Splits the log source handling with a pluggable interface.

Provides a cleaner split between log sources, specifically for not
compiling with systemd libraries.

This is in preparation for a new log source to read from Docker.

logsource.go

@@ -0,0 +1,63 @@
+package main
+
+import (
+	"fmt"
+	"io"
+
+	"github.com/alecthomas/kingpin"
+)
+
+// A LogSourceFactory provides a repository of log sources that can be
+// instantiated from command line flags.
+type LogSourceFactory interface {
+	// Init adds the factory's struct fields as flags in the
+	// application.
+	Init(*kingpin.Application)
+
+	// New attempts to create a new log source. This is called after
+	// flags have been parsed. Returning `nil, nil`, means the user
+	// didn't want this log source.
+	New() (LogSourceCloser, error)
+}
+
+type LogSourceCloser interface {
+	io.Closer
+	LogSource
+}
+
+var logSourceFactories []LogSourceFactory
+
+// RegisterLogSourceFactory can be called from module `init` functions
+// to register factories.
+func RegisterLogSourceFactory(lsf LogSourceFactory) {
+	logSourceFactories = append(logSourceFactories, lsf)
+}
+
+// InitLogSourceFactories runs Init on all factories. The
+// initialization order is arbitrary, except `fileLogSourceFactory` is
+// always last (the fallback). The file log source must be last since
+// it's enabled by default.
+func InitLogSourceFactories(app *kingpin.Application) {
+	RegisterLogSourceFactory(&fileLogSourceFactory{})
+
+	for _, f := range logSourceFactories {
+		f.Init(app)
+	}
+}
+
+// NewLogSourceFromFactories iterates through the factories and
+// attempts to instantiate a log source. The first factory to return
+// success wins.
+func NewLogSourceFromFactories() (LogSourceCloser, error) {
+	for _, f := range logSourceFactories {
+		src, err := f.New()
+		if err != nil {
+			return nil, err
+		}
+		if src != nil {
+			return src, nil
+		}
+	}
+
+	return nil, fmt.Errorf("no log source configured")
+}

logsource_file.go

@@ -0,0 +1,78 @@
+package main
+
+import (
+	"context"
+	"io"
+	"log"
+
+	"github.com/alecthomas/kingpin"
+	"github.com/hpcloud/tail"
+)
+
+// A FileLogSource can read lines from a file.
+type FileLogSource struct {
+	tailer *tail.Tail
+}
+
+// NewFileLogSource creates a new log source, tailing the given file.
+func NewFileLogSource(path string) (*FileLogSource, error) {
+	tailer, err := tail.TailFile(path, tail.Config{
+		ReOpen:    true,                               // reopen the file if it's rotated
+		MustExist: true,                               // fail immediately if the file is missing or has incorrect permissions
+		Follow:    true,                               // run in follow mode
+		Location:  &tail.SeekInfo{Whence: io.SeekEnd}, // seek to end of file
+		Logger:    tail.DiscardingLogger,
+	})
+	if err != nil {
+		return nil, err
+	}
+	return &FileLogSource{tailer}, nil
+}
+
+func (s *FileLogSource) Close() error {
+	defer s.tailer.Cleanup()
+	go func() {
+		// Stop() waits for the tailer goroutine to shut down, but it
+		// can be blocking on sending on the Lines channel...
+		for range s.tailer.Lines {
+		}
+	}()
+	return s.tailer.Stop()
+}
+
+func (s *FileLogSource) Path() string {
+	return s.tailer.Filename
+}
+
+func (s *FileLogSource) Read(ctx context.Context) (string, error) {
+	select {
+	case line, ok := <-s.tailer.Lines:
+		if !ok {
+			return "", io.EOF
+		}
+		return line.Text, nil
+	case <-ctx.Done():
+		return "", ctx.Err()
+	}
+}
+
+// A fileLogSourceFactory is a factory than can create log sources
+// from command line flags.
+//
+// Because this factory is enabled by default, it must always be
+// registered last.
+type fileLogSourceFactory struct {
+	path string
+}
+
+func (f *fileLogSourceFactory) Init(app *kingpin.Application) {
+	app.Flag("postfix.logfile_path", "Path where Postfix writes log entries.").Default("/var/log/maillog").StringVar(&f.path)
+}
+
+func (f *fileLogSourceFactory) New() (LogSourceCloser, error) {
+	if f.path == "" {
+		return nil, nil
+	}
+	log.Printf("Reading log events from %s", f.path)
+	return NewFileLogSource(f.path)
+}

