Spanner: Add trivial sequence number generator. (#101777)

* Add trivial sequence number generator.

pkg/util/xorm/sequence.go

@@ -0,0 +1,69 @@
+package xorm
+
+import (
+	"context"
+	"database/sql"
+	"errors"
+	"fmt"
+)
+
+type sequenceGenerator struct {
+	db             *sql.DB
+	sequencesTable string
+}
+
+func newSequenceGenerator(db *sql.DB) *sequenceGenerator {
+	return &sequenceGenerator{
+		db:             db,
+		sequencesTable: "autoincrement_sequences",
+	}
+}
+
+func (sg *sequenceGenerator) Next(ctx context.Context, table, column string) (int64, error) {
+	// Current implementation fetches new value for each Next call.
+	key := fmt.Sprintf("%s:%s", table, column)
+
+	tx, err := sg.db.BeginTx(ctx, &sql.TxOptions{Isolation: sql.LevelSerializable})
+	if err != nil {
+		return 0, err
+	}
+
+	// TODO: "FOR UPDATE". (Somehow this doesn't seem to be supported in Spanner emulator?)
+	r, err := tx.QueryContext(ctx, "SELECT next_value FROM "+sg.sequencesTable+" WHERE name = ?", key)
+	if err != nil {
+		err2 := tx.Rollback()
+		return 0, errors.Join(err, err2)
+	}
+	defer r.Close()
+
+	// Sequence doesn't exist yet. Return 1, and put 2 into the table.
+	if !r.Next() {
+		if err := r.Err(); err != nil {
+			return 0, errors.Join(err, tx.Rollback())
+		}
+		val := int64(1)
+
+		_, err := tx.ExecContext(ctx, "INSERT INTO "+sg.sequencesTable+" (name, next_value) VALUES(?, ?)", key, val+1)
+		if err != nil {
+			return 0, errors.Join(err, tx.Rollback())
+		}
+
+		return val, tx.Commit()
+	}
+
+	var val int64
+	if err := r.Scan(&val); err != nil {
+		return 0, errors.Join(err, tx.Rollback())
+	}
+
+	_, err = tx.ExecContext(ctx, "UPDATE "+sg.sequencesTable+" SET next_value = ? WHERE name = ?", val+1, key)
+	if err != nil {
+		return 0, errors.Join(err, tx.Rollback())
+	}
+
+	return val, tx.Commit()
+}
+
+func (sg *sequenceGenerator) close() {
+	// Nothing to do just yet.
+}