From 7d612ab344a7f03cbb4961bc39b88b79874ac540 Mon Sep 17 00:00:00 2001
From: John McLear <john@mclear.co.uk>
Date: Sat, 16 May 2026 08:48:41 +0100
Subject: [PATCH] perf: avoid throw-as-control-flow in SessionManager hot path
 (#7756)
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CPU profile of the SUT at the 100-400 author dive sweep
(load-test workflow run 25956384097) attributed about 6% of total
process CPU to the throw + catch around getSessionInfo:

  - ~1.82% to `new CustomError('sessionID does not exist', 'apierror')`
    construction (stack trace capture)
  - ~4.12% downstream, via the catch block's `console.debug(...)`
    routed through log4js -> sendToListeners -> sendLogEventToAppender

Both call sites (`findAuthorID` on every CLIENT_READY, and
`listSessionsWithDBKey` on session listing) immediately caught
`apierror` and discarded it. The public `exports.getSessionInfo`
contract still has to throw for the HTTP API (returning code:1 for
missing sessionID), so introduce a private `getSessionInfoOrNull`
helper that returns null and have the hot-path callers use it
directly. `exports.getSessionInfo` is kept as a thin wrapper that
preserves the existing throw semantics.

No behaviour change for the HTTP API — sessionsAndGroups.ts test
file (32 cases, including "getSessionInfo of deleted session"
expecting code:1) passes unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
---
 src/node/db/SessionManager.ts | 42 +++++++++++++++++------------------
 1 file changed, 20 insertions(+), 22 deletions(-)

diff --git a/src/node/db/SessionManager.ts b/src/node/db/SessionManager.ts
index b8b1b2562dc..e2f785d82e1 100644
--- a/src/node/db/SessionManager.ts
+++ b/src/node/db/SessionManager.ts
@@ -62,18 +62,8 @@ exports.findAuthorID = async (groupID:string, sessionCookie: string) => {
    * Also, see #3820.
    */
   const sessionIDs = sessionCookie.replace(/^"|"$/g, '').split(',');
-  const sessionInfoPromises = sessionIDs.map(async (id) => {
-    try {
-      return await exports.getSessionInfo(id);
-    } catch (err:any) {
-      if (err.message === 'sessionID does not exist') {
-        console.debug(`SessionManager getAuthorID: no session exists with ID ${id}`);
-      } else {
-        throw err;
-      }
-    }
-    return undefined;
-  });
+  const sessionInfoPromises = sessionIDs.map(async (id) =>
+      (await getSessionInfoOrNull(id)) || undefined);
   const now = Math.floor(Date.now() / 1000);
   const isMatch = (si: {
     groupID: string;
@@ -163,9 +153,20 @@ exports.createSession = async (groupID: string, authorID: string, validUntil: nu
  * @param {String} sessionID The id of the session
  * @return {Promise<Object>} the sessioninfos
  */
+// Non-throwing variant for hot-path callers. Hot path uses
+// `findAuthorID` on every CLIENT_READY and `listSessionsWithDBKey` on
+// session listing; both wrap getSessionInfo in try/catch and discard
+// "sessionID does not exist" CustomError. Profiling against develop at
+// 100-400 author sweep (ether/etherpad#7756) attributed ~6% of total
+// CPU to that throw+catch pair: ~1.8% to CustomError construction and
+// ~4% to the cascading `console.debug` call routed through log4js. A
+// null return collapses the cost to a single nullable check.
+const getSessionInfoOrNull = async (sessionID: string) =>
+  await db.get(`session:${sessionID}`);
+
 exports.getSessionInfo = async (sessionID:string) => {
   // check if the database entry of this session exists
-  const session = await db.get(`session:${sessionID}`);
+  const session = await getSessionInfoOrNull(sessionID);
 
   if (session == null) {
     // session does not exist
@@ -250,15 +251,12 @@ const listSessionsWithDBKey = async (dbkey: string) => {
 
   // iterate through the sessions and get the sessioninfos
   for (const sessionID of Object.keys(sessions || {})) {
-    try {
-      sessions[sessionID] = await exports.getSessionInfo(sessionID);
-    } catch (err:any) {
-      if (err.name === 'apierror') {
-        console.warn(`Found bad session ${sessionID} in ${dbkey}`);
-        sessions[sessionID] = null;
-      } else {
-        throw err;
-      }
+    const info = await getSessionInfoOrNull(sessionID);
+    if (info == null) {
+      console.warn(`Found bad session ${sessionID} in ${dbkey}`);
+      sessions[sessionID] = null;
+    } else {
+      sessions[sessionID] = info;
     }
   }