logsource_file_test.go

@@ -0,0 +1,87 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestFileLogSource_Path(t *testing.T) {
+	path, close, err := setupFakeLogFile()
+	if err != nil {
+		t.Fatalf("setupFakeTailer failed: %v", err)
+	}
+	defer close()
+
+	src, err := NewFileLogSource(path)
+	if err != nil {
+		t.Fatalf("NewFileLogSource failed: %v", err)
+	}
+	defer src.Close()
+
+	assert.Equal(t, path, src.Path(), "Path should be set by New.")
+}
+
+func TestFileLogSource_Read(t *testing.T) {
+	ctx := context.Background()
+
+	path, close, err := setupFakeLogFile()
+	if err != nil {
+		t.Fatalf("setupFakeTailer failed: %v", err)
+	}
+	defer close()
+
+	src, err := NewFileLogSource(path)
+	if err != nil {
+		t.Fatalf("NewFileLogSource failed: %v", err)
+	}
+	defer src.Close()
+
+	s, err := src.Read(ctx)
+	if err != nil {
+		t.Fatalf("Read failed: %v", err)
+	}
+	assert.Equal(t, "Feb 13 23:31:30 ahost anid[123]: aline", s, "Read should get data from the journal entry.")
+}
+
+func setupFakeLogFile() (string, func(), error) {
+	f, err := ioutil.TempFile("", "filelogsource")
+	if err != nil {
+		return "", nil, err
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	var wg sync.WaitGroup
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		defer os.Remove(f.Name())
+		defer f.Close()
+
+		for {
+			// The tailer seeks to the end and then does a
+			// follow. Keep writing lines so we know it wakes up and
+			// returns lines.
+			fmt.Fprintln(f, "Feb 13 23:31:30 ahost anid[123]: aline")
+
+			select {
+			case <-time.After(10 * time.Millisecond):
+				// continue
+			case <-ctx.Done():
+				return
+			}
+		}
+	}()
+
+	return f.Name(), func() {
+		cancel()
+		wg.Wait()
+	}, nil
+}

