client-go: refactor retry logic so it can be reused

staging/src/k8s.io/client-go/rest/request.go

@@ -93,7 +93,6 @@ type Request struct {
 	rateLimiter flowcontrol.RateLimiter
 	backoff     BackoffManager
 	timeout     time.Duration
-	maxRetries  int
 
 	// generic components accessible via method setters
 	verb       string
@@ -112,6 +111,7 @@ type Request struct {
 	// output
 	err   error
 	body  io.Reader
+	retry WithRetry
 }
 
 // NewRequest creates a new request helper object for accessing runtime.Objects on a server.
@@ -142,7 +142,7 @@ func NewRequest(c *RESTClient) *Request {
 		backoff:        backoff,
 		timeout:        timeout,
 		pathPrefix:     pathPrefix,
-		maxRetries:     10,
+		retry:          &withRetry{maxRetries: 10},
 		warningHandler: c.warningHandler,
 	}
 
@@ -408,10 +408,7 @@ func (r *Request) Timeout(d time.Duration) *Request {
 // function is specifically called with a different value.
 // A zero maxRetries prevent it from doing retires and return an error immediately.
 func (r *Request) MaxRetries(maxRetries int) *Request {
-	if maxRetries < 0 {
+	r.retry.SetMaxRetries(maxRetries)
-		maxRetries = 0
-	}
-	r.maxRetries = maxRetries
 	return r
 }
 
@@ -842,6 +839,17 @@ func (r *Request) requestPreflightCheck() error {
 	return nil
 }
 
+func (r *Request) newHTTPRequest(ctx context.Context) (*http.Request, error) {
+	url := r.URL().String()
+	req, err := http.NewRequest(r.verb, url, r.body)
+	if err != nil {
+		return nil, err
+	}
+	req = req.WithContext(ctx)
+	req.Header = r.headers
+	return req, nil
+}
+
 // request connects to the server and invokes the provided function when a server response is
 // received. It handles retry behavior and up front validation of requests. It will invoke
 // fn at most once. It will return an error if a problem occurred prior to connecting to the
@@ -881,27 +889,22 @@ func (r *Request) request(ctx context.Context, fn func(*http.Request, *http.Resp
 	}
 
 	// Right now we make about ten retry attempts if we get a Retry-After response.
-	retries := 0
+	var retryAfter *RetryAfter
-	var retryInfo string
 	for {
-
+		req, err := r.newHTTPRequest(ctx)
-		url := r.URL().String()
-		req, err := http.NewRequest(r.verb, url, r.body)
 		if err != nil {
 			return err
 		}
-		req = req.WithContext(ctx)
-		req.Header = r.headers
 
