[magnusp]

Why

Issue #294's acceptance asks for a read-then-write transaction with immediate that commits while a second connection waits. #295 shipped the URL parameter and a test that the immediate begin holds the write lock, but not the read-to-write upgrade itself.

What changed

• Added urlImmediateTransactionModeUpgradesAReadToAWriteAndCommitsWhileAWriterWaits to TransactionModeIT. It reads, starts a second connection whose insert must time out behind the lock, inserts and commits in the first transaction, then checks both rows and that the waiting writer completed.

Scope

• Test only. No production code and no release.

Verification

• TransactionModeIT runs 3 tests against sqld v0.24.33, all passing.
• spotless:check passes.

[github-actions]

Performance benchmark

Base 601e8d32b815 → head 10ceade173ff. Medians of 3 base runs and 3 head runs, interleaved on the same runner.

Driver-side microbenchmarks only; server and network time are excluded. Shared-runner noise is about ±10%, so only larger regressions are flagged.

JDK 25.0.5, Linux amd64, 4 CPUs.

Throughput

Workload	Base	Head	Δ
Decode 5k rows (direct)	1.07M rows/s	1.05M rows/s	-1.8%
Decode 5k rows (Jackson baseline)	920.4k rows/s	929.3k rows/s	+1.0%
Decode 20k rows (direct)	1.03M rows/s	1.03M rows/s	-0.2%
Encode one execute (direct)	985.1k req/s	943.5k req/s	-4.2%
Encode one execute (Jackson baseline)	917.8k req/s	961.2k req/s	+4.7%
JDBC scan + getters (20k rows)	19.02M rows/s	7.47M rows/s	-60.7% ⚠️
JDBC scan + getObject (20k rows)	8.80M rows/s	13.22M rows/s	+50.2%

Allocation

Workload	Base	Head	Δ
Decode 5k rows (direct)	8.18 MB/op	8.18 MB/op	+0.0%
Decode 5k rows (Jackson baseline)	5.22 MB/op	5.22 MB/op	+0.0%
Decode 20k rows (direct)	32.66 MB/op	32.66 MB/op	+0.0%
Encode one execute (direct)	1.38 kB/op	1.38 kB/op	+0.0%
Encode one execute (Jackson baseline)	1.84 kB/op	1.84 kB/op	+0.0%
JDBC scan + getters (20k rows)	1.14 kB/op	1.14 kB/op	+0.0%
JDBC scan + getObject (20k rows)	1.12 MB/op	1.12 MB/op	+0.0%

_{Workloads and thresholds live in tools/perf/render-comment.py. Run locally with ./mvnw -pl libsql-perf -am -DskipTests package and see libsql-perf/README.md.}