logsource_systemd.go

@@ -0,0 +1,143 @@
+// +build !nosystemd,linux
+
+package main
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"log"
+	"time"
+
+	"github.com/alecthomas/kingpin"
+	"github.com/coreos/go-systemd/v22/sdjournal"
+)
+
+// timeNow is a test fake injection point.
+var timeNow = time.Now
+
+// A SystemdLogSource reads log records from the given Systemd
+// journal.
+type SystemdLogSource struct {
+	journal SystemdJournal
+	path    string
+}
+
+// A SystemdJournal is the journal interface that sdjournal.Journal
+// provides. See https://pkg.go.dev/github.com/coreos/go-systemd/sdjournal?tab=doc
+type SystemdJournal interface {
+	io.Closer
+	AddMatch(match string) error
+	GetEntry() (*sdjournal.JournalEntry, error)
+	Next() (uint64, error)
+	SeekRealtimeUsec(usec uint64) error
+	Wait(timeout time.Duration) int
+}
+
+// NewSystemdLogSource returns a log source for reading Systemd
+// journal entries. `unit` and `slice` provide filtering if non-empty
+// (with `slice` taking precedence).
+func NewSystemdLogSource(j SystemdJournal, path, unit, slice string) (*SystemdLogSource, error) {
+	logSrc := &SystemdLogSource{journal: j, path: path}
+
+	var err error
+	if slice != "" {
+		err = logSrc.journal.AddMatch("_SYSTEMD_SLICE=" + slice)
+	} else if unit != "" {
+		err = logSrc.journal.AddMatch("_SYSTEMD_UNIT=" + unit)
+	}
+	if err != nil {
+		logSrc.journal.Close()
+		return nil, err
+	}
+
+	// Start at end of journal
+	if err := logSrc.journal.SeekRealtimeUsec(uint64(timeNow().UnixNano() / 1000)); err != nil {
+		logSrc.journal.Close()
+		return nil, err
+	}
+
+	if r := logSrc.journal.Wait(1 * time.Second); r < 0 {
+		logSrc.journal.Close()
+		return nil, err
+	}
+
+	return logSrc, nil
+}
+
+func (s *SystemdLogSource) Close() error {
+	return s.journal.Close()
+}
+
+func (s *SystemdLogSource) Path() string {
+	return s.path
+}
+
+func (s *SystemdLogSource) Read(ctx context.Context) (string, error) {
+	c, err := s.journal.Next()
+	if err != nil {
+		return "", err
+	}
+	if c == 0 {
+		return "", io.EOF
+	}
+
+	e, err := s.journal.GetEntry()
+	if err != nil {
+		return "", err
+	}
+	ts := time.Unix(0, int64(e.RealtimeTimestamp)*int64(time.Microsecond))
+
+	return fmt.Sprintf(
+		"%s %s %s[%s]: %s",
+		ts.Format(time.Stamp),
+		e.Fields["_HOSTNAME"],
+		e.Fields["SYSLOG_IDENTIFIER"],
+		e.Fields["_PID"],
+		e.Fields["MESSAGE"],
+	), nil
+}
+
+// A systemdLogSourceFactory is a factory that can create
+// SystemdLogSources from command line flags.
+type systemdLogSourceFactory struct {
+	enable            bool
+	unit, slice, path string
+}
+
+func (f *systemdLogSourceFactory) Init(app *kingpin.Application) {
+	app.Flag("systemd.enable", "Read from the systemd journal instead of log").Default("false").BoolVar(&f.enable)
+	app.Flag("systemd.unit", "Name of the Postfix systemd unit.").Default("postfix.service").StringVar(&f.unit)
+	app.Flag("systemd.slice", "Name of the Postfix systemd slice. Overrides the systemd unit.").Default("").StringVar(&f.slice)
+	app.Flag("systemd.journal_path", "Path to the systemd journal").Default("").StringVar(&f.path)
+}
+
+func (f *systemdLogSourceFactory) New() (LogSourceCloser, error) {
+	if !f.enable {
+		return nil, nil
+	}
+
+	log.Println("Reading log events from systemd")
+	j, path, err := newSystemdJournal(f.path)
+	if err != nil {
+		return nil, err
+	}
+	return NewSystemdLogSource(j, path, f.unit, f.slice)
+}
+
+// newSystemdJournal creates a journal handle. It returns the handle
+// and a string representation of it. If `path` is empty, it connects
+// to the local journald.
+func newSystemdJournal(path string) (*sdjournal.Journal, string, error) {
+	if path != "" {
+		j, err := sdjournal.NewJournalFromDir(path)
+		return j, path, err
+	}
+
+	j, err := sdjournal.NewJournal()
+	return j, "journald", err
+}
+
+func init() {
+	RegisterLogSourceFactory(&systemdLogSourceFactory{})
+}

