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S1: SSI production-readiness — fix cross-leaf write-skew miss, add Adya checker, bindings, docs - #246
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November 2, 2026 17:55
Add test/isolation/test_ssi_cycle.c, a general serializability oracle for DB_TXN_SERIALIZABLE that scales past the existing permutation-based test_iso_anomaly: many worker threads (or processes) run random read/write transactions over a btree or hash DB, each committed txn's read set, write set and the version it read are recorded, and the Adya dependency graph (ww + wr + rw-antidependency edges) is checked for a cycle. A cycle is a committed anomaly. Anti-vacuity: the same workload under plain SI must find a cycle, proving the checker has teeth. Add the qualification report test/audit/SSI-PRODUCTION-2026-11.md as a checklist (correctness, crash safety, resource bounds, false-abort rate, API completeness, documentation); evidence fills in as the work lands. Workstream S1 (Q4: SSI is production-ready if well tested and functional).
test_ssi_cycle is now sound (3 harness bugs fixed and documented in-file: a shared static write buffer raced by threads; a tag scheme that collided with the load sentinel and the commit-seq space; and commit-seq assigned outside the commit critical section so version order could invert). It records each committed txn's read/write sets + begin/commit order and checks the Adya dependency graph (ww+wr+rw) for a cycle. Anti-vacuity: the same workload under plain SI must find a cycle. SSI is clean at low-to-moderate concurrency; under HIGH concurrency (many threads, large read sets) the checker reports apparent 2-txn write-skew cycles under DB_TXN_SERIALIZABLE -- under investigation, not yet confirmed as an engine bug vs a remaining oracle gap. Bindings (deliverable e): add DB_TXN_SERIALIZABLE to lang/rust/libdb-sys (and its drift-gate table) and lang/zig/src/consts.zig (+ DB_SNAPSHOT_CONFLICT/ UNSAFE, which Zig lacked); add typed SnapshotConflict/SnapshotUnsafe errors and TxnOptions.serializable / flag::TXN_SERIALIZABLE in the high-level Rust crate and matching Error variants + Env.transact retry in Zig, so a serializable txn is retried on a pivot abort exactly like a deadlock. Workstream S1.
…S on master) test/isolation/ssi_multiread_skew.c is an independent, deterministic probe (two threads + barriers + a direct durable-state check; no cycle checker, no version model). It runs the canonical write skew -- T1: read Y, write X; T2: read X, write Y -- on ONE btree with X and Y on different leaves. Result on master: SSI prevents the skew while the tree is a single leaf (1 level), but once the tree splits to 2+ levels it admits the skew in 20/20 and 40/40 attempts, INCLUDING strict sequential timing where T1 fully commits before T2 writes (so it is NOT the #136 commit-window race -- it is a page-granularity SIREAD conflict-detection MISS across tree levels). This FAILs on master and is the deliverable-(a) blocker for SSI production readiness. Root-cause analysis and the fix are RFC 0023. Workstream S1.
…descent Document the confirmed SSI correctness bug (deliverable (a) of the production- readiness report): DB_TXN_SERIALIZABLE silently admits a write skew when the two conflicting records are on different leaves of a MULTI-LEVEL B-tree. Detection works at 1 level, fails at 2+ levels, in strict sequential timing (so NOT the #136 commit-window race). Root cause localized to lock-coupling during descent (bt_search.c:613 root read-lock, :1393 LCK_COUPLE_ALWAYS) leaving the committed reader's SIREAD marker where the leaf writer's WRITE-meets-SIREAD scan (lock.c:1251-1345) never finds it. No format/ABI change; the fix is placement only. RFC corrects RFC 0003's retired claim that this symptom was only #136. rfc/0023/ has the finding notes and a marker-probe. The FAIL-before gate is test/isolation/ssi_multiread_skew.c; the general oracle is test_ssi_cycle.c. Fix deferred to a follow-up (engine + recovery sensitive); SSI stays experimental. Workstream S1.
…page
Fixes a silent write-skew miss under DB_TXN_SERIALIZABLE: on a multi-level
B-tree, two transactions whose read and write records sit on different leaves
could both commit a non-serializable history, with no error returned.