 		r.backoff.Sleep(r.backoff.CalculateBackoff(r.URL()))
-		if retries > 0 {
+		if retryAfter != nil {
 			// We are retrying the request that we already send to apiserver
 			// at least once before.
 			// This request should also be throttled with the client-internal rate limiter.
-			if err := r.tryThrottleWithInfo(ctx, retryInfo); err != nil {
+			if err := r.tryThrottleWithInfo(ctx, retryAfter.Reason); err != nil {
 				return err
 			}
-			retryInfo = ""
+			retryAfter = nil
 		}
 		resp, err := client.Do(req)
 		updateURLMetrics(ctx, r, resp, err)
@@ -910,61 +913,46 @@ func (r *Request) request(ctx context.Context, fn func(*http.Request, *http.Resp
 		} else {
 			r.backoff.UpdateBackoff(r.URL(), err, resp.StatusCode)
 		}
-		if err != nil {
+
+		done := func() bool {
+			defer readAndCloseResponseBody(resp)
+
+			// if the the server returns an error in err, the response will be nil.
+			f := func(req *http.Request, resp *http.Response) {
+				if resp == nil {
+					return
+				}
+				fn(req, resp)
+			}
+
+			var retry bool
+			retryAfter, retry = r.retry.NextRetry(req, resp, err, func(req *http.Request, err error) bool {
 				// "Connection reset by peer" or "apiserver is shutting down" are usually a transient errors.
 				// Thus in case of "GET" operations, we simply retry it.
-			// We are not automatically retrying "write" operations, as
+				// We are not automatically retrying "write" operations, as they are not idempotent.
-			// they are not idempotent.
 				if r.verb != "GET" {
-				return err
+					return false
 				}
 				// For connection errors and apiserver shutdown errors retry.
 				if net.IsConnectionReset(err) || net.IsProbableEOF(err) {
-				// For the purpose of retry, we set the artificial "retry-after" response.
-				// TODO: Should we clean the original response if it exists?
-				resp = &http.Response{
-					StatusCode: http.StatusInternalServerError,
-					Header:     http.Header{"Retry-After": []string{"1"}},
-					Body:       ioutil.NopCloser(bytes.NewReader([]byte{})),
-				}
-			} else {
-				return err
-			}
-		}
-
-		done := func() bool {
-			// Ensure the response body is fully read and closed
-			// before we reconnect, so that we reuse the same TCP
-			// connection.
-			defer func() {
-				const maxBodySlurpSize = 2 << 10
-				if resp.ContentLength <= maxBodySlurpSize {
-					io.Copy(ioutil.Discard, &io.LimitedReader{R: resp.Body, N: maxBodySlurpSize})
-				}
-				resp.Body.Close()
-			}()
-
-			retries++
-			if seconds, wait := checkWait(resp); wait && retries <= r.maxRetries {
-				retryInfo = getRetryReason(retries, seconds, resp, err)
-				if seeker, ok := r.body.(io.Seeker); ok && r.body != nil {
-					_, err := seeker.Seek(0, 0)
-					if err != nil {
-						klog.V(4).Infof("Could not retry request, can't Seek() back to beginning of body for %T", r.body)
-						fn(req, resp)
 					return true
 				}
+				return false
+			})
+			if retry {
+				if err := r.retry.BeforeNextRetry(ctx, r.backoff, retryAfter, req.URL.String(), r.body); err != nil {
+					klog.V(4).Infof("Could not retry request - %v", err)
+					f(req, resp)
+					return true
 				}
-
-				klog.V(4).Infof("Got a Retry-After %ds response for attempt %d to %v", seconds, retries, url)
-				r.backoff.Sleep(time.Duration(seconds) * time.Second)
 				return false
 			}
-			fn(req, resp)
+
+			f(req, resp)
 			return true
 		}()
 		if done {
-			return nil
+			return err
 		}
 	}
 }
@@ -1196,19 +1184,6 @@ func isTextResponse(resp *http.Response) bool {
 	return strings.HasPrefix(media, "text/")
 }
 
-// checkWait returns true along with a number of seconds if the server instructed us to wait
-// before retrying.
-func checkWait(resp *http.Response) (int, bool) {
-	switch r := resp.StatusCode; {
-	// any 500 error code and 429 can trigger a wait
-	case r == http.StatusTooManyRequests, r >= 500:
-	default:
-		return 0, false
-	}
-	i, ok := retryAfterSeconds(resp)
-	return i, ok
-}
-
 // retryAfterSeconds returns the value of the Retry-After header and true, or 0 and false if
 // the header was missing or not a valid number.
 func retryAfterSeconds(resp *http.Response) (int, bool) {
@@ -1220,26 +1195,6 @@ func retryAfterSeconds(resp *http.Response) (int, bool) {
 	return 0, false
 }
 
-func getRetryReason(retries, seconds int, resp *http.Response, err error) string {
-	// priority and fairness sets the UID of the FlowSchema associated with a request
-	// in the following response Header.
-	const responseHeaderMatchedFlowSchemaUID = "X-Kubernetes-PF-FlowSchema-UID"
-
-	message := fmt.Sprintf("retries: %d, retry-after: %ds", retries, seconds)
-
-	switch {
-	case resp.StatusCode == http.StatusTooManyRequests:
-		// it is server-side throttling from priority and fairness
-		flowSchemaUID := resp.Header.Get(responseHeaderMatchedFlowSchemaUID)
-		return fmt.Sprintf("%s - retry-reason: due to server-side throttling, FlowSchema UID: %q", message, flowSchemaUID)
-	case err != nil:
-		// it's a retriable error
-		return fmt.Sprintf("%s - retry-reason: due to retriable error, error: %v", message, err)
-	default:
-		return fmt.Sprintf("%s - retry-reason: %d", message, resp.StatusCode)
-	}
-}
-
 // Result contains the result of calling Request.Do().
 type Result struct {
 	body        []byte

