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

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" {