Add flag to ignore Systemd version and improve tests

This commit adds a flag to ignore the Systemd version check for the
Journald input. The mage targets are updated so the Journald input
unit tests always run. The tests disable the Systemd version check so
we can run the tests in our current hosts.

filebeat/cmd/root.go

@@ -63,6 +63,7 @@ func Filebeat(inputs beater.PluginFactory, settings instance.Settings) *cmd.Beat
 	command.PersistentFlags().AddGoFlag(flag.CommandLine.Lookup("M"))
 	command.TestCmd.Flags().AddGoFlag(flag.CommandLine.Lookup("modules"))
 	command.SetupCmd.Flags().AddGoFlag(flag.CommandLine.Lookup("modules"))
+	command.Flags().AddGoFlag(flag.CommandLine.Lookup("ignore-journald-version"))
 	command.AddCommand(cmd.GenModulesCmd(Name, "", buildModulesManager))
 	command.AddCommand(genGenerateCmd())
 	return command

filebeat/input/journald/input.go

@@ -21,6 +21,7 @@ package journald
 
 import (
 	"errors"
+	"flag"
 	"fmt"
 	"strconv"
 	"strings"
@@ -42,6 +43,15 @@ import (
 	"github.com/elastic/elastic-agent-libs/logp"
 )
 
+var noVersionCheck bool
+
+func init() {
+	flag.BoolVar(&noVersionCheck,
+		"ignore-journald-version",
+		false,
+		"Does not check Journald version when starting the Journald input. This might cause Filebeat to crash!")
+}
+
 type journald struct {
 	Backoff            time.Duration
 	MaxBackoff         time.Duration
@@ -92,6 +102,11 @@ func Plugin(log *logp.Logger, store cursor.StateStore) input.Plugin {
 		Manager:    m,
 	}
 
+	if noVersionCheck {
+		log.Warn("Journald version check has been DISABLED! Filebeat might crash if Journald version is < 255.")
+		return p
+	}
+
 	version, err := systemdVersion()
 	if err != nil {
 		configErr := fmt.Errorf("%w: %s", ErrCannotGetSystemdVersion, err)
@@ -358,9 +373,14 @@ func parseSystemdVersion(output string) (int, error) {
 	return version, err
 }
 
-// getSystemdVersionViaDBus foo
+// getSystemdVersionViaDBus gets the Systemd version from D-Bus
+//
+// We get the version by reading the property
+// `org.freedesktop.systemd1.Manager.Version`. Even though this property is
+// is documented as not being part of the official API and having an unstable
+// scheme, on our tests it proved to be stable enough.
 //
-// Version string should not be parsed:
+// The Systemd D-Bus documentation states:
 //
 //	 Version encodes the version string of the running systemd
 //	 instance. Note that the version string is purely informational,

filebeat/input/journald/input_test.go

@@ -21,13 +21,21 @@ package journald
 
 import (
 	"context"
+	"flag"
 	"fmt"
+	"os"
 	"path"
 	"testing"
 
 	"github.com/elastic/elastic-agent-libs/mapstr"
 )
 
+func TestMain(m *testing.M) {
+	flag.Parse()
+	noVersionCheck = true
+	os.Exit(m.Run())
+}
+
 func TestInputFieldsTranslation(t *testing.T) {
 	// A few random keys to verify
 	keysToCheck := map[string]string{

filebeat/magefile.go

@@ -28,6 +28,7 @@ import (
 
 	devtools "github.com/elastic/beats/v7/dev-tools/mage"
 	"github.com/elastic/beats/v7/dev-tools/mage/target/build"
+	"github.com/elastic/beats/v7/dev-tools/mage/target/unittest"
 	filebeat "github.com/elastic/beats/v7/filebeat/scripts/mage"
 
 	//mage:import
@@ -45,6 +46,7 @@ import (
 func init() {
 	common.RegisterCheckDeps(Update)
 	test.RegisterDeps(IntegTest)
+	unittest.RegisterGoTestDeps(TestJournaldInput)
 
 	devtools.BeatDescription = "Filebeat sends log files to Logstash or directly to Elasticsearch."
 }
@@ -215,15 +217,20 @@ func PythonIntegTest(ctx context.Context) error {
 	return devtools.PythonIntegTestFromHost(devtools.DefaultPythonTestIntegrationFromHostArgs())
 }
 
-// TestJournald executes the Journald input tests
+// TestJournaldInput executes the Journald input unit tests.
+//
+// It requires Systemd >= 255 and D-Bus to be installed
+// on the host.
+//
 // Use TEST_COVERAGE=true to enable code coverage profiling.
 // Use RACE_DETECTOR=true to enable the race detector.
-func TestJournald(ctx context.Context) error {
-	utArgs := devtools.DefaultGoTestUnitArgs()
-	utArgs.Packages = []string{"./input/journald"}
+func TestJournaldInput(ctx context.Context) error {
 	if devtools.Platform.GOOS == "linux" {
-		utArgs.ExtraFlags = append(utArgs.ExtraFlags, "-tags=withjournald")
+		testArgs := devtools.DefaultGoTestUnitArgs()
+		testArgs.Packages = []string{"./input/journald"}
+		testArgs.ExtraFlags = append(testArgs.ExtraFlags, "-tags=withjournald")
+		return devtools.GoTest(ctx, testArgs)
 	}
 
-	return devtools.GoTest(ctx, utArgs)
+	return nil
 }

libbeat/docs/command-reference.asciidoc

@@ -744,12 +744,19 @@ the end of the file is reached. By default harvesters are closed after
 The `--once` option is not currently supported with the
 {filebeat-ref}/filebeat-input-filestream.html[`filestream`] input type.
 
+*`--ignore-journald-version`*::
+When the `--ignore-journald-version` is used, the Journald input
+**will not** validate the minimum Systemd version during the input
+initialisation. Running the Journald input with an unsupported version
+of Systemd might cause {beatname_uc} to crash.
 endif::[]
 
+ifeval::["{beatname_lc}"=="metricbeat"]
 *`--system.hostfs MOUNT_POINT`*::
 
 Specifies the mount point of the host's filesystem for use in monitoring a host. 
 This flag is depricated, and an alternate hostfs should be specified via the `hostfs` module config value.
+endif::[]
 
 
 ifeval::["{beatname_lc}"=="packetbeat"]