staging/src/k8s.io/client-go/rest/request_test.go

@@ -1093,6 +1093,7 @@ func TestRequestWatch(t *testing.T) {
 	for _, testCase := range testCases {
 		t.Run("", func(t *testing.T) {
 			testCase.Request.backoff = &NoBackoff{}
+			testCase.Request.retry = &withRetry{}
 			watch, err := testCase.Request.Watch(context.Background())
 			hasErr := err != nil
 			if hasErr != testCase.Err {
@@ -1194,8 +1195,10 @@ func TestRequestStream(t *testing.T) {
 			},
 		},
 	}
+
 	for i, testCase := range testCases {
 		testCase.Request.backoff = &NoBackoff{}
+		testCase.Request.retry = &withRetry{maxRetries: 0}
 		body, err := testCase.Request.Stream(context.Background())
 		hasErr := err != nil
 		if hasErr != testCase.Err {
@@ -1274,6 +1277,7 @@ func TestRequestDo(t *testing.T) {
 	}
 	for i, testCase := range testCases {
 		testCase.Request.backoff = &NoBackoff{}
+		testCase.Request.retry = &withRetry{}
 		body, err := testCase.Request.Do(context.Background()).Raw()
 		hasErr := err != nil
 		if hasErr != testCase.Err {
@@ -1437,7 +1441,7 @@ func TestConnectionResetByPeerIsRetried(t *testing.T) {
 			}),
 		},
 		backoff: backoff,
-		maxRetries: 10,
+		retry:   &withRetry{maxRetries: 10},
 	}
 	// We expect two retries of "connection reset by peer" and the success.
 	_, err := req.Do(context.Background()).Raw()
@@ -2315,3 +2319,285 @@ func TestRequestMaxRetries(t *testing.T) {
 		})
 	}
 }
+
+type responseErr struct {
+	response *http.Response
+	err      error
+}
+
+type seek struct {
+	offset int64
+	whence int
+}
+
+type count struct {
+	// keeps track of the number of Seek(offset, whence) calls.
+	seeks []seek
+	// how many times {Request|Response}.Body.Close() has been invoked
+	closes int
+}
+
+// used to track {Request|Response}.Body
+type readTracker struct {
+	count     *count
+	delegated io.Reader
+}
+
+func (r *readTracker) Seek(offset int64, whence int) (int64, error) {
+	if seeker, ok := r.delegated.(io.Seeker); ok {
+		r.count.seeks = append(r.count.seeks, seek{offset: offset, whence: whence})
+		return seeker.Seek(offset, whence)
+	}
+	return 0, io.EOF
+}
+
+func (r *readTracker) Read(p []byte) (n int, err error) {
+	return r.delegated.Read(p)
+}
+
+func (r *readTracker) Close() error {
+	if closer, ok := r.delegated.(io.Closer); ok {
+		r.count.closes++
+		return closer.Close()
+	}
+	return nil
+}
+
+func newReadTracker(count *count) *readTracker {
+	return &readTracker{
+		count: count,
+	}
+}
+
+func newCount() *count {
+	return &count{
+		closes: 0,
+		seeks:  make([]seek, 0),
+	}
+}
+
+type readSeeker struct{ err error }
+
+func (rs readSeeker) Read([]byte) (int, error)       { return 0, rs.err }
+func (rs readSeeker) Seek(int64, int) (int64, error) { return 0, rs.err }
+
+func unWrap(err error) error {
+	if uerr, ok := err.(*url.Error); ok {
+		return uerr.Err
+	}
+	return err
+}
+
+// noSleepBackOff is a NoBackoff except it does not sleep,
+// used for faster execution of the unit tests.
+type noSleepBackOff struct {
+	*NoBackoff
+}
+
+func (n *noSleepBackOff) Sleep(d time.Duration) {}
+
+func TestRequestWithRetry(t *testing.T) {
+	tests := []struct {
+		name                         string
+		body                         io.Reader
+		serverReturns                responseErr
+		errExpected                  error
+		transformFuncInvokedExpected int
+		roundTripInvokedExpected     int
+	}{
+		{
+			name:                         "server returns retry-after response, request body is not io.Seeker, retry goes ahead",
