AutopilotMonitor/docs/agent/cmtrace-time-resolution.md

type, title, description, resource, tags, timestamp

type	title	description	resource	tags	timestamp
concept	CMTrace Time Resolution — Per-Line Self-Anchoring	How the agent converts bias-less CMTrace local times to UTC without trusting any process's timezone belief — each provably fresh line anchors its own offset against the agent clock on the 15-minute grid; everything not provably fresh falls back uniformly and says so.	src/Agent/AutopilotMonitor.Agent.V2.Core/Monitoring/Enrollment/Ime	agent ime cmtrace timestamps timezone calibration provenance	2026-08-30

Problem

A CMTrace line carries local time and no offset — IME 1.104 writes DateTime.Now.TimeOfDay in both of its trace listeners. Converting such a line with the reader's own TimeZoneInfo.Local silently assumes the writing and reading processes agree on the zone. They do not: each caches its zone at process start and follows neither a later tzutil nor a Windows auto-timezone change. The error is Offset_writer − Offset_reader — zero only when both believe the same thing, right or wrong. Field measurement over 11,068 sessions found 26 sessions with a real error, from +1 h to −17 h.

Two designs failed before this one:

• Fix the reader's belief (TimeZoneInfo.ClearCachedData, live OS offset): makes the agent isolated-correct and thereby breaks the majority of sessions, where two equally stale beliefs cancel.
• One measured offset per file (agent 2.0.1410, reverted in 04b1a7c6): a single log file holds lines from multiple writer eras. AgentExecutor.log is written by short-lived child processes whose zone belief flips per process — interleaved in one file, and flipping back (fixture: tests/fixtures/cmtrace-logs/agentexecutor-two-writer-eras-v1.cmtrace). Any cross-line anchor — per file, or nearest-in-write-order — applies one era's offset to the other era's lines. In the field this shifted script_started events by −9 h against their correct completions.

Schema

The resolution rule, in order:

1. Writer-declared bias (+480 suffix): authoritative, used as-is. Never a calibration anchor — it needs no measurement and must not overwrite one.

2. Per-line self-anchoring — only for lines read in a provably fresh pass:

   offset_line = round((local − agentUtcNow) / 15 min) · 15 min
   lineUtc     = local − offset_line
   

   Every real UTC offset is a whole multiple of 15 minutes, so the rounding absorbs poll and flush latency. No cross-line state exists, so interleaved eras resolve line by line — the property the fixture test pins. The line's own timestamp still contributes sub-poll precision and ordering (two lines 6 ms apart stay 6 ms apart; a plain occurredAt = now could not do that). Guards: grid residual ≤ 2 min, offset within UTC−12 … UTC+14.

3. Uniform reader-zone fallback for everything else: wrong in absolute terms whenever the writer held a different belief, but self-consistent — both ends of a duration are wrong by the same amount, so derived durations stay right. The revert's core lesson: partially corrected is strictly worse than uniformly wrong.

Freshness is what makes rule 2 sound, and it is strict:

• Every poll look at a file stamps LastCheckedUtc (LogFilePositionTracker.MarkChecked), including "no new data" and an empty first sight — so a file first seen empty (AgentExecutor.log appearing mid-enrollment) anchors from its very first content.
• A pass's lines are fresh only if the file was previously observed in this process and the gap since the last look is ≤ FreshLineMaxAge (30 s — far above the 100 ms poll, far below the 13-minute zone where a line's age could round onto the offset grid).
• A restart-restored position bookmark deliberately carries no freshness: the first pass after a restart reads downtime backlog and must never anchor.

Provenance (sourceOffsetOrigin in event DataJson): bias | line-anchored | reader-zone-fallback (plus retired calibrated from 2.0.1410 events). The applied offset is always recorded (sourceOffsetMinutes), the raw local time too (sourceLocalTs), so any event can be re-derived later. The per-file measurement still runs, purely observationally (measuredWriterOffsetMinutes, Info log when it disagrees with the process zone).

Constraints

• An anchored line resolves to now ± 2 min by construction. The accepted edge: a freshly written line carrying a replayed old timestamp whose age is exactly N×15 min ± 2 min anchors to ≈now instead of its replayed past — bounded error, and sub-24 h replays are treated as current by the historic-replay guard anyway.
• Snapshot readers (StallProbeCollector, LogParserCollector) cannot tail and therefore cannot prove freshness — they stay on the marked fallback. This is a documented limit, not an omission; sharing the per-file measurement with them would re-apply exactly what the revert removed.
• Mixed pairs remain possible across a restart (start resolved from backlog fallback, completion line-anchored) and are recognizable via provenance.

