Maybe it should be replaced with:

table.insert(serf_search_paths, 1, kong_config.serf_path)

[subnetmarco]

I thought about it - if I explicitly tell Kong to look into that path and it's not found, shouldn't an error be returned even if it's found somewhere else?

Does the serf_path property mean:

• search into serf_path in addition to the standard paths?
• look explicitly into serf_path and if it doesn't exist, return an error?

Seems like (2) would be a more expected behavior?

[thibaultcha]

(1) would be a more expected behavior, similar to the shell's $PATH but also to the Lua's LUA_PATH and LUA_CPATH properties, also configurable from the Kong configuration.

[thibaultcha]

It's also less error prone.

[Tieske]

in other cases we said that if the user specifies a value, then not being able to honor that value is an error. We've had this discussion before.

If I would specify an argument "xyz" and the application starts and ends up running with "123" instead, that would be bad imo.

so it ought to be the nr 2. My 2cts.

[thibaultcha]

This goes against all the other path variables in the shell and this project -_-