+			body:                         ioutil.NopCloser(bytes.NewReader([]byte{})),
+			serverReturns:                responseErr{response: retryAfterResponse(), err: nil},
+			errExpected:                  nil,
+			transformFuncInvokedExpected: 1,
+			roundTripInvokedExpected:     2,
+		},
+		{
+			name:                         "server returns retry-after response, request body Seek returns error, retry aborted",
+			body:                         &readSeeker{err: io.EOF},
+			serverReturns:                responseErr{response: retryAfterResponse(), err: nil},
+			errExpected:                  nil,
+			transformFuncInvokedExpected: 1,
+			roundTripInvokedExpected:     1,
+		},
+		{
+			name:                         "server returns retry-after response, request body Seek returns no error, retry goes ahead",
+			body:                         &readSeeker{err: nil},
+			serverReturns:                responseErr{response: retryAfterResponse(), err: nil},
+			errExpected:                  nil,
+			transformFuncInvokedExpected: 1,
+			roundTripInvokedExpected:     2,
+		},
+		{
+			name:                         "server returns retryable err, request body is not io.Seek, retry goes ahead",
+			body:                         ioutil.NopCloser(bytes.NewReader([]byte{})),
+			serverReturns:                responseErr{response: nil, err: io.ErrUnexpectedEOF},
+			errExpected:                  io.ErrUnexpectedEOF,
+			transformFuncInvokedExpected: 0,
+			roundTripInvokedExpected:     2,
+		},
+		{
+			name:                         "server returns retryable err, request body Seek returns error, retry aborted",
+			body:                         &readSeeker{err: io.EOF},
+			serverReturns:                responseErr{response: nil, err: io.ErrUnexpectedEOF},
+			errExpected:                  io.ErrUnexpectedEOF,
+			transformFuncInvokedExpected: 0,
+			roundTripInvokedExpected:     1,
+		},
+		{
+			name:                         "server returns retryable err, request body Seek returns no err, retry goes ahead",
+			body:                         &readSeeker{err: nil},
+			serverReturns:                responseErr{response: nil, err: io.ErrUnexpectedEOF},
+			errExpected:                  io.ErrUnexpectedEOF,
+			transformFuncInvokedExpected: 0,
+			roundTripInvokedExpected:     2,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			var roundTripInvoked int
+			client := clientForFunc(func(req *http.Request) (*http.Response, error) {
+				roundTripInvoked++
+				return test.serverReturns.response, test.serverReturns.err
+			})
+
+			req := &Request{
+				verb: "GET",
+				body: test.body,
+				c: &RESTClient{
+					Client: client,
+				},
+				backoff: &noSleepBackOff{},
+				retry:   &withRetry{maxRetries: 1},
+			}
+
+			var transformFuncInvoked int
+			err := req.request(context.Background(), func(request *http.Request, response *http.Response) {
+				transformFuncInvoked++
+			})
+
+			if test.roundTripInvokedExpected != roundTripInvoked {
+				t.Errorf("Expected RoundTrip to be invoked %d times, but got: %d", test.roundTripInvokedExpected, roundTripInvoked)
+			}
+			if test.transformFuncInvokedExpected != transformFuncInvoked {
+				t.Errorf("Expected transform func to be invoked %d times, but got: %d", test.transformFuncInvokedExpected, transformFuncInvoked)
+			}
+			if test.errExpected != unWrap(err) {
+				t.Errorf("Expected error: %v, but got: %v", test.errExpected, unWrap(err))
+			}
+		})
+	}
+}
+
+func TestRequestDoWithRetry(t *testing.T) {
