Logging

[mitchellh]

Logging is critical in my ability to debug things on Vagrant, and it wasn't added until like... 0.7. I don't want to make that same mistake with Packer. Let's put logging in RIGHT AWAY.

Key ideas:

• PACKER_LOG environmental variable for controlling verbosity.
• Multiple levels: debug, info, warn, etc. I don't actually care what they're named, just that they exist.
• Must be easily available to all parts of the packer program and libraries.

Thoughts on implementation:

• I think a global "Logger" like variable would be nice so ANY library component can hook into loggers.
• However, it might be nice to have one logger per packer Environment. But is an Environment necessary for using other components that might want to log? Probably not. Ugh.
• If we are running multiple packer builds in parallel, we'll want to be able to differentiate log output from the different builds.

[mitchellh]

For simplicity, I just decided to use the standard log package. In the future we can differentiate between log levels using Unix tools like "grep". Some key points on how things work:

• For packer, we'll configure log output depending on PACKER_LOG
• For packer/plugin, log output ALWAYS goes to STDERR. The plugin client reads this in and appends it to the main app log, so we can even get the log output of plugins. Hizzah!

[mitchellh]

Done? I think. WEE!