logsource_systemd_test.go

@@ -0,0 +1,150 @@
+// +build !nosystemd,linux
+
+package main
+
+import (
+	"context"
+	"io"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/coreos/go-systemd/v22/sdjournal"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNewSystemdLogSource(t *testing.T) {
+	j := &fakeSystemdJournal{}
+	src, err := NewSystemdLogSource(j, "apath", "aunit", "aslice")
+	if err != nil {
+		t.Fatalf("NewSystemdLogSource failed: %v", err)
+	}
+
+	assert.Equal(t, []string{"_SYSTEMD_SLICE=aslice"}, j.addMatchCalls, "A match should be added for slice.")
+	assert.Equal(t, []uint64{1234567890000000}, j.seekRealtimeUsecCalls, "A call to SeekRealtimeUsec should be made.")
+	assert.Equal(t, []time.Duration{1 * time.Second}, j.waitCalls, "A call to Wait should be made.")
+
+	if err := src.Close(); err != nil {
+		t.Fatalf("Close failed: %v", err)
+	}
+
+	assert.Equal(t, 1, j.closeCalls, "A call to Close should be made.")
+}
+
+func TestSystemdLogSource_Path(t *testing.T) {
+	j := &fakeSystemdJournal{}
+	src, err := NewSystemdLogSource(j, "apath", "aunit", "aslice")
+	if err != nil {
+		t.Fatalf("NewSystemdLogSource failed: %v", err)
+	}
+	defer src.Close()
+
+	assert.Equal(t, "apath", src.Path(), "Path should be set by New.")
+}
+
+func TestSystemdLogSource_Read(t *testing.T) {
+	ctx := context.Background()
+
+	j := &fakeSystemdJournal{
+		getEntryValues: []sdjournal.JournalEntry{
+			{
+				Fields: map[string]string{
+					"_HOSTNAME":         "ahost",
+					"SYSLOG_IDENTIFIER": "anid",
+					"_PID":              "123",
+					"MESSAGE":           "aline",
+				},
+				RealtimeTimestamp: 1234567890000000,
+			},
+		},
+		nextValues: []uint64{1},
+	}
+	src, err := NewSystemdLogSource(j, "apath", "aunit", "aslice")
+	if err != nil {
+		t.Fatalf("NewSystemdLogSource failed: %v", err)
+	}
+	defer src.Close()
+
+	s, err := src.Read(ctx)
+	if err != nil {
+		t.Fatalf("Read failed: %v", err)
+	}
+	assert.Equal(t, "Feb 13 23:31:30 ahost anid[123]: aline", s, "Read should get data from the journal entry.")
+}
+
+func TestSystemdLogSource_ReadEOF(t *testing.T) {
+	ctx := context.Background()
+
+	j := &fakeSystemdJournal{
+		nextValues: []uint64{0},
+	}
+	src, err := NewSystemdLogSource(j, "apath", "aunit", "aslice")
+	if err != nil {
+		t.Fatalf("NewSystemdLogSource failed: %v", err)
+	}
+	defer src.Close()
+
+	_, err = src.Read(ctx)
+	assert.Equal(t, io.EOF, err, "Should interpret Next 0 as EOF.")
+}
+
+func TestMain(m *testing.M) {
+	// We compare Unix timestamps to date strings, so make it deterministic.
+	os.Setenv("TZ", "UTC")
+	timeNow = func() time.Time { return time.Date(2009, 2, 13, 23, 31, 30, 0, time.UTC) }
+	defer func() {
+		timeNow = time.Now
+	}()
+
+	os.Exit(m.Run())
+}
+
+type fakeSystemdJournal struct {
+	getEntryValues []sdjournal.JournalEntry
+	getEntryError  error
+	nextValues     []uint64
+	nextError      error
+
+	addMatchCalls         []string
+	closeCalls            int
+	seekRealtimeUsecCalls []uint64
+	waitCalls             []time.Duration
+}
+
+func (j *fakeSystemdJournal) AddMatch(match string) error {
+	j.addMatchCalls = append(j.addMatchCalls, match)
+	return nil
+}
+
+func (j *fakeSystemdJournal) Close() error {
+	j.closeCalls++
+	return nil
+}
+
+func (j *fakeSystemdJournal) GetEntry() (*sdjournal.JournalEntry, error) {
+	if len(j.getEntryValues) == 0 {
+		return nil, j.getEntryError
+	}
+	e := j.getEntryValues[0]
+	j.getEntryValues = j.getEntryValues[1:]
+	return &e, nil
+}
+
+func (j *fakeSystemdJournal) Next() (uint64, error) {
+	if len(j.nextValues) == 0 {
+		return 0, j.nextError
+	}
+	v := j.nextValues[0]
+	j.nextValues = j.nextValues[1:]
+	return v, nil
+}
+
+func (j *fakeSystemdJournal) SeekRealtimeUsec(usec uint64) error {
+	j.seekRealtimeUsecCalls = append(j.seekRealtimeUsecCalls, usec)
+	return nil
+}
+
+func (j *fakeSystemdJournal) Wait(timeout time.Duration) int {
+	j.waitCalls = append(j.waitCalls, timeout)
+	return 0
+}