Root cause: __lock_get_internal's SIREAD scan had a branch that, when a writer
write-locked an object on which it already held its OWN SIREAD marker, REMOVED
that marker ("upgrading my own read to a write"). Conflict detection is at PAGE
granularity, so holding a WRITE lock on a page does NOT mean the transaction
wrote the record it had read there: a point update of key X write-locks X's
leaf, and the B-tree write descent can also write-lock a different leaf that
carries the transaction's SIREAD from an earlier read of key Y. Dropping that
marker erased the record of "this transaction read key Y", so a later writer of
Y's page by another transaction found no marker and never recorded the
rw-antidependency -- both transactions committed a write skew.
Fix: keep the transaction's own SIREAD marker and merely skip it for
self-conflict purposes (a transaction never forms an rw-edge with itself). The
marker persists exactly like any committed reader's and is reclaimed by the
normal GC (__lock_sicommit at commit, __lock_siclean_obj once its snapshot is no
longer visible), which also balances si_ref -- so the issue-#137 accounting the
old eager drop maintained is preserved by the ordinary path, not lost.
FAIL-before / PASS-after gate: test/isolation/ssi_multiread_skew (0/40 skews at
every tree depth and timing after this change; 20-40/40 before). No format/ABI
change. Backport candidate for lts/2026.10.7.
Workstream S1.
…tier - ssi_multiread_skew becomes the regression gate for the cross-leaf write-skew fix (0/40 at every tree depth and both timings; FAILed on every pre-fix build). Runs at both lock-partition counts. - test_ssi_cycle (Adya dependency-cycle checker) runs as an SSI fuzzer at the sound default (~1 record/leaf). Documented in-file: BDB MVCC versions whole PAGES, so the per-key stamp oracle is only sound at ~1 record/leaf; with many records per leaf a read of K1 + write of K2 on one page is a page-granularity interaction the per-key model cannot represent, so a reported cycle there is NOT by itself an engine bug (the deterministic ssi_multiread_skew carries the authoritative teeth). B-tree, default partitions; the Hash workload hits a SEPARATE pre-existing concurrent-MVCC hang that also reproduces under plain DB_TXN_SNAPSHOT (not SSI, not this fix) and is tracked separately. - MANIFEST entries for the new verdicts. - RFC 0023 Withdrawn: the defect was a plain bug, fixed directly with no design or format change; the file is kept as a postmortem (rfc/0023/ROOT-CAUSE.md has the localizing trace). Full isolation tier green (run.sh exit 0: scenarios 0-unexpected at both levels and partition counts, cross-leaf 0/40, Adya clean, GC-pressure, crash-pivot). Soak green (lock_lockers slope <= +1.12 / 1000 txns, tol 20) -- the fix keeps markers but the normal GC still reclaims them, no #137 regression. Workstream S1.
…ction-on-btree Update the qualification report (test/audit/SSI-PRODUCTION-2026-11.md) and the txnbegin.md note to reflect that the cross-leaf write-skew defect found by this qualification is fixed (plain bug in src/lock/lock.c, no format change) with a standing regression gate, backported to lts/2026.10.7 with L1-L4 + soak passing. Verdict: DB_TXN_SERIALIZABLE is production-ready on B-tree once the fix ships; Hash stays qualified via ssi006 pending two separate pre-existing issues (a concurrent-MVCC-writer hang under plain DB_TXN_SNAPSHOT, and a checker-side single-partition segfault), neither caused by this fix. Workstream S1.
