add a Close() method to Backend

backend.go

@@ -14,6 +14,7 @@ type Backend interface {
 	SetLevel(Level)
 	Level() Level
 	Reopen() error
+	Close() error
 }
 
 //
@@ -95,6 +96,22 @@ func (b *FileBackend) SetFormatter(f *Formatter) {
 // Reopen closes and reopens the file it writes to. It should be used after log
 // rotation
 func (b *FileBackend) Reopen() error {
+	if err := b.Close(); err != nil {
+		return err
+	}
+
+	fd, err := os.OpenFile(b.filepath, os.O_WRONLY|os.O_APPEND|os.O_CREATE, 0644) //nolint: gosec
+	if err != nil {
+		return fmt.Errorf("Cannot open log file %s: %w", b.filepath, err)
+	}
+
+	b.l = fd
+
+	return nil
+}
+
+// Close closes the underlying file used by the backend
+func (b *FileBackend) Close() error {
 	if b.filepath == "" {
 		return nil
 	}
@@ -105,13 +122,6 @@ func (b *FileBackend) Reopen() error {
 		}
 	}
 
-	fd, err := os.OpenFile(b.filepath, os.O_WRONLY|os.O_APPEND|os.O_CREATE, 0644) //nolint: gosec
-	if err != nil {
-		return fmt.Errorf("Cannot open log file %s: %w", b.filepath, err)
-	}
-
-	b.l = fd
-
 	return nil
 }
 
@@ -147,3 +157,8 @@ func (nb *NoopBackend) Level() Level {
 func (nb *NoopBackend) Reopen() error {
 	return nil
 }
+
+// Close is a noop
+func (nb *NoopBackend) Close() error {
+	return nil
+}

backend_syslog_linux.go

@@ -78,6 +78,11 @@ func (sb *SyslogBackend) Reopen() error {
 	return nil
 }
 
+// Close closes the connection to the syslog daemon
+func (sb *SyslogBackend) Close() error {
+	return sb.w.Close()
+}
+
 var facilities = map[string]syslog.Priority{
 	"kern":     syslog.LOG_KERN,
 	"user":     syslog.LOG_USER,

logger.go

@@ -32,7 +32,6 @@ func (l *Logger) AddBackend(b Backend) {
 }
 
 // SetBackend sets the backend list to the logger. Any existing backend will be lost.
-// FIXME must close file backends to avoid fd leaks
 func (l *Logger) SetBackend(b ...Backend) {
 	l.backends = b
 }

logging.go

@@ -1,3 +1,21 @@
+// Package logging provides a multi-backend leveled logging facility.
+/*
+
+Backends
+
+package logging defines the following builtin backends:
+* NoopBackend, which discards all messages
+* FileBackend, which redirects logs to an io.Writer object. It has constructors
+  for files, stdout and stderr
+* SyslogBackend, only available in unix-like systems, writes log entryies to a
+  syslog daemon.
+
+A backend can safely be used by multiple loggers.
+
+It is the caller's responsability to call Close on backends when they are not
+used anymore to free their resources.
+
+*/
 package logging
 
 import (