main.go

@@ -13,36 +13,25 @@ import (
 
 func main() {
 	var (
-		app                                           = kingpin.New("postfix_exporter", "Prometheus metrics exporter for postfix")
-		listenAddress                                 = app.Flag("web.listen-address", "Address to listen on for web interface and telemetry.").Default(":9154").String()
-		metricsPath                                   = app.Flag("web.telemetry-path", "Path under which to expose metrics.").Default("/metrics").String()
-		postfixShowqPath                              = app.Flag("postfix.showq_path", "Path at which Postfix places its showq socket.").Default("/var/spool/postfix/public/showq").String()
-		postfixLogfilePath                            = app.Flag("postfix.logfile_path", "Path where Postfix writes log entries.").Default("/var/log/maillog").String()
-		logUnsupportedLines                           = app.Flag("log.unsupported", "Log all unsupported lines.").Bool()
-		systemdEnable                                 bool
-		systemdUnit, systemdSlice, systemdJournalPath string
+		app                 = kingpin.New("postfix_exporter", "Prometheus metrics exporter for postfix")
+		listenAddress       = app.Flag("web.listen-address", "Address to listen on for web interface and telemetry.").Default(":9154").String()
+		metricsPath         = app.Flag("web.telemetry-path", "Path under which to expose metrics.").Default("/metrics").String()
+		postfixShowqPath    = app.Flag("postfix.showq_path", "Path at which Postfix places its showq socket.").Default("/var/spool/postfix/public/showq").String()
+		logUnsupportedLines = app.Flag("log.unsupported", "Log all unsupported lines.").Bool()
 	)
-	systemdFlags(&systemdEnable, &systemdUnit, &systemdSlice, &systemdJournalPath, app)
 
+	InitLogSourceFactories(app)
 	kingpin.MustParse(app.Parse(os.Args[1:]))
 
-	var journal *Journal
-	if systemdEnable {
-		var err error
-		journal, err = NewJournal(systemdUnit, systemdSlice, systemdJournalPath)
-		if err != nil {
-			log.Fatalf("Error opening systemd journal: %s", err)
-		}
-		defer journal.Close()
-		log.Println("Reading log events from systemd")
-	} else {
-		log.Printf("Reading log events from %v", *postfixLogfilePath)
+	logSrc, err := NewLogSourceFromFactories()
+	if err != nil {
+		log.Fatalf("Error opening log source: %s", err)
 	}
+	defer logSrc.Close()
 
 	exporter, err := NewPostfixExporter(
 		*postfixShowqPath,
-		*postfixLogfilePath,
-		journal,
+		logSrc,
 		*logUnsupportedLines,
 	)
 	if err != nil {

nosystemd.go

@@ -1,25 +0,0 @@
-// +build nosystemd !linux
-// This file contains stubs to support non-systemd use
-
-package main
-
-import (
-	"io"
-
-	"github.com/alecthomas/kingpin"
-)
-
-type Journal struct {
-	io.Closer
-	Path string
-}
-
-func systemdFlags(enable *bool, unit, slice, path *string, app *kingpin.Application) {}
-
-func NewJournal(unit, slice, path string) (*Journal, error) {
-	return nil, nil
-}
-
-func (e *PostfixExporter) CollectLogfileFromJournal() error {
-	return nil
-}

postfix_exporter.go

@@ -27,7 +27,6 @@ import (
 	"strings"
 	"time"
 
-	"github.com/hpcloud/tail"
 	"github.com/prometheus/client_golang/prometheus"
 )
 
@@ -42,8 +41,7 @@ var (
 // Postfix Prometheus metrics exporter across scrapes.
 type PostfixExporter struct {
 	showqPath           string
-	journal             *Journal
-	tailer              *tail.Tail
+	logSrc              LogSource
 	logUnsupportedLines bool
 
 	// Metrics that should persist after refreshes, based on logs.
@@ -72,6 +70,16 @@ type PostfixExporter struct {
 	opendkimSignatureAdded          *prometheus.CounterVec
 }
 
