fix: Added multisubnetfailover option that can be set to false to pre…

…vent issue microsoft#158

README.md

@@ -65,6 +65,9 @@ Other supported formats are listed below.
 * `ApplicationIntent` - Can be given the value `ReadOnly` to initiate a read-only connection to an Availability Group listener. The `database` must be specified when connecting with `Application Intent` set to `ReadOnly`.
 * `protocol` - forces use of a protocol. Make sure the corresponding package is imported.
 * `columnencryption` or `column encryption setting` - a boolean value indicating whether Always Encrypted should be enabled on the connection.
+* `multisubnetfailover`
+  * `true` (Default) Client library will attempt to connect to all IPs simultaneously. For non multi subnet configurations, servers with multiple IPs, may cause SQL Server to log error 17830 due to the abrupt closure of initial connections during the parallel attempts "fire and forget pattern". Consider false in those scenarios to avoid the misleading error "Network error code 0x2746 occurred while establishing a connection; the connection has been closed. This may have been caused by client or server login timeout expiration".
+  * `false` Client library attempts to connect to IPs in serial.
 
 ### Connection parameters for namedpipe package
 * `pipe`  - If set, no Browser query is made and named pipe used will be `\\<host>\pipe\<pipe>`

msdsn/conn_str.go

@@ -78,6 +78,7 @@ const (
 	Protocol               = "protocol"
 	DialTimeout            = "dial timeout"
 	Pipe                   = "pipe"
+	MultiSubnetFailover    = "multisubnetfailover"
 )
 
 type Config struct {
@@ -128,6 +129,8 @@ type Config struct {
 	ChangePassword string
 	//ColumnEncryption is true if the application needs to decrypt or encrypt Always Encrypted values
 	ColumnEncryption bool
+	// Attempt to connect to all IPs in parallel when MultiSubnetFailover is true
+	MultiSubnetFailover bool
 }
 
 func readDERFile(filename string) ([]byte, error) {
@@ -483,6 +486,24 @@ func Parse(dsn string) (Config, error) {
 		}
 		p.ColumnEncryption = columnEncryption
 	}
+
+	msf, ok := params[MultiSubnetFailover]
+	if ok {
+		multiSubnetFailover, err := strconv.ParseBool(msf)
+		if err != nil {
+			if strings.EqualFold(msf, "Enabled") {
+				multiSubnetFailover = true
+			} else if strings.EqualFold(msf, "Disabled") {
+				multiSubnetFailover = false
+			} else {
+				return p, fmt.Errorf("invalid multiSubnetFailover value '%v': %v", multiSubnetFailover, err.Error())
+			}
+		}
+		p.MultiSubnetFailover = multiSubnetFailover
+	} else {
+		// Defaulting to true to prevent breaking change although other client libraries default to false
+		p.MultiSubnetFailover = true
+	}
 	return p, nil
 }
 

msdsn/conn_str_test.go

@@ -25,6 +25,7 @@ func TestInvalidConnectionString(t *testing.T) {
 		"failoverport=invalid",
 		"applicationintent=ReadOnly",
 		"disableretry=invalid",
+		"multisubnetfailover=invalid",
 
 		// ODBC mode
 		"odbc:password={",
@@ -104,6 +105,8 @@ func TestValidConnectionString(t *testing.T) {
 		{"disableretry=1", func(p Config) bool { return p.DisableRetry }},
 		{"disableretry=0", func(p Config) bool { return !p.DisableRetry }},
 		{"", func(p Config) bool { return p.DisableRetry == disableRetryDefault }},
+		{"MultiSubnetFailover=true", func(p Config) bool { return p.MultiSubnetFailover }},
+		{"MultiSubnetFailover=false", func(p Config) bool { return !p.MultiSubnetFailover }},
 
 		// those are supported currently, but maybe should not be
 		{"someparam", func(p Config) bool { return true }},

protocol.go

@@ -85,11 +85,19 @@ func (t tcpDialer) DialSqlConnection(ctx context.Context, c *Connector, p *msdsn
 	} else {
 		ips = []net.IP{ip}
 	}
-	if len(ips) == 1 {
-		d := c.getDialer(p)
-		addr := net.JoinHostPort(ips[0].String(), strconv.Itoa(int(resolveServerPort(p.Port))))
-		conn, err = d.DialContext(ctx, "tcp", addr)
-
+	if len(ips) == 1 || !p.MultiSubnetFailover {
+		// Attempt IPs sequentially when MultiSubnetFailover is false (or we only have one)
+		// to avoid triggering a 17830 error on SQL Server from a "fire and forget" pattern
+		// caused by connecting and immediately closing without doing any work
+		for _, ipaddress := range ips {
+			d := c.getDialer(p)
+			addr := net.JoinHostPort(ipaddress.String(), strconv.Itoa(int(resolveServerPort(p.Port))))
+			fmt.Println(ipaddress.String())
+			conn, err = d.DialContext(ctx, "tcp", addr)
+			if err == nil {
+				break
+			}
+		}
 	} else {
 		//Try Dials in parallel to avoid waiting for timeouts.
 		connChan := make(chan net.Conn, len(ips))