+	testRequestWithRetry(t, func(ctx context.Context, r *Request) {
+		r.Do(ctx)
+	})
+}
+
+func TestRequestDORawWithRetry(t *testing.T) {
+	testRequestWithRetry(t, func(ctx context.Context, r *Request) {
+		r.DoRaw(ctx)
+	})
+}
+
+func testRequestWithRetry(t *testing.T, doFunc func(ctx context.Context, r *Request)) {
+	tests := []struct {
+		name              string
+		verb              string
+		body              func() io.Reader
+		maxRetries        int
+		serverReturns     []responseErr
+		reqCountExpected  *count
+		respCountExpected *count
+	}{
+		{
+			name:       "server always returns retry-after response",
+			verb:       "GET",
+			body:       func() io.Reader { return bytes.NewReader([]byte{}) },
+			maxRetries: 2,
+			serverReturns: []responseErr{
+				{response: retryAfterResponse(), err: nil},
+				{response: retryAfterResponse(), err: nil},
+				{response: retryAfterResponse(), err: nil},
+			},
+			reqCountExpected:  &count{closes: 0, seeks: make([]seek, 2)},
+			respCountExpected: &count{closes: 3, seeks: []seek{}},
+		},
+		{
+			name:       "server always returns retryable error",
+			verb:       "GET",
+			body:       func() io.Reader { return bytes.NewReader([]byte{}) },
+			maxRetries: 2,
+			serverReturns: []responseErr{
+				{response: nil, err: io.EOF},
+				{response: nil, err: io.EOF},
+				{response: nil, err: io.EOF},
+			},
+			reqCountExpected:  &count{closes: 0, seeks: make([]seek, 2)},
+			respCountExpected: &count{closes: 0, seeks: []seek{}},
+		},
+		{
+			name:       "server returns success on the final retry",
+			verb:       "GET",
+			body:       func() io.Reader { return bytes.NewReader([]byte{}) },
+			maxRetries: 2,
+			serverReturns: []responseErr{
+				{response: retryAfterResponse(), err: nil},
+				{response: nil, err: io.EOF},
+				{response: &http.Response{StatusCode: http.StatusOK}, err: nil},
+			},
+			reqCountExpected:  &count{closes: 0, seeks: make([]seek, 2)},
+			respCountExpected: &count{closes: 2, seeks: []seek{}},
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			respCountGot := newCount()
+			responseRecorder := newReadTracker(respCountGot)
+			var attempts int
+			client := clientForFunc(func(req *http.Request) (*http.Response, error) {
+				defer func() {
+					attempts++
+				}()
+
+				resp := test.serverReturns[attempts].response
+				if resp != nil {
+					responseRecorder.delegated = ioutil.NopCloser(bytes.NewReader([]byte{}))
+					resp.Body = responseRecorder
+				}
+				return resp, test.serverReturns[attempts].err
+			})
+
+			reqCountGot := newCount()
+			reqRecorder := newReadTracker(reqCountGot)
+			reqRecorder.delegated = test.body()
+
+			req := &Request{
+				verb: test.verb,
+				body: reqRecorder,
+				c: &RESTClient{
+					Client: client,
+				},
+				backoff: &noSleepBackOff{},
+				retry:   &withRetry{maxRetries: test.maxRetries},
+			}
+
+			doFunc(context.Background(), req)
+
+			attemptsExpected := test.maxRetries + 1
+			if attemptsExpected != attempts {
+				t.Errorf("Expected retries: %d, but got: %d", attemptsExpected, attempts)
+			}
+			if !reflect.DeepEqual(test.reqCountExpected.seeks, reqCountGot.seeks) {
+				t.Errorf("Expected request body to have seek invocation: %v, but got: %v", test.reqCountExpected.seeks, reqCountGot.seeks)
+			}
+			if test.respCountExpected.closes != respCountGot.closes {
+				t.Errorf("Expected response body Close to be invoked %d times, but got: %d", test.respCountExpected.closes, respCountGot.closes)
+			}
+		})
+	}
+}

