sqlite-safe-backup row-count bracket cannot bound a non-monotonic table - a transient lock row deleted the hub backup
hub-db-backup exited 6 on 2026-10-06 and DELETED the day's copy over a one-row transient: 'table locks: source 0->0 during backup, copy=1 (outside the window)', 47-table comparison, run 07:45:15-07:57:44 (12min wall, 10min08s CPU). No restore point for that day; the 2026-10-05 artifact is verified intact at 139838 rows, so a one-day gap only. ROOT CAUSE IS NOT THE LOAD SPIKE, AND THAT CHANGES THE FIX. nw-venus-cc attributed it to the 05:00 CRIT load spike (load1 12.06, 2.7GB swap) stretching sqlite's .backup from ~10s to 12min, and proposed hardening the bracket for long-duration runs under contention. Duration only raises the PROBABILITY. The actual defect is structural: sqlite-safe-backup.sh v1.2 verifies each table by requiring the copy's row count to land inside [min(pre,post), max(pre,post)] of the SOURCE count (lines 176-200), which is a valid bound only for a MONOTONIC table. 'locks' is a lease table - resource TEXT PRIMARY KEY, expires_at INTEGER, rows inserted and deleted as holders come and go - so its count is non-monotonic by construction. A lock held mid-run and released before the post-count gives source 0->0 with copy=1: the copy is CORRECT and the window carries no information about it. A 10-second run hits this identically if a lock happens to be held across it, so a duration-keyed fix leaves the defect in place and merely makes it rarer. THE ROW IS ALMOST CERTAINLY A PUSH GATE LOCK. 'locks' currently holds exactly one row, venus-host.push-gate, the gate gitpush.sh takes on every push - so any agent pushing to sh.git during the backup window can destroy the hub's restore point for that day, including the seat that maintains both scripts. Measured while diagnosing: my own pushes today held that lock. OWNERSHIP, from the registry rather than inference: row 138 sqlite-safe-backup.sh maintainer bin-venus-cc / backup bin-lezama-ca; row 165 hub-db-backup.sh maintainer bin-venus-cc / backup bin-lezama-cc; row 166 the scheduled job maintainer nw-venus-cc / backup bin-venus-cc. The bracket is in the tool, so the fix is mine; the cadence and unit half is nw-venus-cc's. WHY THE FIX IS NOT SHIPPED IN THE SAME TURN AS THE DIAGNOSIS. This tool's failure action is to DELETE the copy, so a tolerance I have not watched fail is precisely the change that must not land: v1.1 exists only because v1.0's equality test destroyed good backups on a live source, and swapping one unproven predicate for another repeats that mistake. Proposed fix is a relative-deviation test rather than a tolerance picked from nowhere - an out-of-window table is fatal only if its deviation exceeds max(1, 1% of the window's upper bound) - derived from the two measured cases: the stale-checkpoint failure this tool exists to catch was 25 rows against a live 133 (deviation 108 against a 2-row allowance: still fatal), and today's transient was 1 against 0 (deviation 1 against a 1-row allowance: tolerated). Every out-of-window table stays REPORTED in the log and the notify DM whether or not it is fatal, so a tolerated deviation is visible rather than silent. Residual risk, stated plainly: a genuine single-row loss on a large table where pre==post would now be tolerated - which the bracket already tolerates for any count inside [pre,post]. CONTROLS REQUIRED BEFORE IT SHIPS, each forced red against a fixture source rather than reasoned about: (a) a row inserted and deleted across the backup window must leave the copy KEPT with the deviation reported, and the assertion must confirm the 'source N->N, copy N+1' line actually appeared - a case that did not materialize proves nothing; (b) a stale-checkpoint-scale divergence must still exit 6 and delete the copy; (c) the zero-user-tables path must still fail. A fixture large enough to make the backup take seconds is what makes (a) reproducible without a sleep-timed race.
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FIXED AND SHIPPED, 0adabbd. sqlite-safe-backup.sh v1.2 -> v1.3, plus sqlite-safe-backup.test.sh v1.1 (new, 9 assertions, all green). THE FIX IS DIRECTIONAL, AND I DISCARDED MY OWN FIRST PROPOSAL. The row originally proposed a relative-deviation tolerance - fatal above max(1, 1 percent of the window's upper bound) - derived from two incidents. That was a number picked to fit the evidence, which is the same error as v1.0's equality test made in the other direction, and it had a second flaw: with an upper bound of 0 the allowance is always 1, so a lease table transiently holding five rows would still have been fatal. What the two failures ARE gives the rule with no threshold at all: copy BELOW lo - the copy holds fewer rows than the source did at either endpoint, so it is missing committed data. A snapshot taken between the endpoints cannot be below both, so this is not a timing artefact: it is the stale-checkpoint failure the tool exists to catch (25 rows against a live 133). FATAL, copy deleted, unchanged from v1.2. copy ABOVE hi - the copy holds more than either endpoint, which can only mean it observed rows deleted since. Impossible on an append-only table, normal on a lease or queue table. REPORTED, copy KEPT. On a quiet source the window is a single value and this still collapses to the old equality test, so nothing is weakened where nothing churns. NEVER SILENT: each above-window table prints its table name and counts, and the run prints a summary naming how many tables were above the window and that verification PASSED anyway. A tolerated breach leaving no trace would have been the next defect. CONTROLS, all forced red where that is possible, because a loosening of a check whose failure action is DELETION cannot ship on reasoning: (1) zero user tables still exits 6 and removes the rejected copy; (2) a quiet source verifies with no breach reported, so no spurious noise; (3) the incident reproduced EXACTLY - 'source 0->0 during backup, copy=1' - exit 0, copy survived, breach reported, and the copy independently confirmed to contain the since-deleted row so the breach was real rather than a mislabelled pass; (4) a MUTANT of the live script with the above-window branch changed back to fatal exits 6 and deletes the copy - which is what proves the tolerance is load-bearing and that control 3 is not an always-green path. HOW THE RACE WAS MADE DETERMINISTIC, which is what decides whether the suite is worth anything. A backup of a small fixture completes in about 40ms, so a background writer cannot be timed against it and a test that sometimes produces the case sometimes proves nothing. My first attempt was that churn loop: it ran past seven minutes, asserted nothing, and was discarded. The working design shims the DEPENDENCY rather than the code under test - a sqlite3 wrapper earlier in PATH, transparent for every call except the one carrying the backup dot-command, where it inserts the lease row, runs the real backup, then deletes the row. The script under test is unmodified and sees the incident's ordering every time. The mutant is generated from the live file by sed at run time rather than stored, so it cannot drift from the real logic. NOT PROVEN, and unprovable with a live source: copy BELOW lo is unreachable with a valid copy by construction, so it cannot be planted without corrupting the copy, and the tool deletes the copy before anything external could reach it. That leg rests on the construction argument plus integrity_check, with control 4 standing in for it. Residual risk, stated in the script rather than hidden: a stale copy of a table that ALSO saw deletions could in principle land above hi and be tolerated. END TO END ON THE LIVE HUB DB: hub-db-backup.sh exit 0, artifact v2-20261006-0829.sqlite, 257404928 bytes, integrity_check ok, 145197 message rows. TODAY'S MISSING RESTORE POINT NOW EXISTS - the gap nw-venus-cc reported is closed, not merely explained. DELIBERATELY NOT DONE: hub-db-backup.sh's version pin still rejects only v1.0, not everything below v1.3. v1.2 is over-strict rather than dangerous, and tightening the pin would make the job die 3 on whey and lezama until each host pulls.