add ProviderClient.Reauthenticate() function

If a user wants to do their own HTTP requests and reauthenticate in case
of 401 responses, they can already use ProviderClient.ReauthFunc(), but
that function is not thread-safe. This commit provides a safer
alternative by pulling the relevant piece of code out of
ProviderClient.Request().

provider_client.go

@@ -126,6 +126,36 @@ func (client *ProviderClient) SetToken(t string) {
 	client.TokenID = t
 }
 
+//Reauthenticate calls client.ReauthFunc in a thread-safe way. If this is
+//called because of a 401 response, the caller may pass the previous token. In
+//this case, the reauthentication can be skipped if another thread has already
+//reauthenticated in the meantime. If no previous token is known, an empty
+//string should be passed instead to force unconditional reauthentication.
+func (client *ProviderClient) Reauthenticate(previousToken string) (err error) {
+	if client.ReauthFunc == nil {
+		return nil
+	}
+
+	if client.mut == nil {
+		return client.ReauthFunc()
+	}
+	client.mut.Lock()
+	defer client.mut.Unlock()
+
+	client.reauthmut.Lock()
+	client.reauthmut.reauthing = true
+	client.reauthmut.Unlock()
+
+	if previousToken == "" || client.TokenID == previousToken {
+		err = client.ReauthFunc()
+	}
+
+	client.reauthmut.Lock()
+	client.reauthmut.reauthing = false
+	client.reauthmut.Unlock()
+	return
+}
+
 // RequestOpts customizes the behavior of the provider.Request() method.
 type RequestOpts struct {
 	// JSONBody, if provided, will be encoded as JSON and used as the body of the HTTP request. The
@@ -254,21 +284,7 @@ func (client *ProviderClient) Request(method, url string, options *RequestOpts)
 			}
 		case http.StatusUnauthorized:
 			if client.ReauthFunc != nil {
-				if client.mut != nil {
-					client.mut.Lock()
-					client.reauthmut.Lock()
-					client.reauthmut.reauthing = true
-					client.reauthmut.Unlock()
-					if curtok := client.TokenID; curtok == prereqtok {
-						err = client.ReauthFunc()
-					}
-					client.reauthmut.Lock()
-					client.reauthmut.reauthing = false
-					client.reauthmut.Unlock()
-					client.mut.Unlock()
-				} else {
-					err = client.ReauthFunc()
-				}
+				err = client.Reauthenticate(prereqtok)
 				if err != nil {
 					e := &ErrUnableToReauthenticate{}
 					e.ErrOriginal = respErr