Make retries cancelable

When using a request.WithContext it is possible to cancel a long running
request.  However, when using a cancelable request with retries enabled,
we want to quit the retry loop.

This is impactful when using a conservative backoff strategy and a
large number of retries.

pester.go

@@ -300,6 +300,16 @@ func (c *Client) pester(p params) (*http.Response, error) {
 					return
 				}
 
+				//If the request has been cancelled, skip retries
+				if p.req != nil {
+					ctx := p.req.Context()
+					select {
+					case <-ctx.Done():
+						multiplexCh <- result{resp: resp, err: ctx.Err()}
+						return
+					}
+				}
+
 				// if we are retrying, we should close this response body to free the fd
 				if resp != nil {
 					resp.Body.Close()

pester_test.go

@@ -1,6 +1,7 @@
 package pester
 
 import (
+	"context"
 	"fmt"
 	"log"
 	"net"
@@ -397,6 +398,48 @@ func TestConcurrentRequestsNotRacyAndDontLeak_SuccessfulRequest(t *testing.T) {
 	}
 }
 
+func TestRetriesNotAttemptedIfContextIsCancelled(t *testing.T) {
+	t.Parallel()
+
+	ctx, cancel := context.WithCancel(context.Background())
+
+	port, err := timeoutServer(1 * time.Second)
+	if err != nil {
+		t.Fatal("unable to start timeout server", err)
+	}
+
+	timeoutURL := fmt.Sprintf("http://localhost:%d", port)
+	req, err := http.NewRequest("GET", timeoutURL, nil)
+	if err != nil {
+		t.Fatalf("unable to create request %v", err)
+	}
+	req = req.WithContext(ctx)
+
+	c := New()
+	c.MaxRetries = 10
+	c.KeepLog = true
+	c.Backoff = ExponentialBackoff
+
+	//Cancel the context in another routine (eg: user interrupt)
+	go func() {
+		cancel()
+		t.Logf("\n%d - cancelled", time.Now().Unix())
+	}()
+
+	_, err = c.Do(req)
+	if err == nil {
+		t.Fatal("expected to get an error")
+	}
+	c.Wait()
+
+	// in the event of an error, let's see what the logs were
+	t.Log("\n", c.LogString())
+
+	if got, want := c.LogErrCount(), 1; got != want {
+		t.Fatalf("got %d errors, want %d", got, want)
+	}
+}
+
 func withinEpsilon(got, want int64, epslion float64) bool {
 	if want <= int64(epslion*float64(got)) || want >= int64(epslion*float64(got)) {
 		return false