kvdb/postgres: use readonly db transaction if possible

kvdb/postgres/readwrite_tx.go

@@ -4,6 +4,7 @@
 package postgres
 
 import (
+	"context"
 	"database/sql"
 	"sync"
 
@@ -39,7 +40,16 @@ func newReadWriteTx(db *db, readOnly bool) (*readWriteTx, error) {
 	}
 	locker.Lock()
 
-	tx, err := db.db.Begin()
+	// Start the transaction. Don't use the timeout context because it would
+	// be applied to the transaction as a whole. If possible, mark the
+	// transaction as read-only to make sure that potential programming
+	// errors cannot cause changes to the database.
+	tx, err := db.db.BeginTx(
+		context.Background(),
+		&sql.TxOptions{
+			ReadOnly: readOnly,
+		},
+	)
 	if err != nil {
 		locker.Unlock()
 		return nil, err