perf: skip per-key clone when Onyx.init() hydrates the cache

[WojtekBoman]

Details

Onyx.init() loaded the persisted database into OnyxCache through merge(), which runs fastMerge(existing, value) per key. At init time existing is always undefined, so fastMerge never actually merges anything - it just walks and deep-clones every nested object of every row for no reason. On a heavy account this clone-heavy cache-write phase was measured at ~29% of total Onyx.init() time.

Adds OnyxCache.hydrate(), a bulk-load variant used only by Onyx.init(). Values are stored by reference when safe (utils.needsNormalization() is a read-only, non-allocating check for nested null/undefined and the internal ONYX_INTERNALS__REPLACE_OBJECT_MARK). Falls back to the existing fastMerge path in two cases: the key already has a value in cache (a cross-tab sync listener can race ahead of init and write a key before hydrate() reaches it), or the value itself needs normalizing.

OnyxUtils.ts's initializeWithDefaultKeyStates() now calls cache.hydrate(allDataFromStorage) instead of cache.merge(allDataFromStorage).

Related Issues

GH_LINK

Linked E/App PR

Automated Tests

No automated test added in this diff yet. Needed before merge: an equivalence test asserting hydrate() produces identical cache state to merge() on an empty cache, with fixtures for nested nulls, nested undefined, the replace-object mark, and plain values.

Manual Tests

1. Sign in to an account with a reasonably large Onyx database (many reports/reportActions), force-quit and cold-start the app.
   Verify that the app boots normally, the LHN and reports render with the same data as before the change, and no console errors appear.
2. Cold-start the app, open a report with actions containing deleted/optimistic-cleared fields (values that were null on disk).
   Verify that the report renders correctly with those fields absent, matching pre-change behavior.
3. On web, open the app in two browser tabs signed into the same account. In tab A, trigger a write (e.g. send a message) right as tab B is cold-starting/reloading.
   Verify that tab B's cache ends up consistent with tab A's write - i.e. the value isn't clobbered by hydrate()'s bulk load racing the cross-tab sync listener.

Offline tests

1. Put the device in airplane mode, force-quit and cold-start the app.
   Verify that the app boots with the last-persisted cache (same as before this change) and no data is lost or duplicated.
2. While offline, open a report and confirm previously-persisted reportActions/transactions render correctly, then go back online and verify no reconciliation errors occur.

QA Steps

Same as tests.

Author Checklist

• I linked the correct issue in the ### Related Issues section above
• I linked the corresponding Expensify/App PR in the ### Linked E/App PR section above, and verified this change against it (E/App CI passed and manual testing completed)
• I wrote clear testing steps that cover the changes made in this PR
  • I added steps for local testing in the Tests section
  • I tested this PR with a High Traffic account against the staging or production API to ensure there are no regressions (e.g. long loading states that impact usability).
• I included screenshots or videos for tests on all platforms
• I ran the tests on all platforms & verified they passed on:
  • Android / native
  • Android / Chrome
  • iOS / native
  • iOS / Safari
  • MacOS / Chrome / Safari
• I verified there are no console errors (if there's a console error not related to the PR, report it or open an issue for it to be fixed)
• I followed proper code patterns (see Reviewing the code)
  • I verified that any callback methods that were added or modified are named for what the method does and never what callback they handle (i.e. toggleReport and not onIconClick)
  • I verified that the left part of a conditional rendering a React component is a boolean and NOT a string, e.g. myBool && <MyComponent />.
  • I verified that comments were added to code that is not self explanatory
  • I verified that any new or modified comments were clear, correct English, and explained "why" the code was doing something instead of only explaining "what" the code was doing.
  • I verified proper file naming conventions were followed for any new files or renamed files. All non-platform specific files are named after what they export and are not named "index.js". All platform-specific files are named for the platform the code supports as outlined in the README.
  • I verified the JSDocs style guidelines (in STYLE.md) were followed
• If a new code pattern is added I verified it was agreed to be used by multiple Expensify engineers
• I followed the guidelines as stated in the Review Guidelines
• I tested other components that can be impacted by my changes (i.e. if the PR modifies a shared library or component like Avatar, I verified the components using Avatar are working as expected)
• I verified all code is DRY (the PR doesn't include any logic written more than once, with the exception of tests)
• I verified any variables that can be defined as constants (ie. in CONST.js or at the top of the file that uses the constant) are defined as such
• I verified that if a function's arguments changed that all usages have also been updated correctly
• If a new component is created I verified that:
  • A similar component doesn't exist in the codebase
  • All props are defined accurately and each prop has a /** comment above it */
  • The file is named correctly
  • The component has a clear name that is non-ambiguous and the purpose of the component can be inferred from the name alone
  • The only data being stored in the state is data necessary for rendering and nothing else
  • If we are not using the full Onyx data that we loaded, I've added the proper selector in order to ensure the component only re-renders when the data it is using changes
  • For Class Components, any internal methods passed to components event handlers are bound to this properly so there are no scoping issues (i.e. for onClick={this.submit} the method this.submit should be bound to this in the constructor)
  • Any internal methods bound to this are necessary to be bound (i.e. avoid this.submit = this.submit.bind(this); if this.submit is never passed to a component event handler like onClick)
  • All JSX used for rendering exists in the render method
  • The component has the minimum amount of code necessary for its purpose, and it is broken down into smaller components in order to separate concerns and functions
• If any new file was added I verified that:
  • The file has a description of what it does and/or why is needed at the top of the file if the code is not self explanatory
• If the PR modifies a generic component, I tested and verified that those changes do not break usages of that component in the rest of the App (i.e. if a shared library or component like Avatar is modified, I verified that Avatar is working as expected in all cases)
• If the main branch was merged into this PR after a review, I tested again and verified the outcome was still expected according to the Test steps.
• I have checked off every checkbox in the PR author checklist, including those that don't apply to this PR.

Screenshots/Videos

Android: Native

Android: mWeb Chrome

iOS: Native

iOS: mWeb Safari

MacOS: Chrome / Safari

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