Add `-db_connect_timeout_ms` for timing out mysqld conns.

While testing vttablet-mysqld over TCP, we noticed mysql.Connect
sometimes blocks forever if the backend mysqld hit max conns. This
patch adds a timeout to this codepath. It defaults to 0 (no timeout)
for back compatibility.

Signed-off-by: Adam Saponara <as@php.net>

go/mysql/conn_params.go

@@ -30,11 +30,12 @@ type ConnParams struct {
 
 	// The following SSL flags are only used when flags |= 2048
 	// is set (CapabilityClientSSL).
-	SslCa      string `json:"ssl_ca"`
-	SslCaPath  string `json:"ssl_ca_path"`
-	SslCert    string `json:"ssl_cert"`
-	SslKey     string `json:"ssl_key"`
-	ServerName string `json:"server_name"`
+	SslCa            string `json:"ssl_ca"`
+	SslCaPath        string `json:"ssl_ca_path"`
+	SslCert          string `json:"ssl_cert"`
+	SslKey           string `json:"ssl_key"`
+	ServerName       string `json:"server_name"`
+	ConnectTimeoutMs uint64 `json:"connect_timeout_ms"`
 
 	// The following is only set when the deprecated "dbname" flags are
 	// supplied and will be removed.

go/vt/dbconfigs/dbconfigs.go

@@ -103,7 +103,7 @@ func registerBaseFlags() {
 	flag.StringVar(&baseConfig.SslCert, "db_ssl_cert", "", "connection ssl certificate")
 	flag.StringVar(&baseConfig.SslKey, "db_ssl_key", "", "connection ssl key")
 	flag.StringVar(&baseConfig.ServerName, "db_server_name", "", "server name of the DB we are connecting to.")
-
+	flag.Uint64Var(&baseConfig.ConnectTimeoutMs, "db_connect_timeout_ms", 0, "connection timeout to mysqld in milliseconds (0 for no timeout)")
 }
 
 // The flags will change the global singleton
@@ -287,6 +287,7 @@ func Init(defaultSocketFile string) (*DBConfigs, error) {
 			uc.param.SslKey = baseConfig.SslKey
 			uc.param.ServerName = baseConfig.ServerName
 		}
+		uc.param.ConnectTimeoutMs = baseConfig.ConnectTimeoutMs
 	}
 
 	// See if the CredentialsServer is working. We do not use the

go/vt/dbconfigs/dbconfigs_test.go

@@ -261,6 +261,60 @@ func TestInit(t *testing.T) {
 	}
 }
 
+func TestInitTimeout(t *testing.T) {
+	f := saveDBConfigs()
+	defer f()
+
+	baseConfig = mysql.ConnParams{
+		Host:             "a",
+		Port:             1,
+		Uname:            "b",
+		Pass:             "c",
+		DbName:           "d",
+		UnixSocket:       "e",
+		Charset:          "f",
+		Flags:            2,
+		Flavor:           "flavor",
+		ConnectTimeoutMs: 250,
+	}
+	dbConfigs = DBConfigs{
+		userConfigs: map[string]*userConfig{
+			App: {
+				param: mysql.ConnParams{
+					Uname: "app",
+					Pass:  "apppass",
+				},
+			},
+		},
+	}
+
+	dbc, err := Init("default")
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := &DBConfigs{
+		userConfigs: map[string]*userConfig{
+			App: {
+				param: mysql.ConnParams{
+					Host:             "a",
+					Port:             1,
+					Uname:            "app",
+					Pass:             "apppass",
+					UnixSocket:       "e",
+					Charset:          "f",
+					Flags:            2,
+					Flavor:           "flavor",
+					ConnectTimeoutMs: 250,
+				},
+			},
+		},
+	}
+
+	if !reflect.DeepEqual(dbc.userConfigs[App].param, want.userConfigs[App].param) {
+		t.Errorf("dbc: \n%#v, want \n%#v", dbc.userConfigs[App].param, want.userConfigs[App].param)
+	}
+}
+
 func TestAccessors(t *testing.T) {
 	dbc := &DBConfigs{
 		userConfigs: map[string]*userConfig{

go/vt/dbconnpool/connection.go

@@ -124,6 +124,11 @@ func NewDBConnection(info *mysql.ConnParams, mysqlStats *stats.Timings) (*DBConn
 		return nil, err
 	}
 	ctx := context.Background()
+	if info.ConnectTimeoutMs != 0 {
+		var cancel context.CancelFunc
+		ctx, cancel = context.WithTimeout(ctx, time.Duration(info.ConnectTimeoutMs)*time.Millisecond)
+		defer cancel()
+	}
 	c, err := mysql.Connect(ctx, params)
 	if err != nil {
 		mysqlStats.Record("ConnectError", start)