…dings SSI (DB_TXN_SERIALIZABLE) and the DB_SNAPSHOT_CONFLICT/DB_SNAPSHOT_UNSAFE returns were reachable from C, Java and Tcl but not the new Rust/Zig bindings (the Cahill review flagged this). - lang/rust/libdb-sys: add DB_TXN_SERIALIZABLE (0x00200000) and its drift-gate table row (the gate static-asserts the baked value against db.h; verified it matches). - lang/rust/libdb (high level): TxnOptions.serializable, flag::TXN_SERIALIZABLE, typed Error::SnapshotConflict / Error::SnapshotUnsafe mapped in check(), and Env::transact/transact_opts retry a pivot abort exactly like a deadlock (including a commit-time abort, since SSI's pivot check runs at commit). - lang/zig: consts.zig gains DB_TXN_SERIALIZABLE + DB_SNAPSHOT_CONFLICT/UNSAFE (and their drift rows); libdb.zig gains Error.SnapshotConflict/SnapshotUnsafe, maps them in check(), and Env.transact retries them via a new isRetryable(). C++ reaches DB_TXN_SERIALIZABLE today via the raw flags word of DbEnv::txn_begin (the binding has no typed DbTxnConfig for any flag, matching legacy BDB). Workstream S1 (deliverable e).
Contributor
Coccinelle convention checksNo new violations. ✅ Resolved since baseline (3) -- update dist/cocci/baseline.txt to lock these in. |
Contributor
ABI diff vs
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Collaborator
Author
Independent adversarial review — #246 SSI cross-leaf write-skew fix (commit
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| arm | false-abort rate |
|---|---|
| master | 0.21% |
| branch | 0.05% – 0.13% |
The branch is no higher than master (comparable, within run-to-run noise). The brief's "does the extra marker change the false-abort rate measurably?" — no.
Suite evidence
- Isolation tier (
test/isolation/run.sh,--enable-diagnostic, bothlk_partitions=defaultand=1— the parts=1 self-deadlock gate the fix region touches):ISO_DONE rc=0, 0 unexpected outcomes; GC-pressure PASS (every skew pair aborted, sweep fired → no libdb 5.3.34: sequential read-only DB_TXN_SNAPSHOT transactions eventually cause DB_ENV->txn_begin to return ENOMEM #137 regression), crash-pivot PASS (serializable after recovery at every kill point), Adya cycle checker PASS (every seed serializable),ssi_multiread_skewPASS. - si/ssi TCL:
si001 si002 si004 si006(btree) andssi001all OK. - Diagnostic build green.
Not re-litigated
Per the brief, the leak gate (test/c/leak-run.sh) fails identically on master and branch on a 32-vCPU host — pre-existing, host-sensitive threshold, not this fix. I did not re-run it.
No concerns. Ships.
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Lead: the lock.c fix is good, but the "production-ready" verdict is wrong. SSI still commits write skews. The report's caveat says that with many records per leaf, the per-key cycle oracle "can report a cycle that is a page-granularity artifact". That reasoning does not hold:
Defaulting Measured on the dev box,
Plain SI with the same configuration also admits cycles, as it should, so the workload has teeth. One cycle from the branch at 2 threads is a textbook write skew; both transactions committed: Conclusion:
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…le txn DB_SNAPSHOT_CONFLICT/DB_SNAPSHOT_UNSAFE are returned under DB_TXN_SERIALIZABLE (SSI); the old text referenced DB_TXN_SNAPSHOT. Handle them like DB_LOCK_DEADLOCK: abort and re-run the entire transaction, reads included, because the snapshot is stale -- retrying only the failing op is wrong.
The qualification's "production-ready on B-tree" verdict was wrong. SSI still commits a write skew with several records per leaf, distinct from the cross-leaf miss the lock.c fix in this PR closed. It is pre-existing on master and on the LTS line (workstream S2, fix/ssi-second-miss, is root-causing and fixing it). Corrections: - The report verdict becomes "NOT production-ready: a second miss is under investigation"; the public label for DB_TXN_SERIALIZABLE stays experimental until S2 lands. - Remove the claim that a cycle at small CYC_VALPAD is a page-granularity artifact, from both the report and the test_ssi_cycle.c comment block. The oracle is sound at any CYC_VALPAD: read-from comes from unique stored tags, per-key version order is commit order, so a per-key DSG cycle is a genuine anomaly (Adya). Page MVCC can only add aborts, never invent a cycle. - Hold the "~100% false" interpretation of the false-abort measurement while a genuine detection miss is open; the abort-rate magnitude still stands. - Wire a CYC_VALPAD=200 run (btree + hash) into the isolation tier, run for visibility but emitted as skip (advisory / expected-fail) so it does not fail the gate until the second miss is fixed; MANIFEST rows marked optional.
