perf: skip no-op UserDefaults writes on every session autosave - #14822

Summary

Every session autosave write made three UserDefaults mutations, even when nothing had changed: it removed a legacy geometry key, rewrote the window-geometry blob, and removed the crash-only snapshot marker. UserDefaults posts didChangeNotification for every set/removeObject, including no-op ones. So each save ran every defaults observer in the app on com.cmuxterm.app.sessionPersistence. That queued a main-actor refresh for each addUserDefaultsObserver client (about 15 on main) and ran SwiftUI's @AppStorage observer (176 @AppStorage uses in Sources/). The SwiftUI observer takes SwiftUI's global update lock, so it contends with main-thread rendering.

Evidence from sample of the installed 0.64.25 app on a heavily loaded machine (10 s, one autosave write in the window). On the persistence queue, the defaults writes and their observer fan-out took about 400 samples, against about 120 for the JSON encode plus file write. That included:

209  UserDefaultObserver.userDefaultsDidChange -> Update.enqueueAction -> Update.begin -> _MovableLockLock -> __psynch_mutexwait
 83  UserDefaultObserver.userDefaultsDidChange -> Update.enqueueAction -> Update.begin -> _MovableLockLock -> __psynch_mutexwait

This change writes defaults only when the stored value actually changes. It uses new change-only helpers in CmuxFoundation (setIfChanged(_:forKey:) for Data/Bool, removeObjectIfPresent(forKey:)). The geometry blob is now encoded with .sortedKeys, so the byte comparison is stable. Decoding is unchanged.

Before and after, per steady-state autosave write: 3 didChangeNotification posts before, 0 after. That no-op writes post at all was confirmed with an isolated suite: a second set of identical Data, and removeObject of an absent key, each post one notification. Window geometry and crash-marker semantics are unchanged; real changes still write and notify once.

Testing

• Regression, two commits: 0faeb27727f adds crashOnlyPrimarySnapshotRemovalMarkerSkipsNoOpDefaultsWrites to CrashDiagnosticSessionPolicyTests. It asserts that clearing an absent marker and re-marking a set marker post no notifications, and is expected to fail on that commit. 233ef82df77 adds the fix.
• New package tests: UserDefaultsChangeOnlyWritesTests in CmuxFoundation (Data, Bool, and remove paths).
• python3 scripts/verify-local.py --affected --swift-changed passed swift-syntax, test-wiring, package-groups and feature-flags.
• Not yet verified: compilation and test execution. I didn't build locally because the machine is out of memory; fork CI run: https://github.com/teamleaderleo/cmux/actions/runs/36246652921. No tagged-build dogfood yet.

Checklist

• Behavior changes have added or updated tests, or Testing says why not
• Reviewed with a subagent before merge (cmux-review), and all bot and human review comments resolved

🤖 Generated with Claude Code

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Summary by cubic

Skips no-op UserDefaults writes on session autosave so steady-state saves no longer post didChangeNotification or wake every defaults observer.

Each autosave previously rewrote the window-geometry blob, removed a legacy geometry key, and cleared the crash-only snapshot marker even when nothing changed. UserDefaults posts a change notification for every set/remove, including no-ops, which ran all ~15 defaults observers plus SwiftUI's @AppStorage observer (176 uses), contending with main-thread rendering via SwiftUI's global update lock.

Refactors

• New CmuxFoundation helpers setIfChanged(_:forKey:) and removeObjectIfPresent(forKey:) skip writes when the stored value is unchanged.
• Geometry bytes are now encoded with .sortedKeys so byte comparison is stable across saves.
• Crash-marker clearing, legacy-key removal, and geometry persistence use the change-only helpers.

Behavior is unchanged: real changes still write and notify exactly once, and steady-state autosaves now post zero notifications instead of three.

^{Written for commit 233ef82. Summary will update on new commits.}

Summary by CodeRabbit

• Improvements
  • Window geometry and crash-recovery settings now avoid redundant updates when their stored values are unchanged. Missing settings are also left untouched rather than triggering unnecessary removals.
  • These changes reduce needless settings activity during window saves and crash-recovery state updates, while preserving behavior when values actually change.