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packages/core/src/request-context.ts

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+/**
+ * EmDash Request Context
+ *
+ * Uses AsyncLocalStorage to provide request-scoped state to query functions
+ * without requiring explicit parameter passing. The middleware wraps next()
+ * in als.run(), making the context available to all code during rendering.
+ *
+ * For logged-out users with no CMS signals (no edit cookie, no preview param),
+ * the middleware skips ALS entirely — zero overhead for normal traffic.
+ *
+ * The AsyncLocalStorage instance is stored on globalThis with a Symbol key
+ * to guarantee a singleton even when bundlers duplicate this module across
+ * code-split chunks. Without this, Rollup/Vite may inline the module into
+ * multiple chunks (e.g. middleware and page components), each with its own
+ * ALS instance — breaking request-scoped state propagation.
+ */
+
+import { AsyncLocalStorage } from "node:async_hooks";
+
+export interface EmDashRequestContext {
+	/** Whether the current request is in visual editing mode */
+	editMode: boolean;
+	/** Preview token info, if this is a preview request */
+	preview?: {
+		collection: string;
+		id: string;
+	};
+	/** Current locale from Astro's i18n routing (when configured) */
+	locale?: string;
+	/**
+	 * Per-request database override.
+	 *
+	 * Set by middleware when D1 read replica sessions are enabled.
+	 * The runtime's `db` getter checks this first, falling back to
+	 * the singleton instance. Also used by the DO preview pattern.
+	 */
+	db?: unknown;
+}
+
+const ALS_KEY = Symbol.for("emdash:request-context");
+
+const storage: AsyncLocalStorage<EmDashRequestContext> =
+	// eslint-disable-next-line typescript-eslint(no-unsafe-type-assertion) -- globalThis singleton pattern
+	((globalThis as Record<symbol, unknown>)[ALS_KEY] as
+		| AsyncLocalStorage<EmDashRequestContext>
+		| undefined) ??
+	(() => {
+		const als = new AsyncLocalStorage<EmDashRequestContext>();
+		(globalThis as Record<symbol, unknown>)[ALS_KEY] = als;
+		return als;
+	})();
+
+/**
+ * Run a function within an EmDash request context.
+ * Called by middleware to wrap next().
+ */
+export function runWithContext<T>(ctx: EmDashRequestContext, fn: () => T): T {
+	return storage.run(ctx, fn);
+}
+
+/**
+ * Get the current request context.
+ * Returns undefined if no context is set (logged-out fast path).
+ */
+export function getRequestContext(): EmDashRequestContext | undefined {
+	return storage.getStore();
+}