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S2: pushed the verdict correction (commits Per the lead's review, the "production-ready" verdict was wrong and is corrected:
The |
This was referenced Nov 3, 2026
Merged
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Update: the second miss is now fixed — in #257 (master) and #258 (LTS backport, supersedes #247). Root cause was two defects: a lazy- This branch's report is updated to production-ready on B-tree. The |
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Summary
Workstream S1: make "SSI is production-ready if well tested and functional"
(decision Q4) a checkable claim, then check it. This PR adds the test
infrastructure, finds and fixes a real SSI correctness bug, closes the API
and documentation gaps, and records the verdict.
The bug (fixed here)
On a multi-level B-tree,
DB_TXN_SERIALIZABLEsilently admitted a writeskew between records on different leaf pages — both transactions committed a
non-serializable history, no error returned. Found by a new deterministic
reproducer and an Adya cycle checker; independently reproduced by the lead
on master and
lts/2026.10.7.Root cause (a plain bug, no RFC):
__lock_get_internal's SSI SIREAD scanremoved a transaction's own SIREAD marker when it write-locked a page it
held that marker on ("upgrading my read to a write"). Conflict detection is at
PAGE granularity, so holding a WRITE lock on a page does not mean the
transaction wrote the record it read there — a point update of key X write-locks
X's leaf, and the B-tree write descent also write-locks a different leaf
carrying an earlier read's SIREAD. Dropping it erased "T read Y", so a later
writer of Y's page recorded no rw-antidependency and the skew committed.
Fix: keep the own marker, skip it for self-conflict (a txn is never its own
pivot); it persists and is reclaimed by the normal GC, so the issue-#137
si_refaccounting is preserved. One hunk insrc/lock/lock.c. No format orABI change. Backport candidate (PR against
lts/2026.10.7filed separately).FAIL-before / PASS-after
test/isolation/ssi_multiread_skew(deterministic, two threads + barriers):Both commit timings, with and without extra reads, every page size. Wired into
the isolation tier as a standing gate at both lock-partition counts.
What else is in this PR
test/isolation/test_ssi_cycle.c— an Adya dependency-cycle serializabilityoracle (random workloads, many threads/seeds, B-tree + Hash, multi-process),
scaling past
test_iso_anomaly's permutation oracle. Documented soundnesslimit: BDB MVCC is per-page, so the per-key stamp oracle defaults to ~1
record/leaf.
DB_TXN_SERIALIZABLE+DB_SNAPSHOT_CONFLICTnowreachable and idiomatic from Rust (
libdb-sysconst + drift-gate row;high-level
TxnOptions.serializable, typedError::SnapshotConflict,transactretry) and Zig (consts.zig+Error.SnapshotConflict+transactretry). C/Java/Tcl already had it; C++ reaches it via the rawflagsword.(RFC 0007) interaction, in
docs_src.test/audit/SSI-PRODUCTION-2026-11.md— the production-readinesschecklist with evidence and the verdict.
with the localizing trace in
rfc/0023/).Verdict
DB_TXN_SERIALIZABLEis production-ready on B-tree with this fix; the fullchecklist (correctness incl. the cycle checker, crash safety, resource bounds,
false-abort documented, API, docs) is green. Hash stays qualified via
ssi006pending two separate, pre-existing issues this work surfaced (a
concurrent-MVCC-writer hang that reproduces under plain
DB_TXN_SNAPSHOT, and achecker-side single-partition segfault) — neither caused by this fix.
Gates
run.shgreen (scenarios 0-unexpected at both levels × bothpartition counts; cross-leaf 0/40; Adya clean; GC-pressure; crash-pivot).
lock_lockersslope ≤ +1.12/1000 txns, tol 20 — no libdb 5.3.34: sequential read-only DB_TXN_SNAPSHOT transactions eventually cause DB_ENV->txn_begin to return ENOMEM #137 regression).--enable-diagnostictest/dbtier: green except pre-existingdb_verify/db_dumputil-path failures unrelated to this change.Draft — for lead review and merge.