[bug]: Data Race in Log Rotator During Startup

[Euler-B]

Pre-Submission Checklist

• I have searched the existing issues and believe this is a new bug.
• I am not asking a question about how to use lnd, but reporting a bug (otherwise open a discussion).

---

LND Version

v0.20.99-beta-f21262dd6

---

LND Configuration

Minimal configuration for simnet with btcd backend (no lnd.conf file used, all parameters passed via command line):

./lnd-race \
  --bitcoin.simnet \
  --bitcoin.node=btcd \
  --btcd.rpcuser=devuser \
  --btcd.rpcpass=devpass \
  --btcd.rpchost=127.0.0.1:18556 \
  --btcd.rpccert=/btcd-data/rpc.cert \
  --lnddir=/lnd-data \
  --noseedbackup \
  --debuglevel=info \
  --rpclisten=127.0.0.1:10009 \
  --listen=127.0.0.1:9735

---

Backend Version

btcd version 0.26.0-betarc1

---

Backend Configuration

btcd started with minimal simnet configuration:

btcd --simnet \
  --datadir=/btcd-data \
  --logdir=/btcd-data/logs \
  --rpcuser=devuser \
  --rpcpass=devpass \
  --rpclisten=127.0.0.1:18556 \
  --listen=127.0.0.1:18555 \
  --rpccert=/btcd-data/rpc.cert \
  --rpckey=/btcd-data/rpc.key \
  --txindex \
  --debuglevel=info

---

OS/Distribution

6.12.91-1-MANJARO x86_64 GNU/Linux

Bug Details & Steps to Reproduce

Description

A data race occurs in the log rotator during lnd startup. The race is between:

1. The main goroutine writing logs via RotatingLogWriter.Write()
2. A background goroutine running rotator.Run() that reads from the same rotator instance

Steps to Reproduce

1. Start btcd in simnet mode:

   btcd --simnet \
     --datadir=/btcd-data \
     --logdir=/btcd-data/logs \
     --rpcuser=devuser \
     --rpcpass=devpass \
     --rpclisten=127.0.0.1:18556 \
     --listen=127.0.0.1:18555 \
     --rpccert=/btcd-data/rpc.cert \
     --rpckey=/btcd-data/rpc.key \
     --txindex \
     --debuglevel=info

2. Build lnd with race detector:

   go build -race -o lnd-race ./cmd/lnd

3. Start lnd with race detector in simnet mode:

   ./lnd-race \
     --bitcoin.simnet \
     --bitcoin.node=btcd \
     --btcd.rpcuser=devuser \
     --btcd.rpcpass=devpass \
     --btcd.rpchost=127.0.0.1:18556 \
     --btcd.rpccert=/btcd-data/rpc.cert \
     --lnddir=/lnd-data \
     --noseedbackup \
     --debuglevel=info \
     --rpclisten=127.0.0.1:10009 \
     --listen=127.0.0.1:9735

4. Observe data race warning immediately during startup (no shutdown required)

---

Expected Behavior

The log rotator should be thread-safe, allowing concurrent writes from multiple goroutines without triggering data race warnings when built with -race.

---

Debug Information

==================
WARNING: DATA RACE
Write at 0x00c000723650 by main goroutine:
  github.com/jrick/logrotate/rotator.(*Rotator).Write()
      /home/euler/go/pkg/mod/github.com/jrick/logrotate@v1.1.2/rotator/rotator.go:160 +0x85
  github.com/lightningnetwork/lnd/build.(*RotatingLogWriter).Write()
      /home/euler/Projects/Golang/lnd/build/logrotator.go:92 +0x77
  github.com/btcsuite/btclog/v2.(*DefaultHandler).Handle()
      /home/euler/go/pkg/mod/github.com/btcsuite/btclog/v2@v2.0.1-0.20250728225537-6090e87c6c5b/handler.go:252 +0xf4b
  github.com/lightningnetwork/lnd/build.(*handlerSet).Handle()
      /home/euler/Projects/Golang/lnd/build/handler_sets.go:47 +0x14c
  log/slog.(*Logger).log()
      /usr/lib/go/src/log/slog/logger.go:256 +0x2b6
  log/slog.(*Logger).Log()
      /usr/lib/go/src/log/slog/logger.go:189 +0x156
  github.com/btcsuite/btclog/v2.(*sLogger).Warnf()
      /home/euler/go/pkg/mod/github.com/btcsuite/btclog/v2@v2.0.1-0.20250728225537-6090e87c6c5b/log.go:117 +0xb5
  github.com/lightningnetwork/lnd/build.(*ShutdownLogger).Warnf()
      <autogenerated>:1 +0x75
  github.com/lightningnetwork/lnd.LoadConfig()
      /home/euler/Projects/Golang/lnd/config.go:943 +0xc26
  main.main()
      /home/euler/Projects/Golang/lnd/cmd/lnd/main.go:22 +0x17a

Previous read at 0x00c000723650 by goroutine 28:
  github.com/jrick/logrotate/rotator.(*Rotator).Run()
      /home/euler/go/pkg/mod/github.com/jrick/logrotate@v1.1.2/rotator/rotator.go:118 +0x1bb
  github.com/lightningnetwork/lnd/build.(*RotatingLogWriter).InitLogRotator.func1()
      /home/euler/Projects/Golang/lnd/build/logrotator.go:77 +0x57

Goroutine 28 (running) created at:
  github.com/lightningnetwork/lnd/build.(*RotatingLogWriter).InitLogRotator()
      /home/euler/Projects/Golang/lnd/build/logrotator.go:76 +0x8f2
  github.com/lightningnetwork/lnd.ValidateConfig()
      /home/euler/Projects/Golang/lnd/config.go:1552 +0x8073
  github.com/lightningnetwork/lnd.LoadConfig()
      /home/euler/Projects/Golang/lnd/config.go:913 +0xa99
  main.main()
      /home/euler/Projects/Golang/lnd/cmd/lnd/main.go:22 +0x17a
==================

Environment

• Network: simnet (private simulation network)
• Container: None (running directly on host)
• Other applications: None (clean environment)

---

Note: This is a different data race from the one reported in issue #6137 ("signal: data-race during shutdown"). While issue #6137 involves the signal package's logger during shutdown, this issue involves the log rotator during startup and is consistently reproducible.