runtime/controller: add flags to configure exponential back-off retry

Add two new flags to enable users to configure exponential
back-off for Flux objects. The default values are now
set to 750ms for minimum retry time, and 15min for max.

Signed-off-by: Paulo Gomes <paulo.gomes@weave.works>

runtime/controller/rate_limiter.go

@@ -0,0 +1,62 @@
+/*
+Copyright 2022 The Flux authors
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package controller
+
+import (
+	"flag"
+	"time"
+
+	"github.com/spf13/pflag"
+	"k8s.io/client-go/util/workqueue"
+	"sigs.k8s.io/controller-runtime/pkg/ratelimiter"
+)
+
+const (
+	defaultMinRetryDelay = 750 * time.Millisecond
+	defaultMaxRetryDelay = 15 * time.Minute
+	flagMinRetryDelay    = "min-retry-delay"
+	flagMaxRetryDelay    = "max-retry-delay"
+)
+
+// RateLimiterOptions defines the configurable options for rate limiters
+// used on reconcilers.
+type RateLimiterOptions struct {
+	// MinRetryDelay represents the minimum amount of time in which an
+	// object being reconciled will have to wait before a retry.
+	MinRetryDelay time.Duration
+
+	// MaxRetryDelay represents the maximum amount of time in which an
+	// object being reconciled will have to wait before a retry.
+	MaxRetryDelay time.Duration
+}
+
+// BindFlags will parse the given pflag.FlagSet for the controller and
+// set the RateLimiterOptions accordingly.
+func (o *RateLimiterOptions) BindFlags(fs *pflag.FlagSet) {
+	flag.DurationVar(&o.MinRetryDelay, flagMinRetryDelay, defaultMinRetryDelay,
+		"The minimum amount of time for which an object being reconciled will have to wait before a retry.")
+	flag.DurationVar(&o.MaxRetryDelay, flagMaxRetryDelay, defaultMaxRetryDelay,
+		"The maximum amount of time for which an object being reconciled will have to wait before a retry.")
+}
+
+// GetRateLimiter returns a new exponential failure ratelimiter.RateLimiter
+// based on RateLimiterOptions.
+func GetRateLimiter(opts RateLimiterOptions) ratelimiter.RateLimiter {
+	return workqueue.NewItemExponentialFailureRateLimiter(
+		opts.MinRetryDelay,
+		opts.MaxRetryDelay)
+}