oauth2: document that provided context will be used in refresh requests.

The context given to Client() will be stored and reused for token
refreshes througout the life of the Config. If the context expires e.g.
during application startup, refresh attempts will fail.

Updates golang#388

clientcredentials/clientcredentials.go

@@ -60,7 +60,9 @@ func (c *Config) Token(ctx context.Context) (*oauth2.Token, error) {
 // The token will auto-refresh as necessary.
 //
 // The provided context optionally controls which HTTP client
-// is returned. See the oauth2.HTTPClient variable.
+// is returned. See the oauth2.HTTPClient variable. It will be
+// used for all refresh requests for the life of the Config, so
+// do not pass a context with a deadline.
 //
 // The returned Client and its Transport should not be modified.
 func (c *Config) Client(ctx context.Context) *http.Client {

internal/token.go

@@ -231,7 +231,7 @@ func RetrieveToken(ctx context.Context, clientID, clientSecret, tokenURL string,
 func doTokenRoundTrip(ctx context.Context, req *http.Request) (*Token, error) {
 	r, err := ctxhttp.Do(ctx, ContextClient(ctx), req)
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("oauth2: cannot fetch token: %w", err)
 	}
 	body, err := ioutil.ReadAll(io.LimitReader(r.Body, 1<<20))
 	r.Body.Close()

oauth2_test.go

@@ -13,6 +13,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"net/url"
+	"strings"
 	"testing"
 	"time"
 
@@ -452,6 +453,22 @@ func TestTokenRefreshRequest(t *testing.T) {
 	c.Get(ts.URL + "/somethingelse")
 }
 
+func TestTokenRefreshRequest_Expired(t *testing.T) {
+	internal.ResetAuthCache()
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		t.Errorf("should not make http request")
+	}))
+	defer ts.Close()
+	conf := newConf(ts.URL)
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+	c := conf.Client(ctx, &Token{RefreshToken: "REFRESH_TOKEN"})
+	_, getErr := c.Get(ts.URL + "/somethingelse")
+	if !strings.Contains(getErr.Error(), "oauth2: cannot fetch token: context canceled") {
+		t.Errorf("Unexpected error refreshing with expired context: %s", getErr.Error())
+	}
+}
+
 func TestFetchWithNoRefreshToken(t *testing.T) {
 	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		if r.URL.String() == "/somethingelse" {