New flag --alternate-lua-dir does not account for falco_outputs Lua files

[leodido]

Describe the bug

When running Falco (0.26.1) without using the new --alternate-lua-dir flag (introduced by #1419), Falco is unable to start.

How to reproduce it

Simply run Falco.

Expected behavior

Falco to run successfully.

Screenshots

 sudo ./build/userspace/falco/falco -c falco.yaml -r rules/falco_rules.yaml                                                               0.06  
Mon Oct 12 09:39:23 2020: Falco version 0.26.1-1+8b56360f (driver version 2aa88dcf6243982697811df4c1b484bcbe9488a2)
Mon Oct 12 09:39:23 2020: Falco initialized with configuration file falco.yaml
Mon Oct 12 09:39:23 2020: Loading rules from file rules/falco_rules.yaml:
Mon Oct 12 09:39:23 2020: Runtime error: Could not find Falco Lua entrypoint (tried /usr/share/falco/lua/output.lua, /home/vagrant/workspace/falcosecurity/falco/userspace/engine/lua/output.lua). Exiting.

Additional context

After a bit of investigation, I found the problem.

When --alternate-lua-dir is not specified, it gets the value of FALCO_ENGINE_SOURCE_LUA_DIR constant (its default).

This directory is used by the falco_engine ctor and by the falco_outputs::init.

The problem is that the falco_outputs::init was using FALCO_SOURCE_LUA_DIR (before the PR introducing this flag) by default while now, in the particular case described into this issue, it uses FALCO_ENGINE_SOURCE_LUA_DIR.

Compare line 91/92 in the aforementioned PR to spot the exact location of the problem.

The current build system does not move output.lua into FALCO_ENGINE_SOURCE_LUA_DIR and so Falco is not able to find it.

NOTA BENE

Please notice this does not happen with packaged Falco versions (DEB, etc.) because the build system takes care to move all the Lua files in one directory, in this case.

SOLUTIONS

1. Introduce another --alternate-lua-dir (one for the engine Lua files, one for the outputs Lua files)
2. Wait for PR Falco outputs refactoring #1412 to be merged in (should solve the problem at its root - ie., remove Lua files for outputs).

[leodido]

While reviewing #1412 and talking with its author (@leogr) we also found this is preventing the regression/integration test suite to actually run.

We're sending a fix ASAP