Feature: Allowing relative redirect url though an option (#2183)

* Adding relative redirect url option

* Updating CHANGELOG.md

* tests: adding unit test for getOAuthRedirectURI

---------

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

oauthproxy_test.go

@@ -3257,3 +3257,90 @@ func TestAuthOnlyAllowedEmails(t *testing.T) {
 		})
 	}
 }
+
+func TestGetOAuthRedirectURI(t *testing.T) {
+	tests := []struct {
+		name      string
+		setupOpts func(*options.Options) *options.Options
+		req       *http.Request
+		want      string
+	}{
+		{
+			name: "redirect with https schema",
+			setupOpts: func(baseOpts *options.Options) *options.Options {
+				return baseOpts
+			},
+			req: &http.Request{
+				Host: "example",
+				URL: &url.URL{
+					Scheme: schemeHTTPS,
+				},
+			},
+			want: "https://example/oauth2/callback",
+		},
+		{
+			name: "redirect with http schema",
+			setupOpts: func(baseOpts *options.Options) *options.Options {
+				baseOpts.Cookie.Secure = false
+				return baseOpts
+			},
+			req: &http.Request{
+				Host: "example",
+				URL: &url.URL{
+					Scheme: schemeHTTP,
+				},
+			},
+			want: "http://example/oauth2/callback",
+		},
+		{
+			name: "relative redirect url",
+			setupOpts: func(baseOpts *options.Options) *options.Options {
+				baseOpts.RelativeRedirectURL = true
+				return baseOpts
+			},
+			req:  &http.Request{},
+			want: "/oauth2/callback",
+		},
+		{
+			name: "proxy prefix",
+			setupOpts: func(baseOpts *options.Options) *options.Options {
+				baseOpts.ProxyPrefix = "/prefix"
+				return baseOpts
+			},
+			req: &http.Request{
+				Host: "example",
+				URL: &url.URL{
+					Scheme: schemeHTTP,
+				},
+			},
+			want: "https://example/prefix/callback",
+		},
+		{
+			name: "proxy prefix with relative redirect",
+			setupOpts: func(baseOpts *options.Options) *options.Options {
+				baseOpts.ProxyPrefix = "/prefix"
+				baseOpts.RelativeRedirectURL = true
+				return baseOpts
+			},
+			req: &http.Request{
+				Host: "example",
+				URL: &url.URL{
+					Scheme: schemeHTTP,
+				},
+			},
+			want: "/prefix/callback",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			baseOpts := baseTestOptions()
+			err := validation.Validate(baseOpts)
+			assert.NoError(t, err)
+
+			proxy, err := NewOAuthProxy(tt.setupOpts(baseOpts), func(string) bool { return true })
+			assert.NoError(t, err)
+
+			assert.Equalf(t, tt.want, proxy.getOAuthRedirectURI(tt.req), "getOAuthRedirectURI(%v)", tt.req)
+		})
+	}
+}