logp: don't write to files by default if running in a container environment

logp/config.go

@@ -70,9 +70,20 @@ const (
 // DefaultConfig returns the default config options for a given environment the
 // Beat is supposed to be run within.
 func DefaultConfig(environment Environment) Config {
+	toFiles := true
+	toStderr := false
+
+	// For container and systemd environments, we don't write to files by default.
+	switch environment {
+	case ContainerEnvironment, SystemdEnvironment:
+		toFiles = false
+		toStderr = true
+	}
+
 	return Config{
-		Level:   defaultLevel,
-		ToFiles: true,
+		Level:    defaultLevel,
+		ToFiles:  toFiles,
+		ToStderr: toStderr,
 		Files: FileConfig{
 			MaxSize:         10 * 1024 * 1024,
 			MaxBackups:      7,

logp/defaults_test.go

@@ -19,9 +19,11 @@ package logp_test
 
 import (
 	"bufio"
+	"bytes"
 	"encoding/json"
 	"fmt"
 	"os"
+	"os/exec"
 	"path/filepath"
 	"runtime"
 	"sort"
@@ -117,6 +119,62 @@ func TestDefaultConfig(t *testing.T) {
 	}
 }
 
+func TestDefaultConfigContainerLogsToStderr(t *testing.T) {
+	runTestEnvStderr(t, logp.ContainerEnvironment)
+}
+
+func TestDefaultConfigSystemdLogsToStderr(t *testing.T) {
+	runTestEnvStderr(t, logp.SystemdEnvironment)
+}
+
+func runTestEnvStderr(t *testing.T, envType logp.Environment) {
+	switch runtime.GOOS {
+	case "wasip1", "js", "ios":
+		t.Skipf("cannot exec subprocess on %s/%s", runtime.GOOS, runtime.GOARCH)
+	}
+
+	if os.Getenv("TEST_DEFAULT_CONFIG_STDERR") != "1" {
+		cmd := exec.Command(os.Args[0], fmt.Sprintf("-test.run=^%s$", t.Name()), "-test.v") //nolint:gosec // This is intentionally running a subprocess
+		cmd.Env = append(cmd.Env, "TEST_DEFAULT_CONFIG_STDERR=1")
+
+		var stderr bytes.Buffer
+		cmd.Stderr = &stderr
+
+		err := cmd.Run()
+		data := stderr.Bytes()
+		assert.NoError(t, err, "command failed with error: %s\nstderr: %s", err, data)
+		t.Logf("output:\n%s", data)
+
+		logEntry := struct {
+			LogLevel  string `json:"log.level"`
+			LogOrigin struct {
+				FileName string `json:"file.name"`
+				FileLine int    `json:"file.line"`
+			} `json:"log.origin"`
+			Message string `json:"message"`
+		}{}
+
+		assert.NoError(t, json.Unmarshal(data, &logEntry), "cannot unmarshal log entry from stderr")
+
+		assert.Equal(t, "info", logEntry.LogLevel)
+		assert.Equal(t, "foo", logEntry.Message)
+
+		_, fileName, _, _ := runtime.Caller(0)
+		expectedFileName := filepath.Base(fileName)
+		gotFileName := filepath.Base(logEntry.LogOrigin.FileName)
+		assert.Equal(t, expectedFileName, gotFileName)
+
+		return
+	}
+
+	// This is running in a separate process to make sure we capture stderr.
+	cfg := logp.DefaultConfig(envType)
+	assert.NoError(t, logp.Configure(cfg))
+	logger := logp.L()
+	defer logger.Close()
+	logger.Info("foo")
+}
+
 func TestWith(t *testing.T) {
 	tempDir := t.TempDir()