Line parsing (bounded cost)

CmTraceLogParser.TryParseLine (Shared) is the single CMTrace line parser for the agent (ImeLogTracker, LogParserCollector, StallProbeCollector) and the backend (TestLogPatternFunction). Both feed it input an outsider influences: tenant-supplied sample lines (200 x 8 KB per request) on the backend, and on the agent every tailed log file, including entries that IME assembles from installer or script output. It is therefore not one regex. The former shape — a greedy (?<message>.*) in front of the ]LOG]!><time=" literal, unanchored, no match timeout — re-tried every <![LOG[ occurrence as a fresh start and backtracked the message group across every later character for each: quadratic in line length (an 8 KB line of repeated prefixes and ] filler cost ~7 million engine steps; a 1 MB assembled entry would have spun the SYSTEM agent for hours and blacked out IME tracking for the rest of the enrollment).

The parser reproduces exactly what that regex returned, in linear time: the message begins after the first <![LOG[ (a later start could only match when the first one does, since .* spans the difference) and ends at the last ]LOG]!><time=" occurrence whose trailer parses — earlier occurrences are tried in turn, which is the order greedy backtracking visited them. The trailer is checked with a \G-anchored regex that has no ambiguous quantifier and a 1-second match timeout as a backstop; a timeout means "not a CMTrace line" (callers already have that path: raw-text matching in the tracker, parse_failed on the backend), never an exception. CmTraceLogParserTests pins the equivalence against the legacy regex over the shapes that matter (multiline messages, bias suffix, trailer literal inside the message, a last trailer that fails to parse, BOM prefix) and the reported worst case at 200 lines.

On the agent, ImeLogTracker additionally caps an assembled multiline entry at 100 raw lines and 1 MB of characters (MaxMultiLineBufferChars). A capped entry is dropped and logged at Warning — visible at the default Info level, so a real IME entry that large shows up in the client log and can be acted on. The remaining physical lines of a dropped entry are skipped until it closes or a new <![LOG[ entry begins; previously they fell through to raw-text pattern matching, so fragments of a discarded entry could fire patterns without CMTrace context.

Backend regression tripwire

The backend watches for the failure mode this design eliminates. On every terminal ingest batch, the counter reconcile's existing event-partition scan additionally collects Δ = ReceivedAt − OccurredUtc per event, split by Source == "ImeLogTracker" versus the rest (CmTraceSkewTripwire, hooked in EventIngestProcessor). If median(Δ_IME) − median(Δ_other) is a clean 15-minute multiple (residual < 2 min — the same constants as the anchoring grid guard above) with ≥ 20 samples over ≥ 3 distinct upload batches per side, and ≥ 80 % of the individual IME deltas (relative to the other side's median) themselves sit within 2 min of some grid multiple — added after the 2026-08-28 soak, when relaunched agents re-tailing the IME log produced one burst holding most of a session's IME samples with a continuum of ages whose median hit the grid by chance (7 false fires in a week, 0 true) — a CmTraceTimeSkewRegression ops event fires (category Agent, warning), carrying all numbers plus the session's sourceOffsetOrigin histogram. Bias-dominated sessions are suppressed (writer-declared offsets cannot be an anchoring regression); measuredWriterOffsetMinutes is never consulted (sticky after era flip-backs). Kill switch: app setting CmTraceSkewTripwireDisabled. Goal state: the event never fires — any hit is an anchoring case the per-line design misses, or a detector bug.

Device-clock problems (the customer-actionable cousin) are covered separately by the clock_skew analyze condition behind ANALYZE-DEV-008, which excludes IME-derived events precisely so an anchoring regression can never surface as a customer finding.

Citations

• CmTraceOffsetCalibrator.TryMeasureOffset — the pure grid measurement, shared by anchoring and the observational per-file path.
• CmTraceSkewTripwire + CmTraceSkewTripwireTests — the backend tripwire above.
• RuleEngine.ConditionEvaluators.EvaluateClockSkewCondition + RuleEngineClockSkewTests — the customer-facing device-clock metrics (batch-median frames, spool-spread filter, plateau step detection, end-state persistence).
• ImeLogTracker.LogProcessing.cs ResolveEntryUtc — the resolution order above.
• ImeLogTrackerLineAnchoringTests — resolves-to-T across writer beliefs, interleaved eras in one chunk, freshness boundaries (first sight, empty first sight, poll gap, restart restore), the grid-replay edge, and the field fixture replayed at production poll cadence.
• CmTraceCalibratorFileIsolationTests — pins per-file isolation of the observational measurement (session e9753578 logged a cross-file pairing the committed code could not be shown to produce; the mechanism remains open, the resolution path is immune by design, and CalibrateFrom logs the anchor timestamp as a tripwire).
• tasks/todo.md (2026-08-20) — full evidence trail: field measurements, the revert, the two-writer-era fixture, and the disproven foreign-binary hypothesis (the SHA mismatch was the known release race against the 5-minute AdminConfiguration cache).
• CmTraceLogParser.TryParseLine + CmTraceLogParserTests — the linear parser and its legacy-regex equivalence oracle; ImeLogTracker.LogProcessing.cs multiline buffering (MaxMultiLineBufferLines / MaxMultiLineBufferChars, skippingDroppedEntry).