SetLoggerWithOptions: support flushing

When setting a logger that buffers data in memory, we want klog.Flush to flush
that data. For that the caller has to provide an additional callback because
logr.Logger has no API for flushing.

contextual.go

@@ -42,6 +42,10 @@ var (
 	// none is available.
 	globalLogger *Logger
 
+	// globalLoggerOptions contains the options that were supplied for
+	// globalLogger.
+	globalLoggerOptions loggerOptions
+
 	// contextualLogger defines whether globalLogger may get called
 	// directly.
 	contextualLogger bool
@@ -87,11 +91,10 @@ func SetLogger(logger logr.Logger) {
 // later release.
 func SetLoggerWithOptions(logger logr.Logger, opts ...LoggerOption) {
 	globalLogger = &logger
-	var o loggerOptions
+	globalLoggerOptions = loggerOptions{}
 	for _, opt := range opts {
-		opt(&o)
+		opt(&globalLoggerOptions)
 	}
-	contextualLogger = o.contextualLogger
 }
 
 // ContextualLogger determines whether the logger passed to
@@ -108,6 +111,13 @@ func ContextualLogger(enabled bool) LoggerOption {
 	}
 }
 
+// FlushLogger provides a callback for flushing data buffered by the logger.
+func FlushLogger(flush func()) LoggerOption {
+	return func(o *loggerOptions) {
+		o.flush = flush
+	}
+}
+
 // LoggerOption implements the functional parameter paradigm for
 // SetLoggerWithOptions.
 //
@@ -119,6 +129,7 @@ type LoggerOption func(o *loggerOptions)
 
 type loggerOptions struct {
 	contextualLogger bool
+	flush            func()
 }
 
 // SetContextualLogger does the same as SetLogger, but in addition the
@@ -137,6 +148,7 @@ func SetContextualLogger(logger logr.Logger) {
 // goroutines invoke log calls, usually during program initialization.
 func ClearLogger() {
 	globalLogger = nil
+	globalLoggerOptions = loggerOptions{}
 }
 
 // EnableContextualLogging controls whether contextual logging is enabled.
@@ -196,7 +208,9 @@ func TODO() Logger {
 // Notice: This function is EXPERIMENTAL and may be changed or removed in a
 // later release.
 func Background() Logger {
-	if globalLogger != nil && contextualLogger {
+	if globalLoggerOptions.contextualLogger {
+		// Is non-nil because globalLoggerOptions.contextualLogger is
+		// only true if a logger was set.
 		return *globalLogger
 	}
 

contextual_test.go

@@ -43,3 +43,16 @@ func ExampleSetLogger() {
 	// logger after SetLoggerWithOptions with ContextualLogger(false): *klog.klogger
 	// logger after SetLoggerWithOptions with ContextualLogger(true): logr.discardLogSink
 }
+
+func ExampleFlushLogger() {
+	defer klog.ClearLogger()
+
+	// This simple logger doesn't need flushing, but others might.
+	klog.SetLoggerWithOptions(logr.Discard(), klog.FlushLogger(func() {
+		fmt.Print("flushing...")
+	}))
+	klog.Flush()
+
+	// Output:
+	// flushing...
+}

klog.go

@@ -1097,6 +1097,9 @@ func (l *loggingT) flushAll() {
 			file.Sync()  // ignore error
 		}
 	}
+	if globalLoggerOptions.flush != nil {
+		globalLoggerOptions.flush()
+	}
 }
 
 // CopyStandardLogTo arranges for messages written to the Go "log" package's