Advisory-lock DB migrations against concurrent replica startup

Migrate's check-then-apply loop against schema_migrations had no
locking: two replicas booting at once against a fresh or
partially-migrated database could both pass the "not yet applied"
check for the same file and race applying it, crashing whichever lost
the duplicate-key insert (confirmed: reverting the lock fails the new
test 10/10 on a duplicate-key violation, racing as early as the
CREATE TABLE IF NOT EXISTS schema_migrations statement itself).

Hold a Postgres advisory lock for Migrate's whole run, on a dedicated
connection reserved via db.Conn so lock and unlock happen on the same
session. Blocking (pg_advisory_lock), unlike the archiver/notifier's
pg_try_advisory_lock: on boot there's no later tick to defer to, so a
second replica should wait for the first to finish migrating rather
than skip ahead.

Adds internal/db's first test file, exercising two concurrent Migrate
calls against a fresh schema.

Still open: the new-incident-insert race on a webhook for a brand-new
groupKey, noted in the chart's updated comment. Login rate limiting
staying in-process, diluted across replicas, is an accepted tradeoff.

Co-authored-by: Claude <noreply@anthropic.com>

internal/db/db_test.go

@@ -0,0 +1,125 @@
+package db_test
+
+import (
+	"database/sql"
+	"fmt"
+	"net/url"
+	"os"
+	"strings"
+	"sync"
+	"testing"
+
+	"git.ryuvia.com/niklas/terdut-server/internal/db"
+
+	_ "github.com/jackc/pgx/v5/stdlib"
+)
+
+// TERDUT_TEST_DSN must point at a database the test role may create schemas
+// in; see internal/api/testdb_test.go for the fuller rationale this mirrors.
+// An unset DSN fails rather than skips, deliberately.
+const testDSNEnv = "TERDUT_TEST_DSN"
+
+// TestMigrate_ConcurrentCallersDoNotRace reproduces two replicas starting at
+// once against a brand-new, unmigrated schema: both call db.Migrate at the
+// same time. Before migrationLockKey, the loser could crash on a
+// duplicate-key insert into schema_migrations, or contend with the winner's
+// uncommitted DDL; with the advisory lock, one blocks until the other
+// finishes and both return cleanly.
+func TestMigrate_ConcurrentCallersDoNotRace(t *testing.T) {
+	dsn := os.Getenv(testDSNEnv)
+	if dsn == "" {
+		t.Fatalf("%s is not set: these tests need Postgres.\n"+
+			"Run `make test-db` for a local one, then\n"+
+			"  export %s=postgres://terdut:terdut@localhost:5432/terdut_test?sslmode=disable",
+			testDSNEnv, testDSNEnv)
+	}
+
+	schema := fmt.Sprintf("migrate_race_%d", os.Getpid())
+	admin, err := sql.Open("pgx", dsn)
+	if err != nil {
+		t.Fatalf("connect to %s: %v", testDSNEnv, err)
+	}
+	defer admin.Close()
+	if _, err := admin.Exec("CREATE SCHEMA " + schema); err != nil {
+		t.Fatalf("create schema %s: %v", schema, err)
+	}
+	t.Cleanup(func() {
+		cleanup, err := sql.Open("pgx", dsn)
+		if err != nil {
+			return
+		}
+		defer cleanup.Close()
+		if _, err := cleanup.Exec("DROP SCHEMA " + schema + " CASCADE"); err != nil {
+			t.Logf("drop schema %s: %v", schema, err)
+		}
+	})
+
+	scoped := withSearchPath(dsn, schema)
+
+	const callers = 2
+	errs := make([]error, callers)
+	var wg sync.WaitGroup
+	for i := range callers {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			database, err := db.Open(scoped)
+			if err != nil {
+				errs[i] = fmt.Errorf("open: %w", err)
+				return
+			}
+			defer database.Close()
+			errs[i] = db.Migrate(database)
+		}(i)
+	}
+	wg.Wait()
+
+	for i, err := range errs {
+		if err != nil {
+			t.Fatalf("Migrate #%d: %v", i, err)
+		}
+	}
+
+	entries, err := os.ReadDir("migrations")
+	if err != nil {
+		t.Fatalf("read migrations dir: %v", err)
+	}
+	var want int
+	for _, e := range entries {
+		if !e.IsDir() && strings.HasSuffix(e.Name(), ".sql") {
+			want++
+		}
+	}
+
+	check, err := sql.Open("pgx", scoped)
+	if err != nil {
+		t.Fatalf("connect for verification: %v", err)
+	}
+	defer check.Close()
+
+	var got int
+	if err := check.QueryRow("SELECT COUNT(*) FROM schema_migrations").Scan(&got); err != nil {
+		t.Fatalf("count schema_migrations: %v", err)
+	}
+	if got != want {
+		t.Fatalf("schema_migrations has %d row(s) after two concurrent Migrate calls, want %d (one per migration file, no duplicates)", got, want)
+	}
+}
+
+// withSearchPath pins a DSN to one schema. Copied from
+// internal/api/testdb_test.go rather than shared: that helper lives in the
+// api_test package, a separate compiled package this one cannot import.
+func withSearchPath(dsn, schema string) string {
+	opt := "-csearch_path=" + schema
+
+	if strings.HasPrefix(dsn, "postgres://") || strings.HasPrefix(dsn, "postgresql://") {
+		u, err := url.Parse(dsn)
+		if err == nil {
+			q := u.Query()
+			q.Set("options", opt)
+			u.RawQuery = q.Encode()
+			return u.String()
+		}
+	}
+	return dsn + " options='" + opt + "'"
+}