+// A LogSource is an interface to read log lines.
+type LogSource interface {
+	// Path returns a representation of the log location.
+	Path() string
+
+	// Read returns the next log line. Returns `io.EOF` at the end of
+	// the log.
+	Read(context.Context) (string, error)
+}
+
 // CollectShowqFromReader parses the output of Postfix's 'showq' command
 // and turns it into metrics.
 //
@@ -420,50 +428,13 @@ func addToHistogramVec(h *prometheus.HistogramVec, value, fieldName string, labe
 	h.WithLabelValues(labels...).Observe(float)
 }
 
-// CollectLogfileFromFile tails a Postfix log file and collects entries from it.
-func (e *PostfixExporter) CollectLogfileFromFile(ctx context.Context) {
-	gaugeVec := prometheus.NewGaugeVec(
-		prometheus.GaugeOpts{
-			Namespace: "postfix",
-			Subsystem: "",
-			Name:      "up",
-			Help:      "Whether scraping Postfix's metrics was successful.",
-		},
-		[]string{"path"})
-	gauge := gaugeVec.WithLabelValues(e.tailer.Filename)
-	for {
-		select {
-		case line := <-e.tailer.Lines:
-			e.CollectFromLogLine(line.Text)
-		case <-ctx.Done():
-			gauge.Set(0)
-			return
-		}
-		gauge.Set(1)
-	}
-}
-
 // NewPostfixExporter creates a new Postfix exporter instance.
-func NewPostfixExporter(showqPath string, logfilePath string, journal *Journal, logUnsupportedLines bool) (*PostfixExporter, error) {
-	var tailer *tail.Tail
-	if logfilePath != "" {
-		var err error
-		tailer, err = tail.TailFile(logfilePath, tail.Config{
-			ReOpen:    true,                               // reopen the file if it's rotated
-			MustExist: true,                               // fail immediately if the file is missing or has incorrect permissions
-			Follow:    true,                               // run in follow mode
-			Location:  &tail.SeekInfo{Whence: io.SeekEnd}, // seek to end of file
-		})
-		if err != nil {
-			return nil, err
-		}
-	}
+func NewPostfixExporter(showqPath string, logSrc LogSource, logUnsupportedLines bool) (*PostfixExporter, error) {
 	timeBuckets := []float64{1e-3, 1e-2, 1e-1, 1.0, 10, 1 * 60, 1 * 60 * 60, 24 * 60 * 60, 2 * 24 * 60 * 60}
 	return &PostfixExporter{
 		logUnsupportedLines: logUnsupportedLines,
 		showqPath:           showqPath,
-		tailer:              tailer,
-		journal:             journal,
+		logSrc:              logSrc,
 
 		cleanupProcesses: prometheus.NewCounter(prometheus.CounterOpts{
 			Namespace: "postfix",
@@ -613,7 +584,7 @@ func NewPostfixExporter(showqPath string, logfilePath string, journal *Journal,
 func (e *PostfixExporter) Describe(ch chan<- *prometheus.Desc) {
 	ch <- postfixUpDesc
 
-	if e.tailer == nil && e.journal == nil {
+	if e.logSrc == nil {
 		return
 	}
 	ch <- e.cleanupProcesses.Desc()
@@ -641,38 +612,12 @@ func (e *PostfixExporter) Describe(ch chan<- *prometheus.Desc) {
 	e.opendkimSignatureAdded.Describe(ch)
 }
 
-func (e *PostfixExporter) foreverCollectFromJournal(ctx context.Context) {
-	gauge := prometheus.NewGaugeVec(
-		prometheus.GaugeOpts{
-			Namespace: "postfix",
-			Subsystem: "",
-			Name:      "up",
-			Help:      "Whether scraping Postfix's metrics was successful.",
-		},
-		[]string{"path"}).WithLabelValues(e.journal.Path)
-	select {
-	case <-ctx.Done():
-		gauge.Set(0)
-		return
-	default:
-		err := e.CollectLogfileFromJournal()
-		if err != nil {
-			log.Printf("Couldn't read journal: %v", err)
-			gauge.Set(0)
-		} else {
-			gauge.Set(1)
-		}
-	}
-}
-
 func (e *PostfixExporter) StartMetricCollection(ctx context.Context) {
-	if e.journal != nil {
-		e.foreverCollectFromJournal(ctx)
-	} else if e.tailer != nil {
-		e.CollectLogfileFromFile(ctx)
+	if e.logSrc == nil {
+		return
 	}
 
