writeonce/database/src/CODE-LOGIC.md

database/src — how the engine hangs together

The database engine is its own top-level directory, statically linked into every wovm and every runtime test binary (runtime/Makefile's DBSRC). One binary, unchanged. Format doc: docs/plan/oop-vm/04-db-binding.md. Memory-safety doctrine: the 9b design's section 6.

table.c — rows (iteration 9, Task 1)

VM values ──copy──▶ row slots (engine-owned malloc) ──copy──▶ fresh VM values
        wo_row_insert                              wo_row_read

• No VM pointer ever enters a slab; no slab pointer ever leaves. Encode copies per kind (Texts to db_text, owned objects flattened recursively to db_rec, containers element-wise); decode allocates fresh VM values from the caller's wo_rt. The GCREF kind is refused at encode — the compiler should have made that impossible (the GC bulkhead), the engine refuses it anyway.
• Rows never move. Slabs of 256 are malloc'd and kept for the table's life; the free-slot list recycles removed slots before any slab grows; the id hash maps id → slot. Ids are never reused (per-table counter, shard-interleaved S+1, S+1+N, …), which is also what makes the hash's tombstone sentinel safe.
• Storage is per-table since databasev2 2. @table(durable: false) sets WO_CLASSF_VOLATILE in the class descriptor (.wob v7), and db.c's table_is_durable gates all three mutation sites: a volatile table stages nothing, so it pays none of the fsync cost and is empty after a restart. Measured: 50 inserts wrote 1500 WAL bytes durable, 0 volatile. The three sites stayed three — the predicate is one function, not an inlined condition, precisely so this file's "nothing else may mutate storage" claim keeps holding.
• A mode mismatch refuses, it does not convert. If the log holds records for a class the loaded image now declares volatile, apply_record returns -2 (distinct from -1 corruption) and wo_wal_replay_ex reports the class id so main.c can name it. Silently skipping those records would resurrect nothing but would also hide a real migration; silently applying them would load rows into a table declared not to have any. wo_wal_replay remains as the NULL-out-param wrapper so the 156 WAL unit checks are untouched.
• Choke points: wo_row_insert / wo_row_remove carry the INDEX HOOK comments where Task 4's secondary indexes attach and Task 2's WAL stages its record. Nothing else may mutate storage.
• One deliberate file-static: g_classes for recursive frees (db_val_free has no context parameter). One process, one class table; revisit at iteration 8 (shards share the same immutable table).

wal.c — durability (iteration 9, Task 2)

The commit order IS the module: RAM apply → stage → one pwrite + one fdatasync → ack. wo_wal_commit returning 0 is the only thing "durable" means. Replay never touches the VM heap — payloads decode straight into engine-owned values and re-enter through the row API, so whatever hooks the choke points (indexes, Task 4) applies to replayed rows identically. Torn tails end the intact prefix and get overwritten by the next commit; CRC-valid-but-undecodable records fail replay loudly (corruption is not a tear). The crash battery in runtime/test/test_wal.c is the module's meaning proven: acked-over-a-pipe after commit, SIGKILL mid-stream, replay, zero acked-but-missing.

db.c — statement executors (iteration 9, Task 3)

One dispatcher, the builtin contract (0 ok, else WO_T_* + msg). The engine handles ride wo_rt.db / wo_rt.wal as opaque pointers set by main.c — NULL db traps WO_T_DB, NULL wal means RAM-only (the corpus's mode; WO_DATA opts into durability). Insert's contract: RAM apply through the row API, then stage + commit BEFORE returning — the builtin's return is the acknowledgment, so a failed commit un-applies the row and traps WO_T_IO rather than acknowledging what disk never got.

Verifying a change

• make -C runtime test — test_table is this directory's suite (round trips across kinds, nil encodings, shard interleave, slab growth, slot reuse, misuse), ASan+UBSan like every runtime test.
• just oop-e2e, just log-watcher — regression that linking the engine into wovm changed nothing observable (it is dead code until Task 3 wires the first builtin).

The slot-level surface (arc stage 3, 2026-08-21)

• Why it exists: the transparent DB actor executes a worker's statement on the owner shard, and VM heaps are never read cross-shard — so the requester encodes to engine slots on its own thread and the owner executes from slots, exactly the shape WAL replay already used.
• wo_db_val_encode exposes the in-gate for the RPC marshaler; wo_db_val_clone deep-copies an engine value (a get-field reply must outlive the row: a later serialized statement may free the slot); wo_row_insert_slots / wo_row_update_field_slot are the pre-encoded twins of insert/update (slot values consumed either way — installed on success, freed on failure); wo_db_exec_req (db.c) mirrors wo_builtin_db case for case with slot inputs and plain outputs, so a worker sees byte-identical traps and messages.
• The update refactor extracted row_apply_field_slot (the post-encode half: unique shadow-check, index fix-up, slot swap) shared by both entry points — the VM-value path's behavior is unchanged bit for bit.

The read-path index probe (2026-08-22)

• wo_idx_probe (table.c) answers a single-column equality from the index's hash buckets instead of walking slabs — the O(1) wiring the db-bench numbers demanded (reads were ~1.5k ops/s at p50 600µs on 20k rows; ~1.3M ops/s at p50 1µs after). idx_hash_key1 must reproduce idx_hash's single-column result bit for bit (same FNV over text bytes, same float canonicalization, same position mix) or probes and maintenance disagree on the bucket and rows silently vanish.
• The VERIFY step compares exactly as the slab walk compared (raw words for scalars/floats, byte equality for text; nil text == NULL bytes) — the hash canonicalizes only to FIND the bucket, so probe results are identical to scan results by construction.
• Composite indexes refuse (return 0) and callers keep the slab walk; both probe executors (wo_builtin_db and wo_db_exec_req) carry the same wiring, so worker shards get the speedup through the DB actor.
• The COMPILER half (emit.ml probe_key_of_where): a query whose where list contains var.col == key on a single-column-indexed column lowers its source to DB_PROBE; every where guard still runs over the candidates, so the guard — not the engine — stays the final arbiter. Keys are a plain identifier or an integer literal only; Float/Bytes columns excluded (engine raw-eq is narrower than VM float-eq, and a probe miss cannot be resurrected by a recheck). Pinned by tests/corpus/run/query-index-probe.