Support to set qps and burst via env variable (#2755)

* Support to set client config via env variable

Currently some configuration such as `kube-api-burst` and
`kube-api-qps` must be set via command line option.

Knative deployments usually do not have the command line option,
the command line option is unusual and so knative/operator also does
not support the API.

Hence This patch allows to set client config via env variable.

* only qps and burst

* Add copyright

* Handle float

* Add error handling

environment/client_config.go

@@ -19,8 +19,10 @@ package environment
 import (
 	"flag"
 	"fmt"
+	"log"
 	"math"
 	"os"
+	"strconv"
 
 	"k8s.io/client-go/rest"
 	"k8s.io/client-go/tools/clientcmd"
@@ -45,9 +47,19 @@ func (c *ClientConfig) InitFlags(fs *flag.FlagSet) {
 	fs.StringVar(&c.Kubeconfig, "kubeconfig", os.Getenv("KUBECONFIG"),
 		"Path to a kubeconfig. Only required if out-of-cluster.")
 
-	fs.IntVar(&c.Burst, "kube-api-burst", 0, "Maximum burst for throttle.")
+	fs.IntVar(&c.Burst, "kube-api-burst", int(envVarOrDefault("KUBE_API_BURST", 0)), "Maximum burst for throttle.")
 
-	fs.Float64Var(&c.QPS, "kube-api-qps", 0, "Maximum QPS to the server from the client.")
+	fs.Float64Var(&c.QPS, "kube-api-qps", envVarOrDefault("KUBE_API_QPS", 0.0), "Maximum QPS to the server from the client.")
+}
+
+func envVarOrDefault(key string, val float64) float64 {
+	var err error
+	if v := os.Getenv(key); v != "" {
+		if val, err = strconv.ParseFloat(v, 64); err != nil {
+			log.Fatal(err)
+		}
+	}
+	return val
 }
 
 func (c *ClientConfig) GetRESTConfig() (*rest.Config, error) {

environment/client_config_test.go

@@ -0,0 +1,50 @@
+/*
+Copyright 2023 The Knative 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 environment
+
+import (
+	"flag"
+	"strconv"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestInitFlag(t *testing.T) {
+	t.Setenv("KUBE_API_BURST", "50")
+	t.Setenv("KUBE_API_QPS", "60.1")
+	t.Setenv("KUBECONFIG", "myconfig")
+
+	c := new(ClientConfig)
+	c.InitFlags(flag.CommandLine)
+
+	// Override kube-api-burst via command line option.
+	flag.CommandLine.Set("kube-api-burst", strconv.Itoa(100))
+
+	// Call parse() here as InitFlags does not call it.
+	flag.Parse()
+
+	expect := &ClientConfig{
+		Burst:      100,
+		QPS:        60.1,
+		Kubeconfig: "myconfig",
+	}
+
+	if !cmp.Equal(c, expect) {
+		t.Errorf("ClientConfig mismatch: diff(-want,+got):\n%s", cmp.Diff(expect, c))
+	}
+}