Implement a new shared "Drainer" handler. (#1517)

* Implement a new shared "Drainer" handler.

This implements a new `http.Handler` called `Drainer`, which is intended to wrap some inner `http.Handler` business logic with a new outer handler that can respond to Kubelet probes (successfully until told to "Drain()").

This takes over the webhook's relatively new probe handling and lame duck logic with one key difference.  Previously the webhook waited for a fixed period after SIGTERM before exitting, but the new logic waits for this same grace period AFTER THE LAST REQUEST.  So if the handler keeps getting (non-probe) requests, the timer will continually reset, and once it stops receiving requests for the configured grace period, "Drain()" will return and the webhook will exit.

The goal of this work is to try to better cope with what we believe to be high tail latencies of the API server seeing that a webhook replica is shutting down.

Related: #1509

* Switch to RWLock

network/error_handler.go

@@ -25,6 +25,8 @@ import (
 
 // ErrorHandler sets up a handler suitable for use with the ErrorHandler field on
 // httputil's reverse proxy.
+// TODO(mattmoor): Move the implementation into handlers/error.go once downstream consumers
+// have adopted the alias.
 func ErrorHandler(logger *zap.SugaredLogger) func(http.ResponseWriter, *http.Request, error) {
 	return func(w http.ResponseWriter, req *http.Request, err error) {
 		ss := readSockStat(logger)

network/handlers/doc.go

@@ -0,0 +1,18 @@
+/*
+Copyright 2020 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 handlers holds useful HTTP handler implementations.
+package handlers

network/handlers/drain.go

@@ -0,0 +1,110 @@
+/*
+Copyright 2020 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 handlers
+
+import (
+	"net/http"
+	"sync"
+	"time"
+
+	"knative.dev/pkg/network"
+)
+
+// Drainer wraps an inner http.Handler to support responding to kubelet
+// probes with a "200 OK" until the handler is told to Drain.
+// When the Drainer is told to Drain, it will immediately start to fail
+// probes with a "500 shutting down", and the call will block until no
+// requests have been received for QuietPeriod (defaults to
+// network.DefaultDrainTimeout).
+type Drainer struct {
+	// Mutex guards the initialization and resets of the timer
+	sync.RWMutex
+
+	// Inner is the http.Handler to which we delegate actual requests.
+	Inner http.Handler
+
+	// QuietPeriod is the duration that must elapse without any requests
+	// after Drain is called before it may return.
+	QuietPeriod time.Duration
+
+	// once is used to initialize timer
+	once sync.Once
+
+	// timer is used to orchestrate the drain.
+	timer *time.Timer
+}
+
+// Ensure Drainer implements http.Handler
+var _ http.Handler = (*Drainer)(nil)
+
+// ServeHTTP implements http.Handler
+func (d *Drainer) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	if network.IsKubeletProbe(r) { // Respond to probes regardless of path.
+		if d.draining() {
+			http.Error(w, "shutting down", http.StatusServiceUnavailable)
+		} else {
+			w.WriteHeader(http.StatusOK)
+		}
+		return
+	}
+
+	d.reset()
+	d.Inner.ServeHTTP(w, r)
+}
+
+// Drain blocks until QuietPeriod has elapsed since the last request,
+// starting when this is invoked.
+func (d *Drainer) Drain() {
+	d.once.Do(func() {
+		t := func() *time.Timer {
+			d.Lock()
+			defer d.Unlock()
+			if d.QuietPeriod <= 0 {
+				d.QuietPeriod = network.DefaultDrainTimeout
+			}
+			d.timer = time.NewTimer(d.QuietPeriod)
+			return d.timer
+		}()
+
+		<-t.C
+	})
+}
+
+// reset resets the drain timer to the full amount of time.
+func (d *Drainer) reset() {
+	if func() bool {
+		d.RLock()
+		defer d.RUnlock()
+		return d.timer == nil
+	}() {
+		return
+	}
+
+	d.Lock()
+	defer d.Unlock()
+	if d.timer.Stop() {
+		d.timer.Reset(d.QuietPeriod)
+	}
+}
+
+// draining returns whether we are draining the handler.
+func (d *Drainer) draining() bool {
+	d.RLock()
+	defer d.RUnlock()
+
+	return d.timer != nil
+}

