From 01acacc27b118708701212a5e7e39ccc5e719ac6 Mon Sep 17 00:00:00 2001
From: Linus Rath <139418639+rathlinus@users.noreply.github.com>
Date: Sat, 31 Oct 2026 23:48:40 +0100
Subject: [PATCH] fix: answer 503 instead of 500 when the shared store is full
 or too busy to take a write #1066

---
 .../__tests__/store-errors.test.ts            | 22 +++++++++++++++++++
 lib/shared-store/redis.ts                     | 11 ++++++----
 2 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/lib/shared-store/__tests__/store-errors.test.ts b/lib/shared-store/__tests__/store-errors.test.ts
index a957b63b6..b6f8564ee 100644
--- a/lib/shared-store/__tests__/store-errors.test.ts
+++ b/lib/shared-store/__tests__/store-errors.test.ts
@@ -12,3 +12,25 @@ describe('isStoreUnavailableError', () => {
     expect(second.isStoreUnavailableError(null)).toBe(false);
   });
 });
+
+describe('redis store error mapping', () => {
+  const reply = (message: string) => Object.assign(new Error(message), { name: 'ReplyError' });
+
+  it('treats a full server (OOM under noeviction) and connection errors as unavailable', async () => {
+    const { unavailable } = await import('../redis');
+    const { isStoreUnavailableError } = await import('../types');
+    for (const error of [reply('OOM command not allowed when used memory > \'maxmemory\'.'), reply('LOADING Redis is loading'), new Error('Stream isn\'t writeable')]) {
+      let thrown: unknown;
+      try { unavailable(error); } catch (e) { thrown = e; }
+      expect(isStoreUnavailableError(thrown)).toBe(true);
+    }
+  });
+
+  it('rethrows script and command errors as they are', async () => {
+    const { unavailable } = await import('../redis');
+    const bug = reply('ERR Error running script (call to f_x): attempt to compare nil');
+    let thrown: unknown;
+    try { unavailable(bug); } catch (e) { thrown = e; }
+    expect(thrown).toBe(bug);
+  });
+});
diff --git a/lib/shared-store/redis.ts b/lib/shared-store/redis.ts
index 5483f4e5d..f698facec 100644
--- a/lib/shared-store/redis.ts
+++ b/lib/shared-store/redis.ts
@@ -128,9 +128,11 @@ type ScriptName = keyof typeof SCRIPTS;
 type Scripted = Redis & { [K in ScriptName]: (...args: Array<string | number | Buffer>) => Promise<unknown> };
 
 // Server replies that mean "not available right now", not "bug in our script".
-const UNAVAILABLE_REPLY = /^(NOAUTH|WRONGPASS|LOADING|READONLY|MASTERDOWN|BUSY|CLUSTERDOWN|TRYAGAIN|ERR max number of clients)/;
+// OOM is what a full server with the required noeviction policy answers to writes.
+const UNAVAILABLE_REPLY = /^(NOAUTH|WRONGPASS|LOADING|READONLY|MASTERDOWN|BUSY|CLUSTERDOWN|TRYAGAIN|OOM|NOREPLICAS|ERR max number of clients)/;
 
-function unavailable(error: unknown): never {
+/** Throw `error` as a StoreUnavailableError when it means the store is unavailable; rethrow it as it is otherwise. */
+export function unavailable(error: unknown): never {
   if (isStoreUnavailableError(error)) throw error;
   const name = (error as Error)?.name;
   const message = (error as Error)?.message ?? '';
@@ -233,7 +235,8 @@ export function createRedisStore(options: RedisStoreOptions): SharedStore {
         if (ok === 1) return next;
         await backoff(attempt);
       }
-      throw new Error(`Too many concurrent writes to ${key}`);
+      // Fail closed like an outage: the callers then answer 503 instead of 500.
+      throw new StoreUnavailableError(`Too many concurrent writes to ${key}`);
     },
   };
 
@@ -318,7 +321,7 @@ export function createRedisStore(options: RedisStoreOptions): SharedStore {
           }
           await backoff(attempt);
         }
-        throw new Error(`Too many concurrent writes to ${name}/${id}`);
+        throw new StoreUnavailableError(`Too many concurrent writes to ${name}/${id}`);
       },
       async delete(id, putOpts: PutOptions = {}) {
         assertDocId(id);