wip

pgutil/tx_test.go

@@ -0,0 +1,120 @@
+package pgutil
+
+import (
+	"database/sql"
+	"fmt"
+	"sync"
+	"testing"
+	"time"
+)
+
+// TestExecuteTx verifies transaction retry using the classic
+// example of write skew in bank account balance transfers.
+func TestWithTx(t *testing.T) {
+	db, err := sql.Open(
+		"postgres",
+		"host=127.0.0.1 dbname=test sslmode=disable user=test password=test")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if err := DropAllTables(db); err != nil {
+		t.Fatal(err)
+	}
+
+	defer db.Close()
+
+	initStmt := `
+CREATE TABLE t (acct INT PRIMARY KEY, balance INT);
+INSERT INTO t (acct, balance) VALUES (1, 100), (2, 100);
+`
+	if _, err := db.Exec(initStmt); err != nil {
+		t.Fatal(err)
+	}
+
+	type queryI interface {
+		Query(string, ...interface{}) (*sql.Rows, error)
+	}
+
+	getBalances := func(q queryI) (bal1, bal2 int, err error) {
+		var rows *sql.Rows
+		rows, err = q.Query(`SELECT balance FROM t WHERE acct IN (1, 2);`)
+		if err != nil {
+			return
+		}
+		defer rows.Close()
+		balances := []*int{&bal1, &bal2}
+		i := 0
+		for ; rows.Next(); i++ {
+			if err = rows.Scan(balances[i]); err != nil {
+				return
+			}
+		}
+		if i != 2 {
+			err = fmt.Errorf("expected two balances; got %d", i)
+			return
+		}
+		return
+	}
+
+	txRunner := SerialTxRunner{100 * time.Millisecond, 800 * time.Millisecond}
+
+	runTxn := func(wg *sync.WaitGroup, iter *int) <-chan error {
+		errCh := make(chan error, 1)
+		go func() {
+			*iter = 0
+			errCh <- txRunner.WithTx(db, func(tx *sql.Tx) error {
+				*iter++
+				bal1, bal2, err := getBalances(tx)
+				if err != nil {
+					return err
+				}
+				// If this is the first iteration, wait for the other tx to
+				// also read.
+				if *iter == 1 {
+					wg.Done()
+					wg.Wait()
+				}
+				// Now, subtract from one account and give to the other.
+				if bal1 > bal2 {
+					if _, err := tx.Exec(`
+UPDATE t SET balance=balance-100 WHERE acct=1;
+UPDATE t SET balance=balance+100 WHERE acct=2;
+`); err != nil {
+						return err
+					}
+				} else {
+					if _, err := tx.Exec(`
+UPDATE t SET balance=balance+100 WHERE acct=1;
+UPDATE t SET balance=balance-100 WHERE acct=2;
+`); err != nil {
+						return err
+					}
+				}
+				return nil
+			})
+		}()
+		return errCh
+	}
+
+	var wg sync.WaitGroup
+	wg.Add(2)
+	var iters1, iters2 int
+	txn1Err := runTxn(&wg, &iters1)
+	txn2Err := runTxn(&wg, &iters2)
+	if err := <-txn1Err; err != nil {
+		t.Errorf("expected success in txn1; got %s", err)
+	}
+	if err := <-txn2Err; err != nil {
+		t.Errorf("expected success in txn2; got %s", err)
+	}
+	if iters1+iters2 <= 2 {
+		t.Errorf("expected retries between the competing transactions; "+
+			"got txn1=%d, txn2=%d", iters1, iters2)
+	}
+	bal1, bal2, err := getBalances(db)
+	if err != nil || bal1 != 100 || bal2 != 100 {
+		t.Errorf("expected balances to be restored without error; "+
+			"got acct1=%d, acct2=%d: %s", bal1, bal2, err)
+	}
+}