network/handlers/drain_test.go

@@ -0,0 +1,160 @@
+/*
+Copyright 2020 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 handlers
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+
+	"knative.dev/pkg/network"
+)
+
+func TestDrainMechanics(t *testing.T) {
+	var (
+		w     http.ResponseWriter
+		req   = &http.Request{}
+		probe = &http.Request{
+			Header: http.Header{
+				"User-Agent": []string{network.KubeProbeUAPrefix},
+			},
+		}
+	)
+
+	inner := http.HandlerFunc(func(http.ResponseWriter, *http.Request) {})
+
+	drainer := &Drainer{
+		Inner:       inner,
+		QuietPeriod: 100 * time.Millisecond,
+	}
+
+	// Works before Drain is called.
+	drainer.ServeHTTP(w, req)
+	drainer.ServeHTTP(w, req)
+	drainer.ServeHTTP(w, req)
+
+	// Check for 200 OK
+	resp := httptest.NewRecorder()
+	drainer.ServeHTTP(resp, probe)
+	if got, want := resp.Code, http.StatusOK; got != want {
+		t.Errorf("probe status = %d, wanted %d", got, want)
+	}
+
+	// Start to drain, and cancel the context when it returns.
+	ctx, cancel := context.WithCancel(context.Background())
+	go func() {
+		drainer.Drain()
+		cancel()
+	}()
+
+	select {
+	case <-time.After(40 * time.Millisecond):
+		// Drain is blocking.
+	case <-ctx.Done():
+		t.Error("Drain terminated prematurely.")
+	}
+	// Now send a request to reset things.
+	drainer.ServeHTTP(w, req)
+
+	// Check for 400 shutting down
+	resp = httptest.NewRecorder()
+	drainer.ServeHTTP(resp, probe)
+	if got, want := resp.Code, http.StatusServiceUnavailable; got != want {
+		t.Errorf("probe status = %d, wanted %d", got, want)
+	}
+
+	select {
+	case <-time.After(40 * time.Millisecond):
+		// Drain is blocking.
+	case <-ctx.Done():
+		t.Error("Drain terminated prematurely.")
+	}
+	// Now send a request to reset things.
+	drainer.ServeHTTP(w, req)
+
+	select {
+	case <-time.After(40 * time.Millisecond):
+		// Drain is blocking.
+	case <-ctx.Done():
+		t.Error("Drain terminated prematurely.")
+	}
+	// Now send a request to reset things.
+	drainer.ServeHTTP(w, req)
+
+	select {
+	case <-time.After(40 * time.Millisecond):
+		// Drain is blocking.
+	case <-ctx.Done():
+		t.Error("Drain terminated prematurely.")
+	}
+	// Probing does not reset the clock.
+	// Check for 500 shutting down
+	resp = httptest.NewRecorder()
+	drainer.ServeHTTP(resp, probe)
+	if got, want := resp.Code, http.StatusServiceUnavailable; got != want {
+		t.Errorf("probe status = %d, wanted %d", got, want)
+	}
+
+	// Big finish, test that multiple invocations of Drain all block.
+	ctx1, cancel1 := context.WithCancel(context.Background())
+	go func() {
+		drainer.Drain()
+		cancel1()
+	}()
+	ctx2, cancel2 := context.WithCancel(context.Background())
+	go func() {
+		drainer.Drain()
+		cancel2()
+	}()
+	ctx3, cancel3 := context.WithCancel(context.Background())
+	go func() {
+		drainer.Drain()
+		cancel3()
+	}()
+
+	select {
+	case <-time.After(70 * time.Millisecond):
+		t.Error("Timed out waiting for Drain to return.")
+
+	case <-ctx.Done():
+	case <-ctx1.Done():
+	case <-ctx2.Done():
+	case <-ctx3.Done():
+		// Once the first context is cancelled, check that all of them are cancelled.
+	}
+
+	// Check that a 4th and final one after things complete finishes instantly.
+	ctx4, cancel4 := context.WithCancel(context.Background())
+	go func() {
+		drainer.Drain()
+		cancel4()
+	}()
+
+	// Give the rest a short window to complete.
+	time.Sleep(time.Millisecond)
+
+	for idx, ictx := range []context.Context{ctx, ctx1, ctx2, ctx3, ctx4} {
+		select {
+		case <-ictx.Done():
+			// Should be done.
+		default:
+			t.Errorf("Drain[%d] did not complete.", idx)
+		}
+	}
+}

network/handlers/error.go

@@ -0,0 +1,23 @@
+/*
+Copyright 2020 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 handlers
+
+import "knative.dev/pkg/network"
+
+// Error sets up a handler suitable for use with the ErrorHandler field on
+// httputil's reverse proxy, which logs /proc/net/sockstat data.
+var Error = network.ErrorHandler