feat(outputs.syslog): Implement startup error behavior options

plugins/outputs/syslog/README.md

@@ -23,6 +23,20 @@ See the [CONFIGURATION.md][CONFIGURATION.md] for more details.
 
 [CONFIGURATION.md]: ../../../docs/CONFIGURATION.md#plugins
 
+## Startup error behavior options <!-- @/docs/includes/startup_error_behavior.md -->
+
+In addition to the plugin-specific and global configuration settings the plugin
+supports options for specifying the behavior when experiencing startup errors
+using the `startup_error_behavior` setting. Available values are:
+
+- `error`:  Telegraf with stop and exit in case of startup errors. This is the
+            default behavior.
+- `ignore`: Telegraf will ignore startup errors for this plugin and disables it
+            but continues processing for all other plugins.
+- `retry`:  Telegraf will try to startup the plugin in every gather or write
+            cycle in case of startup errors. The plugin is disabled until
+            the startup succeeds.
+
 ## Configuration
 
 ```toml @sample.conf

plugins/outputs/syslog/syslog.go

@@ -16,6 +16,7 @@ import (
 
 	"github.com/influxdata/telegraf"
 	"github.com/influxdata/telegraf/config"
+	"github.com/influxdata/telegraf/internal"
 	tlsint "github.com/influxdata/telegraf/plugins/common/tls"
 	"github.com/influxdata/telegraf/plugins/outputs"
 )
@@ -75,7 +76,7 @@ func (s *Syslog) Connect() error {
 		c, err = tls.Dial(spl[0], spl[1], tlsCfg)
 	}
 	if err != nil {
-		return err
+		return &internal.StartupError{Err: err, Retry: true}
 	}
 
 	if err := s.setKeepAlive(c); err != nil {

plugins/outputs/syslog/syslog_test.go

@@ -9,7 +9,9 @@ import (
 	"github.com/stretchr/testify/require"
 
 	"github.com/influxdata/telegraf"
+	"github.com/influxdata/telegraf/internal"
 	"github.com/influxdata/telegraf/metric"
+	"github.com/influxdata/telegraf/models"
 	"github.com/influxdata/telegraf/testutil"
 	"github.com/leodido/go-syslog/v4/nontransparent"
 )
@@ -244,3 +246,185 @@ func TestSyslogWriteReconnect(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, string(messageBytesWithFraming), string(buf[:n]))
 }
+
+func TestStartupErrorBehaviorDefault(t *testing.T) {
+	// Setup a dummy listener but do not accept connections
+	listener, err := net.Listen("tcp", "127.0.0.1:0")
+	require.NoError(t, err)
+	address := listener.Addr().String()
+	listener.Close()
+
+	// Setup the plugin and the model to be able to use the startup retry strategy
+	plugin := &Syslog{
+		Address:             "tcp://" + address,
+		Trailer:             nontransparent.LF,
+		Separator:           "_",
+		DefaultSeverityCode: uint8(5), // notice
+		DefaultFacilityCode: uint8(1), // user-level
+		DefaultAppname:      "Telegraf",
+	}
+
+	model := models.NewRunningOutput(
+		plugin,
+		&models.OutputConfig{
+			Name: "syslog",
+		},
+		10, 100,
+	)
+	require.NoError(t, model.Init())
+
+	// Starting the plugin will fail with an error because the server does not listen
+	err = model.Connect()
+	require.Error(t, err, "connection should be refused")
+	var serr *internal.StartupError
+	require.ErrorAs(t, err, &serr)
+}
+
+func TestStartupErrorBehaviorError(t *testing.T) {
+	// Setup a dummy listener but do not accept connections
+	listener, err := net.Listen("tcp", "127.0.0.1:0")
+	require.NoError(t, err)
+	address := listener.Addr().String()
+	listener.Close()
+
+	// Setup the plugin and the model to be able to use the startup retry strategy
+	plugin := &Syslog{
+		Address:             "tcp://" + address,
+		Trailer:             nontransparent.LF,
+		Separator:           "_",
+		DefaultSeverityCode: uint8(5), // notice
+		DefaultFacilityCode: uint8(1), // user-level
+		DefaultAppname:      "Telegraf",
+	}
+
