fix: go-libp2p and slog interop

[lidel]

Problem

go-libp2p migrated from go-log to slog in

• feat: Migrate to log/slog libp2p/go-libp2p#3364

Unfortunately, the gologshim from that PR was not enough. go-libp2p. 0.44 breaks dynamic level control via SetLogLevel() which in turn broke things like ipfs log level and the Diagnostic screen from ipfs-webui (ipfs/ipfs-webui#2392) which allows users to dynamically adjust log level per system to investigate bugs.

Solution

This PR adds automatic slog integration that routes slog logs through go-log's zap core when SetupLogging() is called.

This restores runtime level adjustments for go-libp2p subsystems without requiring daemon restarts, making it possible to debug production issues by changing log levels on the fly.

The integration detects subsystem names via logger attributes and applies per-subsystem level control. Libraries can opt-in via duck typing to avoid adding go-log as a dependency (see go-libp2p's gologshim implementation).

Important

Original version of this PR based on turn-key runtime detection was rejected by go-libp2p maintainers, and instead go-libp2p 0.45 requires go-log users to manually set up wiring. See https://github.com/ipfs/go-log/releases/tag/v2.9.0 and libp2p/go-libp2p#3419 (comment)

Context

• This PR is prerequisite for addressing go-libp2p v0.44 logs not present in ipfs log tail kubo#11035
• This PR depends on go-libp2p to implement improved shim in feat(gologshim): Add SetDefaultHandler libp2p/go-libp2p#3418
• Kubo PR with end-to-end fixes and tests: fix([email protected]/[email protected]): dynamic log level control and tail kubo#11039

[gammazero]

This is brilliant!

[lidel]

Triage notes

• Pushed some extra docs
• Waiting for decision in Gologshim runtime controlled handler libp2p/go-libp2p#3419 (review) (does not impact code in this PR, but tbd if we need to remove extra docs)

[MarcoPolo]

I've not reviewed this PR. Linking this in case you hadn't seen it: https://pkg.go.dev/go.uber.org/zap/exp/zapslog.

[lidel]

@MarcoPolo thx, we've seen it, iirc ended up being not enough, thats why go-log has own opinionated slog bridge.

Update: found my notes from when i was researching our options here:

• Dynamic core support
  • go-log must preserves LIVE core reference (SAME core → pipe readers work)
  • zapslog approach - breaks LIVE reference
    • When libraries call WithAttrs() (like go-libp2p/gologshim does), zapslog would create a NEW wrapped core that loses connection to the multi-core. Dynamically-added pipe readers won't receive logs from these loggers, breaking things like on-demand ipfs log tail
• No per-subsystem level control / No "logger" attribute extraction
  • go-log's subsystemAwareHandler provides early atomic filtering for per-subsystem levels (ipfs log level libp2p debug). zapslog has no equivalent - it always delegates to the core's single level
  • go-log specially handles slog.String("logger", "subsystem") to enable dynamic level control. zapslog treats it as a regular attribute.

[lidel]

Since go-libp2p decided to not use automatic bridge described in libp2p/go-libp2p#3419 (review), and go-libp2p 0.45 (libp2p/go-libp2p#3424) will require every end application that uses both go-log and go-libp2p to explicitly wire them together in own code:

  import (
      logging "github.com/ipfs/go-log/v2"
      "github.com/libp2p/go-libp2p/gologshim"
  )

  func init() {
      // use the explicit go-log bridge      
      gologshim.SetDefaultHandler(golog.SlogHandler())
  }

.. I do not think it makes sense to set slog.Default in go-log's init() anymore. Aytomatic magic and touching global slog made sense ONLY if go-libp2p cooperated with go-log and leveraged automatic slog bridge, to fix users apps without asking them to do extra coding.

Now that every go-libp2p >=0.45 user has to have the above snippet in init() anyway, automatic slog bridge feels like diminishing returns. If user needs automatic slog bridging, they can add one line to override global slog:

+ slog.SetDefault(slog.New(golog.SlogHandler()))
gologshim.SetDefaultHandler(golog.SlogHandler())

I will keep GOLOG_CAPTURE_DEFAULT_SLOG but flip it to be disabled by default just to make the go-log release safer (it will not touch global slog.Default() anymore by default, making it safer for everyone to update).

Update: done in b453280

[lidel]

All green in

• fix([email protected]/[email protected]): dynamic log level control and tail kubo#11039
• feat(diagnostics/logs): include extended details ipfs-webui#2457

I'm going to merge this and then make go-log release to ensure it is available before go-libp2p 0.45 (libp2p/go-libp2p#3424) ships