Add client-side support for datetime value of Retry-After header (#131)

According to specifications, Retry-After header should be either seconds-delay or HTTP-date
https://datatracker.ietf.org/doc/html/rfc7231#section-7.1.3

Resolves #119

internal/retryafter.go

@@ -1,31 +1,61 @@
 package internal
 
 import (
+	"errors"
 	"net/http"
 	"strconv"
-	"strings"
 	"time"
 )
 
 const retryAfterHTTPHeader = "Retry-After"
 
+var errCouldNotParseRetryAfterHeader = errors.New("could not parse" + retryAfterHTTPHeader + "header")
+
 type OptionalDuration struct {
 	Duration time.Duration
 	// true if duration field is defined.
 	Defined bool
 }
 
+func parseDelaySeconds(s string) (time.Duration, error) {
+	n, err := strconv.Atoi(s)
+
+	// Verify duration parsed properly and bigger than 0
+	if err == nil && n > 0 {
+		duration := time.Duration(n) * time.Second
+		return duration, nil
+	}
+	return 0, errCouldNotParseRetryAfterHeader
+}
+
+func parseHTTPDate(s string) (time.Duration, error) {
+	t, err := http.ParseTime(s)
+
+	// Verify duration parsed properly and bigger than 0
+	if err == nil {
+		if duration := time.Until(t); duration > 0 {
+			return duration, nil
+		}
+	}
+	return 0, errCouldNotParseRetryAfterHeader
+}
+
 // ExtractRetryAfterHeader extracts Retry-After response header if the status
 // is 503 or 429. Returns 0 duration if the header is not found or the status
 // is different.
 func ExtractRetryAfterHeader(resp *http.Response) OptionalDuration {
 	if resp.StatusCode == http.StatusServiceUnavailable ||
 		resp.StatusCode == http.StatusTooManyRequests {
-		retryAfter := strings.TrimSpace(resp.Header.Get(retryAfterHTTPHeader))
+		retryAfter := resp.Header.Get(retryAfterHTTPHeader)
 		if retryAfter != "" {
-			retryIntervalSec, err := strconv.Atoi(retryAfter)
+			// Parse delay-seconds https://datatracker.ietf.org/doc/html/rfc7231#section-7.1.3
+			retryInterval, err := parseDelaySeconds(retryAfter)
+			if err == nil {
+				return OptionalDuration{Defined: true, Duration: retryInterval}
+			}
+			// Parse HTTP-date https://datatracker.ietf.org/doc/html/rfc7231#section-7.1.3
+			retryInterval, err = parseHTTPDate(retryAfter)
 			if err == nil {
-				retryInterval := time.Duration(retryIntervalSec) * time.Second
 				return OptionalDuration{Defined: true, Duration: retryInterval}
 			}
 		}

internal/retryafter_test.go

@@ -0,0 +1,88 @@
+package internal
+
+import (
+	"github.com/stretchr/testify/assert"
+	"math/rand"
+	"net/http"
+	"strconv"
+	"testing"
+	"time"
+)
+
+func response503() *http.Response {
+	return &http.Response{
+		StatusCode: http.StatusServiceUnavailable,
+		Header:     map[string][]string{},
+	}
+}
+
+func assertUndefinedDuration(t *testing.T, d OptionalDuration) {
+	assert.NotNil(t, d)
+	assert.Equal(t, false, d.Defined)
+	assert.Equal(t, time.Duration(0), d.Duration)
+}
+
+func assertDuration(t *testing.T, duration OptionalDuration, expected time.Duration) {
+	assert.NotNil(t, duration)
+	assert.Equal(t, true, duration.Defined)
+
+	// LessOrEqual to consider the time passes during the tests (actual duration would decrease in HTTP-date tests)
+	assert.LessOrEqual(t, duration.Duration, expected)
+}
+
+func TestExtractRetryAfterHeaderDelaySeconds(t *testing.T) {
+	// Generate random n > 0 int
+	rand.Seed(time.Now().UnixNano())
+	retryIntervalSec := rand.Intn(9999)
+
+	// Generate a 503 status code response with Retry-After = n header
+	resp := response503()
+	resp.Header.Add(retryAfterHTTPHeader, strconv.Itoa(retryIntervalSec))
+
+	expectedDuration := time.Second * time.Duration(retryIntervalSec)
+	assertDuration(t, ExtractRetryAfterHeader(resp), expectedDuration)
+
+	// Verify status code 429
+	resp.StatusCode = http.StatusTooManyRequests
+	assertDuration(t, ExtractRetryAfterHeader(resp), expectedDuration)
+
+	// Verify different status code than {429, 503}
+	resp.StatusCode = http.StatusBadGateway
+	assertUndefinedDuration(t, ExtractRetryAfterHeader(resp))
+
+	// Verify no duration is created for n < 0
+	resp.Header.Set(retryAfterHTTPHeader, strconv.Itoa(-1))
+	assertUndefinedDuration(t, ExtractRetryAfterHeader(resp))
+}
+
+func TestExtractRetryAfterHeaderHttpDate(t *testing.T) {
+	// Generate a random n > 0 second duration
+	now := time.Now()
+	rand.Seed(now.UnixNano())
+	retryIntervalSec := rand.Intn(9999)
+	expectedDuration := time.Second * time.Duration(retryIntervalSec)
+
+	// Set a response with Retry-After header = random n > 0 int
+	resp := response503()
+	retryAfter := now.Add(time.Second * time.Duration(retryIntervalSec)).UTC()
+
+	// Verify HTTP-date TimeFormat format is being parsed correctly
+	resp.Header.Set(retryAfterHTTPHeader, retryAfter.Format(http.TimeFormat))
+	assertDuration(t, ExtractRetryAfterHeader(resp), expectedDuration)
+
+	// Verify ANSI time format
+	resp.Header.Set(retryAfterHTTPHeader, retryAfter.Format(time.ANSIC))
+	assertDuration(t, ExtractRetryAfterHeader(resp), expectedDuration)
+
+	// Verify RFC850 time format
+	resp.Header.Set(retryAfterHTTPHeader, retryAfter.Format(time.RFC850))
+	assertDuration(t, ExtractRetryAfterHeader(resp), expectedDuration)
+
+	// Verify non HTTP-date RFC1123 format isn't being parsed
+	resp.Header.Set(retryAfterHTTPHeader, retryAfter.Format(time.RFC1123))
+	assertUndefinedDuration(t, ExtractRetryAfterHeader(resp))
+
+	// Verify no duration is created for n < 0
+	resp.Header.Set(retryAfterHTTPHeader, now.Add(-1*time.Second).UTC().Format(http.TimeFormat))
+	assertUndefinedDuration(t, ExtractRetryAfterHeader(resp))
+}