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package sqlutil
import (
"database/sql"
"errors"
"go.uber.org/atomic"
)
// ExclusiveTransactionWriter allows queuing database writes so that you don't
// contend on database locks in, e.g. SQLite. Only one task will run
// at a time on a given ExclusiveTransactionWriter.
type ExclusiveTransactionWriter struct {
running atomic.Bool
todo chan transactionWriterTask
}
func NewTransactionWriter() TransactionWriter {
return &ExclusiveTransactionWriter{
todo: make(chan transactionWriterTask),
}
}
// transactionWriterTask represents a specific task.
type transactionWriterTask struct {
db *sql.DB
txn *sql.Tx
f func(txn *sql.Tx) error
wait chan error
}
// Do queues a task to be run by a TransactionWriter. The function
// provided will be ran within a transaction as supplied by the
// txn parameter if one is supplied, and if not, will take out a
// new transaction from the database supplied in the database
// parameter. Either way, this will block until the task is done.
func (w *ExclusiveTransactionWriter) Do(db *sql.DB, txn *sql.Tx, f func(txn *sql.Tx) error) error {
if w.todo == nil {
return errors.New("not initialised")
}
if !w.running.Load() {
go w.run()
}
task := transactionWriterTask{
db: db,
txn: txn,
f: f,
wait: make(chan error, 1),
}
w.todo <- task
return <-task.wait
}
// run processes the tasks for a given transaction writer. Only one
// of these goroutines will run at a time. A transaction will be
// opened using the database object from the task and then this will
// be passed as a parameter to the task function.
func (w *ExclusiveTransactionWriter) run() {
if !w.running.CAS(false, true) {
return
}
defer w.running.Store(false)
for task := range w.todo {
if task.txn != nil {
task.wait <- task.f(task.txn)
} else if task.db != nil {
task.wait <- WithTransaction(task.db, func(txn *sql.Tx) error {
return task.f(txn)
})
} else {
panic("expected database or transaction but got neither")
}
close(task.wait)
}
}
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