log: fix side effects when importing rclone as a library

Avoid side effects by using own logger instance

- Importing fs/log only sets rclone's private logger via fs.SetLogger,
  so internal rclone logging works from the moment the package is
  imported but the process-wide slog default is left untouched.

- slog.SetDefault and slog.SetLogLoggerLevel move into InitLogging,
  which is called explicitly from the CLI (cmd/cmd.go), the librclone
  wrapper and the integration test framework. So rclone-as-a-program
  keeps capturing log.Print/log.Fatal and slog.Default() output as
  before.

Library consumers that import fs/log without calling InitLogging now
keep their own slog default and can safely route rclone output back
into it via log.Handler.SetOutput without recursing.

Fixes #8907

Co-authored-by: Nick Craig-Wood <nick@craig-wood.com>

fs/log.go

@@ -12,6 +12,10 @@ import (
 	"github.com/rclone/rclone/lib/caller"
 )
 
+// logger represents the slog logging facility and should be overridden by
+// the fs/log handling code.
+var logger *slog.Logger = slog.Default()
+
 // LogLevel describes rclone's logs.  These are a subset of the syslog log levels.
 type LogLevel = Enum[logLevelChoices]
 
@@ -137,7 +141,7 @@ func LogLevelToSlog(level LogLevel) slog.Level {
 }
 
 func logSlog(level LogLevel, text string, attrs []any) {
-	slog.Log(context.Background(), LogLevelToSlog(level), text, attrs...)
+	logger.Log(context.Background(), LogLevelToSlog(level), text, attrs...)
 }
 
 func logSlogWithObject(level LogLevel, o any, text string, attrs []any) {
@@ -337,3 +341,8 @@ func PrettyPrint(in any, label string, level LogLevel) {
 	}
 	LogPrintf(level, label, "\n%s\n", string(inBytes))
 }
+
+// SetLogger overrides the slog logger using the specified handler
+func SetLogger(h slog.Handler) {
+	logger = slog.New(h)
+}