testenv: Allow klog and glog to coexist

This change allows test environments with glog to work with testenv.
The logger initialization is moved from init() to a separate function
that's called when a new testenv Environment is created. This allows a
user to set their custom logger to the controller-runtime logger before
creating testenv because the controller-runtime deferred logger can only
be set once.

Signed-off-by: Sunny <darkowlzz@protonmail.com>

runtime/testenv/testenv.go

@@ -25,6 +25,7 @@ package testenv
 
 import (
 	"context"
+	"flag"
 	"fmt"
 	"sync"
 	"time"
@@ -52,11 +53,23 @@ var (
 	env *envtest.Environment
 )
 
-func init() {
-	klog.InitFlags(nil)
+func initLogger() {
+	// NOTE: Set the klog flags on a separate FlagSet to avoid conflicting with
+	// any flags set by the application code. For example, an application
+	// importing glog would conflict with the flags set by klog, resulting in
+	// redeclaration of the same flags.
+	// This is based on klog's coexist with glog example, refer:
+	// https://github.com/kubernetes/klog/blob/f8e668dbaa5f6f0e6a5c24ffd7667263840d79ae/examples/coexist_glog/coexist_glog.go
+	// The klog flags set below are not available to the klogr logger below
+	// because they aren't set on the global FlagSet. To prevent conflicting
+	// with any flags registered later during the test setup, the flags are left
+	// empty. In the tests, a custom logger can be created with all the
+	// necessary flags and assigned to the controller-runtime logger explicitly
+	// before creating testenv Environment.
+	klogFlags := flag.NewFlagSet("klog", flag.ExitOnError)
+	klog.InitFlags(klogFlags)
 	logger := klogr.New()
 	log.SetLogger(logger)
-	ctrl.SetLogger(logger)
 }
 
 var (
@@ -118,7 +131,31 @@ func WithCRDPath(path ...string) Option {
 //
 // NOTE: This function should be called only once for each package you are running tests within, usually the environment
 // is initialised in a suite_test.go or <package>_test.go file within a `TestMain` function.
+//
+// When a testenv Environment is created, it initializes the
+// controller-runtime's deferred logger with a default logger based on klog. In
+// order to override this behavior, the controller-runtime logger can be
+// initialized before creating testenv Environment.
+//
+//  import (
+//      "testing"
+//
+//      "github.com/fluxcd/pkg/runtime/testenv"
+//      ctrl "sigs.k8s.io/controller-runtime"
+//      "sigs.k8s.io/controller-runtime/pkg/log/zap"
+//  }
+//
+//  func TestMain(m *testing.M) {
+//      zlog := zap.New(zap.UseDevMode(true))
+//      ctrl.SetLogger(zlog)
+//
+//      testEnv = testenv.New()
+//      ...
+//  }
+//
 func New(o ...Option) *Environment {
+	initLogger()
+
 	opts := options{}
 	for _, apply := range o {
 		apply(&opts)