From 2ee23a50b062d49ab210935bd1ef98de6a12bcb5 Mon Sep 17 00:00:00 2001
From: Michael Abair <abairmj@gmail.com>
Date: Thu, 12 Oct 2023 22:56:55 -0400
Subject: [PATCH] fix: Added multisubnetfailover option that can be set to
 false to prevent issue #158

---
 README.md              |  3 +++
 msdsn/conn_str.go      | 21 +++++++++++++++++++++
 msdsn/conn_str_test.go |  3 +++
 protocol.go            | 15 ++++++++++-----
 4 files changed, 37 insertions(+), 5 deletions(-)

diff --git a/README.md b/README.md
index 2a151d2d..a1d05a0a 100644
--- a/README.md
+++ b/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 attempt to connect to all IPs simultaneously. 
+  * `false` Client 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>`
diff --git a/msdsn/conn_str.go b/msdsn/conn_str.go
index 2bdddb57..460dd4e3 100644
--- a/msdsn/conn_str.go
+++ b/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
 }
 
diff --git a/msdsn/conn_str_test.go b/msdsn/conn_str_test.go
index 0645313b..f1bf03eb 100644
--- a/msdsn/conn_str_test.go
+++ b/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 }},
diff --git a/protocol.go b/protocol.go
index 6a83ab25..7d9646b5 100644
--- a/protocol.go
+++ b/protocol.go
@@ -85,11 +85,16 @@ 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 {
+		// Try to connect to IPs sequentially until one is successful per MultiSubnetFailover false rules
+		for _, ipaddress := range ips {
+			d := c.getDialer(p)
+			addr := net.JoinHostPort(ipaddress.String(), strconv.Itoa(int(resolveServerPort(p.Port))))
+			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))