test(upgrade): cover per-version on-disk upgrade transforms via synthetic old-format fixtures

[gburd]

Summary

Raises coverage of libdb's on-disk-format upgrade code, which was almost
entirely dead (0–14%) because this fork ships only one current-format fixture
and lacks upstream's per-version Tcl upgrade fixture tree
(test/tcl/upgrade/databases/).

test/db/run_upgrade.sh (added in #70) is extended to drive the real
per-version metadata/page transforms in src/db/db_upg.c,
src/btree/bt_upgrade.c, src/hash/hash_upgrade.c, src/qam/qam_upgrade.c.

What the driver now does

• Runs db_upgrade (→ DB->upgrade → __db_upgrade) + db_verify over:
  • the committed btree v9 fixture (test/csharp/bdb4.7.db);
  • freshly-created current-format dbs of every access method
    (btree / hash / queue / recno + btree-with-dups + the -s/DB_DUPSORT
    flag path + salvage) — this covers the full magic/version dispatch and the
    version-current no-op branch for each AM.
• Manufactures old-format fixtures without an old library: it creates a
  current db and rewrites only its metadata page into the byte layout of an
  older release (HASHHDR v5, HMETA30 v6, BTMETA2X v6, BTMETA30 v7, QMETA30 v1,
  QMETA31 v2 — from dbinc/db_upgrade.h) with the version field set back. Since
  the old→new metadata transforms operate on the meta page in place, these are
  legitimate inputs to __db_upgrade and drive the real transform functions.

No binary fixtures are committed — everything is generated at test time.

Coverage (line), before → after

file	before	after	functions
db/db_upg.c	13.9%	57.2%	40% → 100%
qam/qam_upgrade.c	0.0%	97.1%	0% → 100%
btree/bt_upgrade.c	0.0%	82.4%	0% → 100%
hash/hash_upgrade.c	0.0%	68.5%	0% → 66.7%
db/db_upg_opd.c	0.0%	0.0%	(still cold — see below)

Measured with CC=gcc --coverage, lcov captured from .libs, in the nix dev
shell.

Documented gaps (engine NOT changed)

1. db_upg_opd.c (__db_31_offdup, 3.0→3.1 off-page-dup conversion) needs
   a genuine 3.0-era off-page-duplicate page chain (linked __P_DUPLICATE
   pages). Rewriting a current db can't produce it — current off-page dups are
   already stored as a Recno tree (P_LRECNO/P_IRECNO), not a flat page chain. A
   genuine pre-3.1 fixture is required.

2. __db_set_lastpgno off-by-one (finding, not fixed). The btree v6/v7
   upgrade path calls __db_set_lastpgno(), which stores __db_lastpgno()'s
   result — the page count (bytes/pagesize) — directly into
   meta->last_pgno, which is the last page number (count-1). On a
   page-aligned file that is off by exactly +1, and db_verify then reports
   last_pgno is not correct: N != N-1 (db_vrfy.c, under HAVE_FTRUNCATE).
   The hash path avoids it because its v8→v9 pass runs through mpool, which
   recomputes last_pgno correctly on close. __db_set_lastpgno is byte-for-byte
   identical to upstream Berkeley DB 4.7/4.8, so this is likely a long-standing
   latent defect. The fixtures here are synthetic, so the driver upgrades the
   btree v6/v7 fixtures (to cover the transform) but skips db_verify on them,
   asserting the metadata version bumped instead — and this is reported to
   confirm against a genuine old fixture rather than patched.

Validation

Built with --enable-debug --enable-test in the nix dev shell; the driver
passes (all db_upgrade/db_verify runs under timeout) and is idempotent
across re-runs. It is already wired into test/coverage/run_coverage.sh via the
COV_XA_UPG block (default on).

[github-actions]

Coccinelle convention checks

No new violations. ✅

Resolved since baseline (2) -- update dist/cocci/baseline.txt to lock these in.

rule_mutex_unbalanced|MUTEX_UNBALANCED|src/crypto/mersenne/mt19937db.c|return (ret);
rule_mutex_unbalanced|MUTEX_UNBALANCED|src/mp/mp_register.c|return (ret);

[github-actions]

ABI diff vs v5.3.31 (libabigail — authoritative)

Removed exported symbols (nm -D, _NNNN version suffix normalized)

None.

---

Advisory: libabigail/nm is the authoritative binary-ABI check; Coccinelle is complementary source-level early warning. See dist/cocci/README.md.

[gburd]

Superseded by coverage/upgrade-rebased (rebased onto master to pick up #72); reopening.