Add *Tx.AfterClose hook

ConnPool now implements its connection with Tx via this hook instead of
manipulating the internals of Tx.

tx_test.go

@@ -3,6 +3,7 @@ package pgx_test
 import (
 	"github.com/jackc/pgx"
 	"testing"
+	"time"
 )
 
 func TestTransactionSuccessfulCommit(t *testing.T) {
@@ -114,3 +115,38 @@ func TestBeginIso(t *testing.T) {
 		}
 	}
 }
+
+func TestTxAfterClose(t *testing.T) {
+	t.Parallel()
+
+	conn := mustConnect(t, *defaultConnConfig)
+	defer closeConn(t, conn)
+
+	tx, err := conn.Begin()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var zeroTime, t1, t2 time.Time
+	tx.AfterClose(func(tx *pgx.Tx) {
+		t1 = time.Now()
+	})
+
+	tx.AfterClose(func(tx *pgx.Tx) {
+		t2 = time.Now()
+	})
+
+	tx.Rollback()
+
+	if t1 == zeroTime {
+		t.Error("First Tx.AfterClose callback not called")
+	}
+
+	if t2 == zeroTime {
+		t.Error("Second Tx.AfterClose callback not called")
+	}
+
+	if t1.After(t2) {
+		t.Error("AfterClose callbacks called out of order: %v, %v", t1, t2)
+	}
+}