staging/src/k8s.io/client-go/rest/with_retry.go

@@ -0,0 +1,228 @@
+/*
+Copyright 2021 The Kubernetes 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 rest
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"net/http"
+	"time"
+
+	"k8s.io/klog/v2"
+)
+
+// IsRetryableErrorFunc allows the client to provide its own function
+// that determines whether the specified err from the server is retryable.
+//
+// request: the original request sent to the server
+// err: the server sent this error to us
+//
+// The function returns true if the error is retryable and the request
+// can be retried, otherwise it returns false.
+// We have four mode of communications - 'Stream', 'Watch', 'Do' and 'DoRaw', this
+// function allows us to customize the retryability aspect of each.
+type IsRetryableErrorFunc func(request *http.Request, err error) bool
+
+func (r IsRetryableErrorFunc) IsErrorRetryable(request *http.Request, err error) bool {
+	return r(request, err)
+}
+
+// WithRetry allows the client to retry a request up to a certain number of times
+// Note that WithRetry is not safe for concurrent use by multiple
+// goroutines without additional locking or coordination.
+type WithRetry interface {
+	// SetMaxRetries makes the request use the specified integer as a ceiling
+	// for retries upon receiving a 429 status code  and the "Retry-After" header
+	// in the response.
+	// A zero maxRetries should prevent from doing any retry and return immediately.
+	SetMaxRetries(maxRetries int)
+
+	// NextRetry advances the retry counter appropriately and returns true if the
+	// request should be retried, otherwise it returns false if:
+	//  - we have already reached the maximum retry threshold.
+	//  - the error does not fall into the retryable category.
+	//  - the server has not sent us a 429, or 5xx status code and the
+	//    'Retry-After' response header is not set with a value.
+	//
+	// if retry is set to true, retryAfter will contain the information
+	// regarding the next retry.
+	//
+	// request: the original request sent to the server
+	// resp: the response sent from the server, it is set if err is nil
+	// err: the server sent this error to us, if err is set then resp is nil.
+	// f: a IsRetryableErrorFunc function provided by the client that determines
+	//    if the err sent by the server is retryable.
+	NextRetry(req *http.Request, resp *http.Response, err error, f IsRetryableErrorFunc) (*RetryAfter, bool)
+
+	// BeforeNextRetry is responsible for carrying out operations that need
+	// to be completed before the next retry is initiated:
+	// - if the request context is already canceled there is no need to
+	//   retry, the function will return ctx.Err().
+	// - we need to seek to the beginning of the request body before we
+	//   initiate the next retry, the function should return an error if
+	//   it fails to do so.
+	// - we should wait the number of seconds the server has asked us to
+	//   in the 'Retry-After' response header.
+	//
+	// If BeforeNextRetry returns an error the client should abort the retry,
+	// otherwise it is safe to initiate the next retry.
+	BeforeNextRetry(ctx context.Context, backoff BackoffManager, retryAfter *RetryAfter, url string, body io.Reader) error
+}
+
+// RetryAfter holds information associated with the next retry.
+type RetryAfter struct {
+	// Wait is the duration the server has asked us to wait before
+	// the next retry is initiated.
+	// This is the value of the 'Retry-After' response header in seconds.
+	Wait time.Duration
+
+	// Attempt is the Nth attempt after which we have received a retryable
+	// error or a 'Retry-After' response header from the server.
+	Attempt int
+
+	// Reason describes why we are retrying the request
+	Reason string
+}
+
+type withRetry struct {
+	maxRetries int
+	attempts   int
+}
+
+func (r *withRetry) SetMaxRetries(maxRetries int) {
+	if maxRetries < 0 {
