8.3 KiB
TD: Pixel Definition: CPM Failure Diagnostics (macOS/iOS)
Author: Alex M
Reviewer: TBD
Stakeholders: Michal, Konrad, Russell (from the first Tech Design; confirm for this proposal)
Project: CPM: root-cause the stuck embedded extension messaging (macOS/iOS)
Previous Tech Design: CPM breakage pixels
Background & Requirements
The first CPM pixels detect initialization failure, a stuck messaging episode and recovery. They do not show whether the background process stopped, the extension failed to load, or the tab used the wrong extension configuration.
Terminating the extension's background process can leave CPM unable to respond, even after page reloads or opening new tabs. Add process, extension and tab state to existing iOS/macOS failure pixels, even when the extension cannot respond.
Problem Statement
Distinguish background-process and extension-context failures from problems affecting one tab, and identify failed reloads. Collect no page content, URLs or persistent identifiers.
This proposal covers diagnostic collection only; it does not change recovery behavior or introduce an experiment.
Recommended Approach
Keep the existing failure/stuck detection. A shared recorder holds lifecycle and critical-memory-pressure events in memory. When a pixel fires, it captures current WebKit/tab state synchronously on the main actor. Earlier context loads cannot add events to this timeline. There are no JavaScript probes or periodic uploads.
A delegate proxy records process termination and responsiveness, then forwards callbacks to WebKit. WebKit's delegate owns the proxy; the proxy holds it weakly. The remote flag enables the proxy on the current view. Disabling restores original delegates on all surviving views, including after unload; other diagnostics remain enabled.
| Pixel | Trigger | Frequency |
|---|---|---|
debug_web_extension_cpm_initialization_failed_after_<reason> |
Existing initialization failure; add diagnostic parameters. | Daily |
debug_web_extension_cpm_messaging_stuck_<reason> |
Existing failure confirmed by a later eligible navigation; add diagnostic parameters. | Daily + count, once per episode |
debug_web_extension_reload_failed |
New: extension unload/load fails during a reload operation. Applies to all supported extension types. | Daily + count |
<reason>: session_restoration, tab_crash, extension_reload, other. PixelKit applies standard frequency/platform suffixes. Standard metadata includes appVersion; macOS also declares pixelSource and channel.
Recovery pixels remain unchanged. The new reload-failure pixel reports a failed reload operation, unlike the existing cpm_messaging_extension_reload_failed, which reports CPM failure after a successful reload.
Failure / stuck parameters
Unavailable optional facts are omitted. The first nine parameters are booleans (true / false); PIDs are never sent.
| Parameter | What is collected |
|---|---|
context_loaded |
Context is loaded. |
bg_view_alive |
Current background WKWebView exists. |
bg_process_alive |
Background process identifier is nonzero. |
bg_process_responsive |
WebKit's process-responsiveness verdict. |
network_restarted |
Network PID differs from the value at context load. |
tab_in_context |
Failing tab is in WebKit's open-tabs set. |
tab_controller_match |
Tab and context use the same extension controller. |
tab_has_ext_scripts |
Registered scripts include the expected extension world; not proof of execution. |
handler_registered |
Autoconsent native-message handler is registered; not proof of message delivery. |
critical_memory_age |
Time since critical memory pressure: none (not observed), 1m (<1 min), 5m (1–<5 min), 30m (5–<30 min), over_30 (≥30 min). |
bg_view_creations |
Creations since context load: 0, 1, 2, 3_to_5, over_5. |
bg_view_leaked_count |
Retained views beyond the expected current view; same buckets. Not proof of a leak. |
context_errors |
none or accumulated error descriptors: raw domain/code pairs, including the immediate underlying error. No localized text; no explicit payload cap. |
bg_events |
Recent lifecycle sequence with relative ages in whole seconds. |
The recorder keeps 40 events. bg_events sends up to the last 12, within 255 characters, dropping older entries first. Format: event_name@-seconds before the snapshot.
Events: load, view, dealloc, unresponsive, responsive, proxy_on, proxy_off, died_<reason>, error_<descriptor>, context_unload_failed_<descriptor>, extension_files_remove_failed_<descriptor>.
Termination reasons: memory, cpu, client, crash, crash_limit, unknown. The error_ events record background-load failures. CPM unload failures (reload, data clearing or uninstall) and uninstall file-removal failures are also recorded. Failed unloads keep the context; file-removal errors can still be recorded after unload. These entries reach the server only with a later failure/stuck pixel; existing standalone error pixels are unchanged.
Timeline labels are lowercased, character-filtered and truncated to 64 characters, which can cut off error codes. context_errors and reload error fields preserve raw domain/code pairs.
Example: view@-60,died_crash@-42 means the view was created 60 seconds before the snapshot and its process crashed 42 seconds before it.
Reload-failure parameters
| Parameter | Values |
|---|---|
extension_type |
embedded, darkMode, adBlocking, searchToken, unknown |
reload_trigger |
data_clearing, scriptlet_update, explicit |
reload_phase |
unload, load, light_load, full_load, fallback_load |
The three fields above are allowlisted enums. The pixel also sends standard error parameters:
| Parameter | Values |
|---|---|
d (errorDomain) |
Original NSError domain. |
e (errorCode) |
Original numeric NSError code. |
ud (underlyingErrorDomain) |
Immediate underlying NSError domain, when present. |
ue (underlyingErrorCode) |
Immediate underlying numeric code, when present. |
Localized descriptions, file paths and other userInfo contents are not serialized. Only the immediate underlying error is included.
Testing
- Unit tests: parameter serialization, buckets, timeline limits, attachment to failure/stuck pixels, reload-error mapping, delegate forwarding, ownership and runtime flag changes.
- Integration validation on iOS/macOS: force background-process termination, check the emitted diagnostics, test proxy on/off and extension reload failures. Confirm unknown SPI values are omitted.
- Privacy validation: inspect actual payloads and run both platform schema validators. Confirm URLs, identifiers and error messages are absent. Review exact-second timelines and error-domain/code values before release.
Additional Considerations (if applicable)
Privacy
New pixel and additional parameters require Privacy Triage. These payloads do not intentionally collect browsing URLs, search queries, page content, message payloads, tab/context IDs or PIDs. Exact-second timelines and unrestricted domain/code pairs create many distinct values; Privacy Triage must decide whether to bucket or restrict them. data_clearing discloses the operation preceding a failed reload.
Local context-error diagnostics log only original and immediate underlying error domains/codes, without localized descriptions or other userInfo contents.
Security
Uses guarded private WebKit selectors for process/script facts. Validate supported OS versions; missing facts must not block pixel delivery.
Site Breakage
No intended page-behavior changes. The proxy changed the bug's reproduction in an earlier test. Test the current proxy on/off to check whether it still affects the failure.
Experimentation
None. Recovery changes and experiments are outside this proposal.
Operational
Update the existing CPM Grafana board for the new fields and reload failures. Agree a diagnostic removal/review date; none is enforced currently. Failure-only snapshots cannot measure the failure rate across all process deaths or prove that all tabs failed. Older events, including the trigger, can drop out of the timeline.
Localization / Internationalization
None.