feat: support reverse proxy path prefix deployments (#257)

* feat: support reverse proxy path prefixes

* fix: respect BASE_URL in SAML callback fallback

* fix: make BASE_URL runtime configurable

src/lib/base-path.ts

@@ -0,0 +1,83 @@
+const URL_SCHEME_REGEX = /^[a-zA-Z][a-zA-Z\d+\-.]*:/;
+const BASE_PATH_WINDOW_KEY = "__GITEA_MIRROR_BASE_PATH__";
+
+function normalizeBasePath(basePath: string | null | undefined): string {
+  if (!basePath) {
+    return "/";
+  }
+
+  let normalized = basePath.trim();
+  if (!normalized) {
+    return "/";
+  }
+
+  if (!normalized.startsWith("/")) {
+    normalized = `/${normalized}`;
+  }
+
+  normalized = normalized.replace(/\/+$/, "");
+  return normalized || "/";
+}
+
+function resolveRuntimeBasePath(): string {
+  if (typeof process !== "undefined" && typeof process.env?.BASE_URL === "string") {
+    return normalizeBasePath(process.env.BASE_URL);
+  }
+
+  if (typeof window !== "undefined") {
+    const runtimeBasePath = (window as Window & { [BASE_PATH_WINDOW_KEY]?: string })[BASE_PATH_WINDOW_KEY];
+    if (typeof runtimeBasePath === "string") {
+      return normalizeBasePath(runtimeBasePath);
+    }
+  }
+
+  return "/";
+}
+
+export function getBasePath(): string {
+  return resolveRuntimeBasePath();
+}
+
+export const BASE_PATH = getBasePath();
+export { BASE_PATH_WINDOW_KEY };
+
+export function withBase(path: string): string {
+  const basePath = getBasePath();
+
+  if (!path) {
+    return basePath === "/" ? "/" : `${basePath}/`;
+  }
+
+  if (URL_SCHEME_REGEX.test(path) || path.startsWith("//")) {
+    return path;
+  }
+
+  const normalizedPath = path.startsWith("/") ? path : `/${path}`;
+  if (basePath === "/") {
+    return normalizedPath;
+  }
+
+  return `${basePath}${normalizedPath}`;
+}
+
+export function stripBasePath(pathname: string): string {
+  const basePath = getBasePath();
+
+  if (!pathname) {
+    return "/";
+  }
+
+  if (basePath === "/") {
+    return pathname;
+  }
+
+  if (pathname === basePath || pathname === `${basePath}/`) {
+    return "/";
+  }
+
+  if (pathname.startsWith(`${basePath}/`)) {
+    return pathname.slice(basePath.length) || "/";
+  }
+
+  return pathname;
+}