+		maxRetries = 0
+	}
+	r.maxRetries = maxRetries
+}
+
+func (r *withRetry) NextRetry(req *http.Request, resp *http.Response, err error, f IsRetryableErrorFunc) (*RetryAfter, bool) {
+	if req == nil || (resp == nil && err == nil) {
+		// bad input, we do nothing.
+		return nil, false
+	}
+
+	r.attempts++
+	retryAfter := &RetryAfter{Attempt: r.attempts}
+	if r.attempts > r.maxRetries {
+		return retryAfter, false
+	}
+
+	// if the server returned an error, it takes precedence over the http response.
+	var errIsRetryable bool
+	if f != nil && err != nil && f.IsErrorRetryable(req, err) {
+		errIsRetryable = true
+		// we have a retryable error, for which we will create an
+		// artificial "Retry-After" response.
+		resp = retryAfterResponse()
+	}
+	if err != nil && !errIsRetryable {
+		return retryAfter, false
+	}
+
+	// if we are here, we have either a or b:
+	//  a: we have a retryable error, for which we already
+	//     have an artificial "Retry-After" response.
+	//  b: we have a response from the server for which we
+	//     need to check if it is retryable
+	seconds, wait := checkWait(resp)
+	if !wait {
+		return retryAfter, false
+	}
+
+	retryAfter.Wait = time.Duration(seconds) * time.Second
+	retryAfter.Reason = getRetryReason(r.attempts, seconds, resp, err)
+	return retryAfter, true
+}
+
+func (r *withRetry) BeforeNextRetry(ctx context.Context, backoff BackoffManager, retryAfter *RetryAfter, url string, body io.Reader) error {
+	// Ensure the response body is fully read and closed before
+	// we reconnect, so that we reuse the same TCP connection.
+	if ctx.Err() != nil {
+		return ctx.Err()
+	}
+
+	if seeker, ok := body.(io.Seeker); ok && body != nil {
+		if _, err := seeker.Seek(0, 0); err != nil {
+			return fmt.Errorf("can't Seek() back to beginning of body for %T", r)
+		}
+	}
+
+	klog.V(4).Infof("Got a Retry-After %s response for attempt %d to %v", retryAfter.Wait, retryAfter.Attempt, url)
+	if backoff != nil {
+		backoff.Sleep(retryAfter.Wait)
+	}
+	return nil
+}
+
+// checkWait returns true along with a number of seconds if
+// the server instructed us to wait before retrying.
+func checkWait(resp *http.Response) (int, bool) {
+	switch r := resp.StatusCode; {
+	// any 500 error code and 429 can trigger a wait
+	case r == http.StatusTooManyRequests, r >= 500:
+	default:
+		return 0, false
+	}
+	i, ok := retryAfterSeconds(resp)
+	return i, ok
+}
+
+func getRetryReason(retries, seconds int, resp *http.Response, err error) string {
+	// priority and fairness sets the UID of the FlowSchema
+	// associated with a request in the following response Header.
+	const responseHeaderMatchedFlowSchemaUID = "X-Kubernetes-PF-FlowSchema-UID"
+
+	message := fmt.Sprintf("retries: %d, retry-after: %ds", retries, seconds)
+
+	switch {
+	case resp.StatusCode == http.StatusTooManyRequests:
+		// it is server-side throttling from priority and fairness
+		flowSchemaUID := resp.Header.Get(responseHeaderMatchedFlowSchemaUID)
+		return fmt.Sprintf("%s - retry-reason: due to server-side throttling, FlowSchema UID: %q", message, flowSchemaUID)
+	case err != nil:
+		// it's a retryable error
+		return fmt.Sprintf("%s - retry-reason: due to retryable error, error: %v", message, err)
+	default:
+		return fmt.Sprintf("%s - retry-reason: %d", message, resp.StatusCode)
+	}
+}
+
+func readAndCloseResponseBody(resp *http.Response) {
+	if resp == nil {
+		return
+	}
+
+	// Ensure the response body is fully read and closed
+	// before we reconnect, so that we reuse the same TCP
+	// connection.
+	const maxBodySlurpSize = 2 << 10
+	defer resp.Body.Close()
+
+	if resp.ContentLength <= maxBodySlurpSize {
+		io.Copy(ioutil.Discard, &io.LimitedReader{R: resp.Body, N: maxBodySlurpSize})
+	}
+}
+
+func retryAfterResponse() *http.Response {
+	return &http.Response{
+		StatusCode: http.StatusInternalServerError,
+		Header:     http.Header{"Retry-After": []string{"1"}},
+	}
+}

