Logging controls #30486

Categories in the logging system currently work as follows:

• You can set a global log level (defaulting to debug), to trace to enable trace logs in all (enabled) categories (-loglevel=trace), or to info to disable debug logs even enabled categories (-debug=all -loglevel=info will still leave debugging disabled)
• You can override the global log level on a per-category basis, either making it more verbose (override info to debug or trace, or override debug to trace), or to make it less verbose (override a global default of debug or trace to a category specific level of info or debug), eg -loglevel=walletdb:trace -loglevel=net:info
• The log levels are only relevant for "enabled" categories (eg, you have to specify -debug=walletdb -loglevel=walletdb:trace to enable wallet db tracing)
• Enabling/disabling categories can be done at startup via the -debug option, or at runtime via the logging RPC
• Setting the log level is currently only possible at startup via the (currently hidden) -loglevel option

Is this behaviour worth preserving, or can we simplify it? I think it would be simpler and almost as powerful if we set things up as:

• Every category has a log level, that can be either info (default), debug, or trace (trace gives you everything, debug gives you everything but LogTrace(), and info skips both LogDebug() and LogTrace())
• We add a new -trace config option that sets a category's log level to trace, analogously to -debug (so you would just specify -trace=walletdb for wallet db tracing.
• We add an optional third argument to the logging rpc that accepts a list of categories to enable trace logging
• We support -debug=all or -trace=all to do all categories at once, and -debug=all -debugexclude=leveldb as usual to set all categories to debug but one left at info, or -trace=all -debug=walletdb to trace all categories but one left at debug (which offer similar behaviour to -debug=all -loglevel=debug -loglevel=leveldb:info or -debug=all -loglevel=trace -loglevel=walletdb:debug)

(Looking for concept acks/feedback)

I think this issue describes the problems with current logging controls very well but a better solution is possible than the one that's proposed here and implemented in #34038 (with some differences and additions). I implemented an alternate approach in #35387, and since AJ suggested discussing the approaches here, the problems I see with #34038 are that it is:

• Complicated. Users need to understand three separate options (-debug, -trace, -debugexclude) and their nontrivial interactions to configure logging, when a single -loglevel option can cover everything.

• Confusing. -trace=net enables debug and trace logging for net (trace implies debug), even though the name suggests it only enables trace. -debugexclude=net disables both debug and trace, even though the name only mentions debug.

• Ambiguous. When -debug and -trace apply to the same category, trace wins and the debug setting is ignored. This isn't apparent from the option names, and the evaluation order (debug args first, then trace, then exclude) isn't something you'd guess.

• Incomplete. There's no way to set trace as the default log level while keeping specific categories at debug. "Trace everything but debug for net" is a normal configuration to want, and in any standard logging framework it's something like -loglevel=trace,net:debug. With these three options it's not possible, because -trace=all clears all per-category level overrides, so any -debug=<cat> setting is ignored. This issue itself lists -trace=all -debug=walletdb as a supported use case, but it doesn't work in the implementation.

• Incompatible. The logging RPC output format changes from {category: bool} to three arrays (excluded, debug, trace).

• Awkward. To check what level a category is logged at, you have to search three arrays instead of doing a map lookup. To change a level, you place the category into an array instead of assigning it a level directly.

• Unusual. Assigning log levels to categories is the norm across logging frameworks. The include/exclude/trace triple with its evaluation ordering rules won't match what anyone coming from other logging contexts would expect.

In #35387 I fixed -loglevel to work without requiring -debug, and added a loglevel RPC. That gives you one option that covers all logging configuration:

bitcoind -loglevel=trace,net:debug,walletdb:info
bitcoin-cli -named loglevel trace net=debug walletdb=info

The loglevel RPC returns a flat map ({"net": "debug", "walletdb": "trace", ...}). The existing -debug, -debugexclude, and logging RPC all continue to work unchanged. #35387 is fully backwards compatible in a way #34038 is not.

I recognize I'm not neutral here since I wrote the alternative. If there are problems with #35387 I haven't seen, I'm interested to hear them.