build: fix data race in RotatingLogWriter during startup

build/logrotator.go

@@ -2,10 +2,12 @@ package build
 
 import (
 	"compress/gzip"
+	"errors"
 	"fmt"
 	"io"
 	"os"
 	"path/filepath"
+	"sync"
 
 	"github.com/jrick/logrotate/rotator"
 	"github.com/klauspost/compress/zstd"
@@ -18,6 +20,13 @@ type RotatingLogWriter struct {
 	pipe *io.PipeWriter
 
 	rotator *rotator.Rotator
+
+	// wg tracks the rotator Run goroutine so that Close can wait for it
+	// to finish before closing the underlying rotator.
+	wg sync.WaitGroup
+
+	// mu protects the pipe field from concurrent access.
+	mu sync.RWMutex
 }
 
 // NewRotatingLogWriter creates a new file rotating log writer.
@@ -73,30 +82,50 @@ func (r *RotatingLogWriter) InitLogRotator(cfg *FileLoggerConfig,
 	// runtime (like running out of disk space or not being allowed to
 	// create a new logfile for whatever reason).
 	pr, pw := io.Pipe()
+
+	r.mu.Lock()
+	r.pipe = pw
+	r.mu.Unlock()
+	r.wg.Add(1)
 	go func() {
+		defer r.wg.Done()
 		err := r.rotator.Run(pr)
-		if err != nil {
+		if err != nil && !errors.Is(err, io.EOF) {
 			_, _ = fmt.Fprintf(os.Stderr,
 				"failed to run file rotator: %v\n", err)
 		}
 	}()
 
-	r.pipe = pw
-
 	return nil
 }
 
 // Write writes the byte slice to the log rotator, if present.
 func (r *RotatingLogWriter) Write(b []byte) (int, error) {
-	if r.rotator != nil {
-		return r.rotator.Write(b)
+	r.mu.RLock()
+	pipe := r.pipe
+	r.mu.RUnlock()
+
+	if pipe != nil {
+		return pipe.Write(b)
 	}
 
 	return len(b), nil
 }
 
 // Close closes the underlying log rotator if it has already been created.
 func (r *RotatingLogWriter) Close() error {
+	r.mu.Lock()
+	pipe := r.pipe
+	r.pipe = nil
+	r.mu.Unlock()
+
+	if pipe != nil {
+		if err := pipe.Close(); err != nil {
+			return err
+		}
+		r.wg.Wait()
+	}
+
 	if r.rotator != nil {
 		return r.rotator.Close()
 	}

docs/release-notes/release-notes-0.21.0.md

@@ -113,6 +113,12 @@
   channel arbitrator suppresses the duplicate event that would otherwise be
   emitted from `MarkChannelClosed` at the final confirmation depth.
 
+* [Fixed a data race in the `RotatingLogWriter`](https://github.com/lightningnetwork/lnd/pull/10886)
+  during startup. The `pipe` field was being read concurrently by `Write` while
+  being written by `InitLogRotator` and `Close`, which could trigger a race
+  condition. A `sync.RWMutex` is now used to synchronize access to the `pipe`
+  field, and `Close` sets `pipe` to `nil` under the lock to ensure idempotency.
+
 # New Features
 
 - [Basic Support](https://github.com/lightningnetwork/lnd/pull/9868) for onion