staging/src/k8s.io/client-go/rest/with_retry_test.go

@@ -0,0 +1,230 @@
+/*
+Copyright 2021 The Kubernetes 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 rest
+
+import (
+	"errors"
+	"net/http"
+	"reflect"
+	"testing"
+	"time"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+var alwaysRetryError = IsRetryableErrorFunc(func(_ *http.Request, _ error) bool {
+	return true
+})
+
+var neverRetryError = IsRetryableErrorFunc(func(_ *http.Request, _ error) bool {
+	return false
+})
+
+func TestNextRetry(t *testing.T) {
+	fakeError := errors.New("fake error")
+	tests := []struct {
+		name               string
+		attempts           int
+		maxRetries         int
+		request            *http.Request
+		response           *http.Response
+		err                error
+		retryableErrFunc   IsRetryableErrorFunc
+		retryExpected      []bool
+		retryAfterExpected []*RetryAfter
+	}{
+		{
+			name:               "bad input, response and err are nil",
+			maxRetries:         2,
+			attempts:           1,
+			request:            &http.Request{},
+			response:           nil,
+			err:                nil,
+			retryExpected:      []bool{false},
+			retryAfterExpected: []*RetryAfter{nil},
+		},
+		{
+			name:          "zero maximum retry",
+			maxRetries:    0,
+			attempts:      1,
+			request:       &http.Request{},
+			response:      retryAfterResponse(),
+			err:           nil,
+			retryExpected: []bool{false},
+			retryAfterExpected: []*RetryAfter{
+				{
+					Attempt: 1,
+				},
+			},
+		},
+		{
+			name:       "server returned a retryable error",
+			maxRetries: 3,
+			attempts:   1,
+			request:    &http.Request{},
+			response:   nil,
+			err:        fakeError,
+			retryableErrFunc: func(_ *http.Request, err error) bool {
+				if err == fakeError {
+					return true
+				}
+				return false
+			},
+			retryExpected: []bool{true},
+			retryAfterExpected: []*RetryAfter{
+				{
+					Attempt: 1,
+					Wait:    time.Second,
+					Reason:  "retries: 1, retry-after: 1s - retry-reason: due to retryable error, error: fake error",
+				},
+			},
+		},
+		{
+			name:       "server returned a retryable HTTP 429 response",
+			maxRetries: 3,
+			attempts:   1,
+			request:    &http.Request{},
+			response: &http.Response{
+				StatusCode: http.StatusTooManyRequests,
+				Header: http.Header{
+					"Retry-After":                    []string{"2"},
+					"X-Kubernetes-Pf-Flowschema-Uid": []string{"fs-1"},
+				},
+			},
+			err:           nil,
+			retryExpected: []bool{true},
+			retryAfterExpected: []*RetryAfter{
+				{
+					Attempt: 1,
+					Wait:    2 * time.Second,
+					Reason:  `retries: 1, retry-after: 2s - retry-reason: due to server-side throttling, FlowSchema UID: "fs-1"`,
+				},
+			},
+		},
+		{
+			name:       "server returned a retryable HTTP 5xx response",
+			maxRetries: 3,
+			attempts:   1,
+			request:    &http.Request{},
+			response: &http.Response{
+				StatusCode: http.StatusServiceUnavailable,
+				Header: http.Header{
+					"Retry-After": []string{"3"},
+				},
+			},
+			err:           nil,
+			retryExpected: []bool{true},
+			retryAfterExpected: []*RetryAfter{
+				{
+					Attempt: 1,
+					Wait:    3 * time.Second,
+					Reason:  "retries: 1, retry-after: 3s - retry-reason: 503",
+				},
+			},
+		},
+		{
+			name:       "server returned a non response without without a Retry-After header",
+			maxRetries: 1,
+			attempts:   1,
+			request:    &http.Request{},
+			response: &http.Response{
+				StatusCode: http.StatusTooManyRequests,
+				Header:     http.Header{},
+			},
+			err:           nil,
+			retryExpected: []bool{false},
+			retryAfterExpected: []*RetryAfter{
+				{
+					Attempt: 1,
+				},
+			},
+		},
+		{
+			name:       "both response and err are set, err takes precedence",
+			maxRetries: 1,
+			attempts:   1,
+			request:    &http.Request{},
+			response:   retryAfterResponse(),
+			err:        fakeError,
+			retryableErrFunc: func(_ *http.Request, err error) bool {
+				if err == fakeError {
+					return true
+				}
+				return false
+			},
+			retryExpected: []bool{true},
+			retryAfterExpected: []*RetryAfter{
+				{
+					Attempt: 1,
+					Wait:    time.Second,
+					Reason:  "retries: 1, retry-after: 1s - retry-reason: due to retryable error, error: fake error",
+				},
+			},
+		},
+		{
+			name:             "all retries are exhausted",
+			maxRetries:       3,
+			attempts:         4,
+			request:          &http.Request{},
+			response:         nil,
+			err:              fakeError,
+			retryableErrFunc: alwaysRetryError,
+			retryExpected:    []bool{true, true, true, false},
+			retryAfterExpected: []*RetryAfter{
+				{
+					Attempt: 1,
+					Wait:    time.Second,
+					Reason:  "retries: 1, retry-after: 1s - retry-reason: due to retryable error, error: fake error",
+				},
+				{
+					Attempt: 2,
+					Wait:    time.Second,
+					Reason:  "retries: 2, retry-after: 1s - retry-reason: due to retryable error, error: fake error",
+				},
+				{
+					Attempt: 3,
+					Wait:    time.Second,
+					Reason:  "retries: 3, retry-after: 1s - retry-reason: due to retryable error, error: fake error",
+				},
+				{
+					Attempt: 4,
+				},
+			},
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			r := &withRetry{maxRetries: test.maxRetries}
+
+			retryGot := make([]bool, 0)
+			retryAfterGot := make([]*RetryAfter, 0)
+			for i := 0; i < test.attempts; i++ {
+				retryAfter, retry := r.NextRetry(test.request, test.response, test.err, test.retryableErrFunc)
+				retryGot = append(retryGot, retry)
+				retryAfterGot = append(retryAfterGot, retryAfter)
+			}
+
+			if !reflect.DeepEqual(test.retryExpected, retryGot) {
+				t.Errorf("Expected retry: %t, but got: %t", test.retryExpected, retryGot)
+			}
+			if !reflect.DeepEqual(test.retryAfterExpected, retryAfterGot) {
+				t.Errorf("Expected retry-after parameters to match, but got: %s", cmp.Diff(test.retryAfterExpected, retryAfterGot))
+			}
+		})
+	}
+}