+	model := models.NewRunningOutput(
+		plugin,
+		&models.OutputConfig{
+			Name:                 "syslog",
+			StartupErrorBehavior: "error",
+		},
+		10, 100,
+	)
+	require.NoError(t, model.Init())
+
+	// Starting the plugin will fail with an error because the server does not listen
+	err = model.Connect()
+	require.Error(t, err, "connection should be refused")
+	var serr *internal.StartupError
+	require.ErrorAs(t, err, &serr)
+}
+
+func TestStartupErrorBehaviorIgnore(t *testing.T) {
+	// Setup a dummy listener but do not accept connections
+	listener, err := net.Listen("tcp", "127.0.0.1:0")
+	require.NoError(t, err)
+	address := listener.Addr().String()
+	listener.Close()
+
+	// Setup the plugin and the model to be able to use the startup retry strategy
+	plugin := &Syslog{
+		Address:             "tcp://" + address,
+		Trailer:             nontransparent.LF,
+		Separator:           "_",
+		DefaultSeverityCode: uint8(5), // notice
+		DefaultFacilityCode: uint8(1), // user-level
+		DefaultAppname:      "Telegraf",
+	}
+
+	model := models.NewRunningOutput(
+		plugin,
+		&models.OutputConfig{
+			Name:                 "syslog",
+			StartupErrorBehavior: "ignore",
+		},
+		10, 100,
+	)
+	require.NoError(t, model.Init())
+
+	// Starting the plugin will fail because the server does not accept connections.
+	// The model code should convert it to a fatal error for the agent to remove
+	// the plugin.
+	err = model.Connect()
+	require.Error(t, err, "connection should be refused")
+	var fatalErr *internal.FatalError
+	require.ErrorAs(t, err, &fatalErr)
+}
+
+func TestStartupErrorBehaviorRetry(t *testing.T) {
+	// Setup a dummy listener but do not accept connections
+	listener, err := net.Listen("tcp", "127.0.0.1:0")
+	require.NoError(t, err)
+	address := listener.Addr().String()
+	listener.Close()
+
+	// Setup the plugin and the model to be able to use the startup retry strategy
+	plugin := &Syslog{
+		Address:             "tcp://" + address,
+		Trailer:             nontransparent.LF,
+		Separator:           "_",
+		DefaultSeverityCode: uint8(5), // notice
+		DefaultFacilityCode: uint8(1), // user-level
+		DefaultAppname:      "Telegraf",
+	}
+
+	model := models.NewRunningOutput(
+		plugin,
+		&models.OutputConfig{
+			Name:                 "syslog",
+			StartupErrorBehavior: "retry",
+		},
+		10, 100,
+	)
+	require.NoError(t, model.Init())
+
+	// Starting the plugin will return no error because the plugin will
+	// retry to connect in every write cycle.
+	require.NoError(t, model.Connect())
+	defer model.Close()
+
+	// Writing metrics in this state should fail because we are not fully
+	// started up
+	metrics := testutil.MockMetrics()
+	for _, m := range metrics {
+		model.AddMetric(m)
+	}
+	require.ErrorIs(t, model.WriteBatch(), internal.ErrNotConnected)
+
+	// Startup an actually working listener we can connect and write to
+	listener, err = net.Listen("tcp", "127.0.0.1:0")
+	require.NoError(t, err)
+	defer listener.Close()
+
+	var wg sync.WaitGroup
+	buf := make([]byte, 256)
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+
+		conn, err := listener.Accept()
+		if err != nil {
+			t.Logf("accepting connection failed: %v", err)
+			t.Fail()
+			return
+		}
+
+		if err := conn.SetReadDeadline(time.Now().Add(3 * time.Second)); err != nil {
+			t.Logf("setting read deadline failed: %v", err)
+			t.Fail()
+			return
+		}
+
+		if _, err := conn.Read(buf); err != nil {
+			t.Logf("reading failed: %v", err)
+			t.Fail()
+		}
+	}()
+
+	// Update the plugin's address and write again. This time the write should
+	// succeed.
+	plugin.Address = "tcp://" + listener.Addr().String()
+	require.NoError(t, model.WriteBatch())
+	wg.Wait()
+	require.NotEmpty(t, string(buf))
+}