Add support for providing clientSecret as a function

API.md

@@ -226,6 +226,8 @@ The `server.auth.strategy()` method requires the following strategy options:
   object will be merged with the Wreck request object used to call the token endpoint. Such an
   object can contain custom HTTP headers or TLS options (e.g.
   `{ agent: new Https.Agent({ cert: myClientCert, key: myClientKey}) }`).
+  To allow dynamically updating secret, this option can be passed as a *function* returning string
+  to be used as `clientSecret`.
 - `forceHttps` - A boolean indicating whether or not you want the redirect_uri to be forced to
   https. Useful if your hapi application runs as http, but is accessed through https.
 - `location` - Set the base redirect_uri manually if it cannot be inferred properly from server

lib/index.js

@@ -72,6 +72,7 @@ internals.schema = Joi.object({
 
     clientSecret: Joi.alternatives()
         .try(Joi.string().allow(''))
+        .try(Joi.function())
         .conditional('provider.protocol', {
             not: 'oauth',
             then: Joi.object()

lib/oauth.js

@@ -263,6 +263,9 @@ exports.v2 = function (settings) {
             if (typeof settings.clientSecret === 'string') {
                 query.client_secret = settings.clientSecret;
             }
+            else if (typeof settings.clientSecret === 'function') {
+                query.client_secret = settings.clientSecret();
+            }
         }
 
         const requestOptions = {

test/oauth.js

@@ -928,6 +928,48 @@ describe('Bell', () => {
             expect(res.headers.location).to.contain(mock.uri + '/auth?special=true&client_id=test&response_type=code&redirect_uri=http%3A%2F%2Flocalhost%3A8080%2Flogin&state=');
         });
 
+        it('uses clientSecret() function', async (flags) => {
+
+            const mock = await Mock.v2(flags);
+            const server = Hapi.server({ host: 'localhost', port: 8080 });
+            await server.register(Bell);
+
+            let callCount = 0;
+            const secretGetter = () => {
+
+                callCount += 1;
+                return 'secret';
+            };
+
+            server.auth.strategy('custom', 'bell', {
+                password: 'cookie_encryption_password_secure',
+                isSecure: false,
+                clientId: 'test',
+                clientSecret: secretGetter,
+                provider: mock.provider
+            });
+
+            server.route({
+                method: '*',
+                path: '/login',
+                options: {
+                    auth: 'custom',
+                    handler: function (request, h) {
+
+                        return request.auth.credentials;
+                    }
+                }
+            });
+
+            const res1 = await server.inject('/login');
+            const cookie = res1.headers['set-cookie'][0].split(';')[0] + ';';
+
+            const res2 = await mock.server.inject(res1.headers.location);
+
+            await server.inject({ url: res2.headers.location, headers: { cookie } });
+            expect(callCount, 'callCount').to.equal(1);
+        });
+
         it('forces https in redirect_uri when set in options', async (flags) => {
 
             const mock = await Mock.v2(flags);