fix(persistence): skip save() when serialized props are unchanged#3687
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This was referenced May 28, 2026
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lricoy
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May 28, 2026
Many internal code paths call `Persistence.save()` after touching a
property. When the resulting props serialize to the same string as the
previous successful write, we still wrote identical bytes to localStorage
(or the cookie). Each identical localStorage write fires a `storage`
event on every other same-origin tab — Chrome's mojo IPC allocates the
payload buffer for that event in `partition_alloc/buffer` regardless of
whether any listener reacts. With a multi-hundred-KB blob, the per-tab
off-heap pressure is significant.
Add a `_lastSavedSerialized` cache in `PostHogPersistence`. In `save()`,
serialize `props` and compare against the cache; skip the storage call
if they match. Update the cache on every successful write.
The cache is cleared in `remove()` because the storage entry no longer
exists — the next save must write through regardless of whether the
in-memory props look unchanged. Tests for the cookie/localStorage modes
had to be adjusted to call `mockClear()` after the setup writes because
`cookieStore` and `localStore` are shared singletons.
The existing 'should write to storage if opt_out_persistence_by_default
and opt_out_capturing_by_default is false' test was calling
`persistence.save()` with no state change and asserting that it wrote;
now correctly asserts via `persistence.register({...})`.
Generated-By: PostHog Code
Task-Id: 3946962f-7365-4b7b-8f5f-8f9309ed88c6
Generated-By: PostHog Code Task-Id: 3946962f-7365-4b7b-8f5f-8f9309ed88c6
…es and stale cookie options
Two regressions introduced by the no-op rejection in save():
1. JSON.stringify rejects BigInt and circular refs. Before, those
values were caught by the storage layer's try/catch and silently
dropped; the new preflight fingerprint threw out of save() into
the caller. Wrap the fingerprint in try/catch and fall through to
_storage._set so the storage layer keeps its existing handling.
2. The fingerprint only covered serialized props, but the call to
_storage._set also takes _expire_days, _cross_subdomain and
_secure. A `set_config({ cookie_expiration: 90 })` that didn't
touch any property would no-op the write, leaving the cookie on
the old Expires header. Include all four arguments in the
fingerprint so changes to cookie options trigger a write.
Generated-By: PostHog Code
Task-Id: 3946962f-7365-4b7b-8f5f-8f9309ed88c6
Generated-By: PostHog Code Task-Id: 3946962f-7365-4b7b-8f5f-8f9309ed88c6
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Problem
there is an open P2 bug in chromium that local storage writes cause a memory leak https://issues.chromium.org/issues/394823902
we write to local storage a lot
the PH memory leak in PostHog/posthog#32479 is fiendish to replicate in a test
Changes
let's write to local storage less since it can't hurt...
second, let's not write duplicates... it's a no-op
Release info Sub-libraries affected
Libraries affected
Checklist
If releasing new changes
pnpm changesetto generate a changeset file