fix(inputs.docker): Allow Telegraf to start when Docker daemon is unavailable

plugins/inputs/docker/docker.go

@@ -143,6 +143,18 @@ func (d *Docker) Init() error {
 }
 
 func (d *Docker) Start(telegraf.Accumulator) error {
+	// Attempt initial connection but don't fail if Docker is unavailable.
+	// This preserves backwards compatibility where Telegraf starts even when
+	// Docker daemon is not running.
+	if err := d.initClient(); err != nil {
+		d.Log.Warnf("Failed to connect to Docker daemon during startup: %v. Will retry on first gather.", err)
+	}
+	return nil
+}
+
+// initClient initializes the Docker client and performs Podman detection.
+// Returns an error if the connection fails, but does not prevent Telegraf from starting.
+func (d *Docker) initClient() error {
 	// Get client
 	c, err := d.getNewClient()
 	if err != nil {
@@ -153,6 +165,8 @@ func (d *Docker) Start(telegraf.Accumulator) error {
 	// Check API version compatibility
 	version, err := semver.NewVersion(d.client.ClientVersion())
 	if err != nil {
+		d.client.Close()
+		d.client = nil
 		return fmt.Errorf("failed to parse client version: %w", err)
 	}
 
@@ -170,6 +184,8 @@ func (d *Docker) Start(telegraf.Accumulator) error {
 
 	info, err := d.client.Info(ctx)
 	if err != nil {
+		d.client.Close()
+		d.client = nil
 		return fmt.Errorf("failed to get Docker info: %w", err)
 	}
 
@@ -197,6 +213,13 @@ func (d *Docker) Stop() {
 }
 
 func (d *Docker) Gather(acc telegraf.Accumulator) error {
+	// If client is not initialized, try to connect now
+	if d.client == nil {
+		if err := d.initClient(); err != nil {
+			return fmt.Errorf("failed to connect to Docker daemon: %w", err)
+		}
+	}
+
 	// Create label filters if not already created
 	if !d.filtersCreated {
 		err := d.createLabelFilters()

plugins/inputs/docker/docker_test.go

@@ -3,6 +3,7 @@ package docker
 import (
 	"context"
 	"crypto/tls"
+	"errors"
 	"io"
 	"reflect"
 	"sort"
@@ -1774,3 +1775,84 @@ func TestPodmanStatsCache(t *testing.T) {
 	require.NotContains(t, d.statsCache, "old-container")
 	require.Contains(t, d.statsCache, testID)
 }
+
+func TestStartWithUnavailableDocker(t *testing.T) {
+	// Test that Telegraf starts successfully even when Docker is unavailable
+	// This is a regression test for https://github.com/influxdata/telegraf/issues/18089
+	var acc testutil.Accumulator
+	d := Docker{
+		Log: testutil.Logger{},
+		newClient: func(string, *tls.Config) (dockerClient, error) {
+			return nil, errors.New("cannot connect to the Docker daemon")
+		},
+		newEnvClient: func() (dockerClient, error) {
+			return nil, errors.New("cannot connect to the Docker daemon")
+		},
+	}
+
+	require.NoError(t, d.Init())
+	// Start should NOT return an error even when Docker is unavailable
+	require.NoError(t, d.Start(&acc))
+	// Client should be nil since connection failed
+	require.Nil(t, d.client)
+
+	// Gather should return an error since Docker is still unavailable
+	err := d.Gather(&acc)
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "failed to connect to Docker daemon")
+}
+
+func TestLazyClientInitialization(t *testing.T) {
+	// Test that client is initialized lazily on first Gather if Start failed to connect
+	var acc testutil.Accumulator
+
+	// Track connection attempts
+	connectionAttempts := 0
+
+	d := Docker{
+		Log: testutil.Logger{},
+		newClient: func(string, *tls.Config) (dockerClient, error) {
+			connectionAttempts++
+			// First attempt fails, subsequent attempts succeed
+			if connectionAttempts == 1 {
+				return nil, errors.New("docker daemon not ready")
+			}
+			return &mockClient{
+				InfoF: func() (system.Info, error) {
+					return system.Info{
+						Name:          "docker-desktop",
+						ServerVersion: "20.10.0",
+					}, nil
+				},
+				ContainerListF: func(container.ListOptions) ([]container.Summary, error) {
+					return nil, nil
+				},
+				ClientVersionF: func() string {
+					return "1.24.0"
+				},
+				CloseF: func() error {
+					return nil
+				},
+			}, nil
+		},
+		newEnvClient: func() (dockerClient, error) {
+			return nil, errors.New("not using env client")
+		},
+	}
+
+	require.NoError(t, d.Init())
+	// Start should succeed even though connection fails
+	require.NoError(t, d.Start(&acc))
+	require.Equal(t, 1, connectionAttempts)
+	require.Nil(t, d.client)
+
+	// First Gather fails because Docker is still unavailable (same mock returns error on attempt 1)
+	// Reset connection attempts to simulate Docker becoming available
+	connectionAttempts = 1 // Set to 1 so next attempt (2) will succeed
+
+	// Second Gather should succeed after lazy initialization
+	err := d.Gather(&acc)
+	require.NoError(t, err)
+	require.Equal(t, 2, connectionAttempts)
+	require.NotNil(t, d.client)
+}