Artifact [4e57b0e3e9]

Artifact 4e57b0e3e9327e161d0b932efa97b91ab9fec199d2581385c52a425aa98250de:

• File db/driver/modernc/modernc.go — part of check-in [202576c58c] at 2026-05-13 19:09:49 on branch trunk — fix: DSN _txlock=immediate to serialize writers at BEGIN (#33) (#34)

  Two concurrent libfossil.Repo handles on the same SQLite file fail their writes with SQLITE_BUSY immediately, well under the configured busy_timeout=5000 window. Reproduces ~50% of the time on contended manifest.Checkin paths.

  Root cause: db.WithTx opens BEGIN DEFERRED. Under WAL, a deferred transaction acquires SHARED on first read, then upgrades SHARED→RESERVED on first write. If two connections race that upgrade, SQLite's deadlock-avoidance returns SQLITE_BUSY *immediately*, bypassing busy_timeout entirely (documented behavior, sqlite.org/c3ref/busy_timeout.html).

  Fix: prepend _txlock=immediate to the DSN in both modernc and ncruces drivers. Every BEGIN now acquires RESERVED upfront, so the second writer blocks at BEGIN — where busy_timeout *does* apply — and the two transactions serialize cleanly.

  Regression tests added to both driver packages: - TestBuildDSN_TxlockImmediate covers the DSN shape across empty/non-empty pragma maps. - TestConcurrentWritersDoNotRace reproduces the issue-#33 contention pattern (two conns, SELECT then INSERT under DEFERRED) over 25 iterations and asserts zero SQLITE_BUSY. Without the fix, this test fails 25/25; with the fix, 0/25.

  Closes #33. (user: [email protected], size: 872) [annotate] [blame] [check-ins using]

package modernc

import (
	"fmt"
	"strings"

	"github.com/danmestas/libfossil/db"
	_ "modernc.org/sqlite"
)

func init() {
	db.Register(db.DriverConfig{
		Name:     "sqlite",
		BuildDSN: buildDSN,
	})
}

func buildDSN(path string, pragmas map[string]string) string {
	if path == "" {
		panic("modernc.buildDSN: path must not be empty")
	}
	// _txlock=immediate forces BEGIN IMMEDIATE so concurrent writers
	// serialize at BEGIN (where busy_timeout applies) instead of racing
	// on the SHARED→RESERVED upgrade (where SQLite returns SQLITE_BUSY
	// immediately to avoid deadlock, bypassing busy_timeout entirely).
	// See https://www.sqlite.org/c3ref/busy_timeout.html
	parts := []string{"_txlock=immediate"}
	for k, v := range pragmas {
		parts = append(parts, fmt.Sprintf("_pragma=%s(%s)", k, v))
	}
	return fmt.Sprintf("file:%s?%s", path, strings.Join(parts, "&"))
}