fix(build-executor): release build log buffers on the cache-hit cleanup path (Closes #107)

[SakethSumanBathini]

Closes #107

I've left a comment on #107 with the full trace. Short version: the sessions Map named in the issue is already cleaned up, but there is a real unbounded-growth bug four lines away, and this PR fixes that one.

Why the stated diagnosis no longer holds

Traced against main @ 2f06e6e:

• cleanup() calls this.sessions.delete(sessionId) on both exits — line 1811 (the cached-directory early return) and line 1825 (the normal path).
• All five this.sessions.set(...) sites (1023, 1064, 1095, 1132, 1178) are each followed by a scheduleCleanup(sessionId) call (1035, 1075, 1106, 1145, 1192), so every session created gets a cleanup timer.
• Session IDs come from crypto.randomBytes(8).toString('hex'), which always satisfies the /^[0-9a-f]{16}$/ guard at line 1785, so that early return never strands an entry either.

What does leak

this.buildLogs. It is populated in _initLog (line 431) and deleted in exactly one place — line 1826, which sits after the isDirCached early return at 1808-1813. That branch deletes sessions and returns, so buildLogs and buildPromises are never cleared on it.

That branch is the cache-hit path, which is the one a long-running server takes most often when rebuilding repos it has seen before — precisely the scenario the issue describes. Each stranded entry holds up to 2000 log-line objects (bounded per entry by the shift() at line 442, but unbounded in the number of entries) plus a subscribers Set.

So the issue's diagnosis is stale, but its thesis is right: there is unbounded map growth in BuildExecutor cleanup that grows memory on long-running instances. It's buildLogs, not sessions.

Change

Releases both maps on the early-return branch, matching what the normal path already does:

if (isDirCached) {
  console.log(`[BuildCache] Keeping ${repoKey} directory (still in cache)`);
  this.sessions.delete(sessionId);
  this.buildLogs.delete(sessionId);
  this.buildPromises.delete(sessionId);
  return;
}

artifactCache.invalidateSession() deliberately stays on the normal path only — invalidating it here would defeat the cache this branch exists to preserve. The on-disk directory is likewise untouched.

Verification

• npx tsc --noEmit in backend/ clean.
• Every mutation site for buildLogs, buildPromises and pdfBuilds audited: pdfBuilds already self-cleans via promise.finally() at line 1778 and buildPromises partially self-cleans at 1496/1508, so buildLogs is the only map with no other release path.

Note: npm test is red on clean main independently of this branch.

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