MySQL: Trying again after a small delay instead of giving up when no more...

MySQL: Trying again after a small delay instead of giving up when no more connections are available.

mysql/database.go

@@ -57,6 +57,8 @@ type cachedStatement struct {
 	query string
 }
 
+var waitForConnMu sync.Mutex
+
 var (
 	_ = db.Database(&database{})
 	_ = db.Tx(&tx{})
@@ -129,7 +131,7 @@ func (d *database) Open() error {
 			conn.Options["charset"] = "utf8"
 		}
 
-		// Connection charset, UTF-8 by default.
+		// parseTime=true changes the output type of DATE and DATETIME values to time.Time instead of []byte / string
 		if conn.Options["parseTime"] == "" {
 			conn.Options["parseTime"] = "true"
 		}
@@ -150,7 +152,12 @@ func (d *database) Open() error {
 		d.connURL = conn
 	}
 
-	if d.session, err = sqlx.Open(`mysql`, d.connURL.String()); err != nil {
+	connFn := func(d **database) (err error) {
+		(*d).session, err = sqlx.Open(`mysql`, (*d).connURL.String())
+		return
+	}
+
+	if err := waitForConnection(func() error { return connFn(&d) }); err != nil {
 		return err
 	}
 
@@ -364,12 +371,16 @@ func (d *database) Transaction() (db.Tx, error) {
 		return nil, err
 	}
 
-	if sqlTx, err = clone.session.Beginx(); err != nil {
+	connFn := func(sqlTx **sqlx.Tx) (err error) {
+		*sqlTx, err = clone.session.Beginx()
+		return
+	}
+
+	if err := waitForConnection(func() error { return connFn(&sqlTx) }); err != nil {
 		return nil, err
 	}
 
 	clone.tx = sqltx.New(sqlTx)
-
 	return &tx{Tx: clone.tx, database: clone}, nil
 }
 
@@ -643,3 +654,37 @@ func (d *database) getPrimaryKey(tableName string) ([]string, error) {
 
 	return tableSchema.PrimaryKey, nil
 }
+
+// waitForConnection tries to execute the connectFn function, if connectFn
+// returns an error, then waitForConnection will keep trying until connectFn
+// returns nil. Maximum waiting time is 5s after having acquired the lock.
+func waitForConnection(connectFn func() error) error {
+	// This lock ensures first-come, first-served and prevents opening too many
+	// file descriptors.
+	waitForConnMu.Lock()
+	defer waitForConnMu.Unlock()
+
+	// Minimum waiting time.
+	waitTime := time.Millisecond * 10
+
+	// Waitig 5 seconds for a successful connection.
+	for timeStart := time.Now(); time.Now().Sub(timeStart) < time.Second*5; {
+		if err := connectFn(); err != nil {
+			if strings.Contains(err.Error(), `many connections`) {
+				// Sleep and try again if, and only if, the server replied with a "too
+				// many clients" error.
+				time.Sleep(waitTime)
+				if waitTime < time.Millisecond*500 {
+					// Wait a bit more next time.
+					waitTime = waitTime * 2
+				}
+				continue
+			}
+			// Return any other error immediately.
+			return err
+		}
+		return nil
+	}
+
+	return db.ErrGivingUpTryingToConnect
+}

mysql/database_test.go

@@ -30,6 +30,7 @@ import (
 	"reflect"
 	"strconv"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 
@@ -1443,3 +1444,49 @@ func TestDataTypes(t *testing.T) {
 		t.Fatalf("Struct is different.")
 	}
 }
+
+// TestExhaustConnections simulates a "too many connections" situation
+// triggered by opening more transactions than available connections.
+// upper.io/db deals with this problem by waiting a bit more for the connection
+// to be established.
+func TestExhaustConnections(t *testing.T) {
+	var err error
+	var sess db.Database
+	var wg sync.WaitGroup
+
+	if sess, err = db.Open(Adapter, settings); err != nil {
+		t.Fatal(err)
+	}
+
+	for i := 0; i < 500; i++ {
+		wg.Add(1)
+		t.Logf("Tx %d: Pending", i)
+		go func(t *testing.T, wg *sync.WaitGroup, i int) {
+			var tx db.Tx
+			defer wg.Done()
+
+			start := time.Now()
+
+			// Requesting a new transaction session.
+			if tx, err = sess.Transaction(); err != nil {
+				panic(err.Error())
+			}
+
+			t.Logf("Tx %d: OK (waiting time: %v)", i, time.Now().Sub(start))
+
+			// Let's suppose that we do some complex stuff and that the transaction
+			// lasts 3 seconds.
+			time.Sleep(time.Second * 3)
+
+			if err := tx.Rollback(); err != nil {
+				panic(err.Error())
+			}
+
+			t.Logf("Tx %d: Done", i)
+		}(t, &wg, i)
+	}
+
+	wg.Wait()
+
+	sess.Close()
+}