Split daemon's logging backends in separate modules

[nurupo]

• log_backend.h defines an interface for logging back-ends, which log_backend_[stdout|syslog].[h|c] implement. These implementations are then put together into a BACKENDS array in log.c, which is indexed by the LOG_BACKEND enum. Haven't really done any such modular implementations in C before, so any corrections are welcome.

• Renamed

  • open_log() --> log_open()
  • close_log() --> log_close()
  • write_log() --> log_write()

  Should probably have been a separate commit/PR, but I happened to do the renaming and modularization at the same time, so there we go.

I meant to make logging modular from the beginning, but at the time of writing it I thought that maybe it's simpler to keep all logging backends relatively separate in log.c as there is so little code, instead of scattering them in multiple files and adding this struct with function pointers glue, at least until there is more code for the logging that would warrant the splitting, but there was some confusion as to why I want to keep backends relatively separate

The part I don't understand is the "keeping backends separate". You still have explicit knowledge of every backend in this function. What is the advantage of having my_foo(...) instead of foo(...)?
-- @iphydf

, so this PR should remove this confusion now that the logging backends are truly separate.

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This change is 

[nurupo]

@Zer0-One says that I might be over-engineering it with this PR, as there is no reason for having more than two backends -- one for running in foreground (stdout) and one for the background (syslog).

I still think that putting stdout/syslog into separate files makes log.c a lot cleaner, and that having the backend abstraction is still useful even for just 2 backends.

If you really insist that it's over-engineering, I could compromise on keeping stdout and syslog bakends in separate files, but removing the the support for adding additional logging backends: remove log_backend.h which defines the common interface for backends, remove the open() and close() functions from log_backend_stdout.[c|h] as they are noops and with the backend interface gone we don't have to have them, and make log.c call the backend functions directly in switch-case on the LOG_BACKEND enum instead of using the BACKENDS array since there is no backend interface anymore.

[Zer0-One]

I like your second suggestion more, but IMO this is simple enough to keep within a single module.

[iphydf]

All of this can be moved to the .c file. It's used only once.

[iphydf]

Check the checkbox for collaborator pushes.

[nurupo]

Checked.

[iphydf]

I'm ok with this change. In fact, I really don't care about the bootstrap daemon's logging backend design. Are we going to merge this?

[nurupo]

Oh, totally forgot about this PR. I guess I have to modify the code to the compromise I have described in #487 (comment).

[nurupo]

@Zer0-One does the last commit look better? If so, I will squash the commits.

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[iphydf]

@nurupo travis is happy. Can you squash the commits?

[nurupo]

Done.

[iphydf]

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Review status: 0 of 11 files reviewed at latest revision, 1 unresolved discussion.

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