feat: support staleMaxAge: -1 to always respond stale value (#857)

docs/content/1.guide/1.introduction/5.cache.md

@@ -16,6 +16,7 @@ const cachedFn = cachedEventHandler(fn, options);
 - `getKey`: A function that accepts same arguments of normal function and should generate cache key. If not provided, a built-in hash function will be used.
 - `integrity`: A value that changing it, will invalidate all caches for function. By default will be computed from **function code**.
 - `maxAge`: Maximum age that cache is valid in seconds. Default is `1` second.
+- `staleMaxAge`: Maximum age that a stale cache is valid in seconds. If set to `-1` a stale value will still be sent to the client, while updating the cache in the background.
 - `swr`: Enable Stale-While-Revalidate behavior. Enabled by default.
 - `base`: Name of the storage mointpoint to use for caching (`/cache` by default)
 - `shouldInvalidateCache`: A function that returns a boolean to invalidate the current cache and create a new one.

src/runtime/cache.ts

@@ -78,11 +78,13 @@ export function defineCachedFunction<T = any>(
 
     const _resolve = async () => {
       if (!pending[key]) {
-        // Remove cached entry to prevent using expired cache on concurrent requests
-        entry.value = undefined;
-        entry.integrity = undefined;
-        entry.mtime = undefined;
-        entry.expires = undefined;
+        if (entry.value !== undefined && (opts.staleMaxAge || 0) >= 0) {
+          // Remove cached entry to prevent using expired cache on concurrent requests
+          entry.value = undefined;
+          entry.integrity = undefined;
+          entry.mtime = undefined;
+          entry.expires = undefined;
+        }
         pending[key] = Promise.resolve(resolver());
       }
       entry.value = await pending[key];