-	prometheus.NewGaugeVec(
+	gaugeVec := prometheus.NewGaugeVec(
 		prometheus.GaugeOpts{
 			Namespace: "postfix",
 			Subsystem: "",
@@ -680,7 +625,20 @@ func (e *PostfixExporter) StartMetricCollection(ctx context.Context) {
 			Help:      "Whether scraping Postfix's metrics was successful.",
 		},
 		[]string{"path"})
-	return
+	gauge := gaugeVec.WithLabelValues(e.logSrc.Path())
+	defer gauge.Set(0)
+
+	for {
+		line, err := e.logSrc.Read(ctx)
+		if err != nil {
+			if err != io.EOF {
+				log.Printf("Couldn't read journal: %v", err)
+			}
+			return
+		}
+		e.CollectFromLogLine(line)
+		gauge.Set(1)
+	}
 }
 
 // Collect metrics from Postfix's showq socket and its log file.
@@ -701,7 +659,7 @@ func (e *PostfixExporter) Collect(ch chan<- prometheus.Metric) {
 			e.showqPath)
 	}
 
-	if e.tailer == nil && e.journal == nil {
+	if e.logSrc == nil {
 		return
 	}
 	ch <- e.cleanupProcesses

postfix_exporter_test.go

@@ -1,18 +1,17 @@
 package main
 
 import (
-	"github.com/hpcloud/tail"
+	"testing"
+
 	"github.com/prometheus/client_golang/prometheus"
 	io_prometheus_client "github.com/prometheus/client_model/go"
 	"github.com/stretchr/testify/assert"
-	"testing"
 )
 
 func TestPostfixExporter_CollectFromLogline(t *testing.T) {
 	type fields struct {
 		showqPath                       string
-		journal                         *Journal
-		tailer                          *tail.Tail
+		logSrc                          LogSource
 		cleanupProcesses                prometheus.Counter
 		cleanupRejects                  prometheus.Counter
 		cleanupNotAccepted              prometheus.Counter
@@ -173,8 +172,7 @@ func TestPostfixExporter_CollectFromLogline(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			e := &PostfixExporter{
 				showqPath:                       tt.fields.showqPath,
-				journal:                         tt.fields.journal,
-				tailer:                          tt.fields.tailer,
+				logSrc:                          tt.fields.logSrc,
 				cleanupProcesses:                tt.fields.cleanupProcesses,
 				cleanupRejects:                  tt.fields.cleanupRejects,
 				cleanupNotAccepted:              tt.fields.cleanupNotAccepted,

systemd.go

@@ -1,119 +0,0 @@
-// +build !nosystemd,linux
-
-package main
-
-import (
-	"fmt"
-	"log"
-	"sync"
-	"time"
-
-	"github.com/alecthomas/kingpin"
-	"github.com/coreos/go-systemd/v22/sdjournal"
-)
-
-// Journal represents a lockable systemd journal.
-type Journal struct {
-	*sdjournal.Journal
-	sync.Mutex
-	Path string
-}
-
-// NewJournal returns a Journal for reading journal entries.
-func NewJournal(unit, slice, path string) (j *Journal, err error) {
-	j = new(Journal)
-	if path != "" {
-		j.Journal, err = sdjournal.NewJournalFromDir(path)
-		j.Path = path
-	} else {
-		j.Journal, err = sdjournal.NewJournal()
-		j.Path = "journald"
-	}
-	if err != nil {
-		return
-	}
-
-	if slice != "" {
-		err = j.AddMatch("_SYSTEMD_SLICE=" + slice)
-		if err != nil {
-			return
-		}
-	} else if unit != "" {
-		err = j.AddMatch("_SYSTEMD_UNIT=" + unit)
-		if err != nil {
-			return
-		}
-	}
-
-	// Start at end of journal
-	err = j.SeekRealtimeUsec(uint64(time.Now().UnixNano() / 1000))
-	if err != nil {
-		log.Printf("%v", err)
-	}
-	return
-}
-
-// NextMessage reads the next message from the journal.
-func (j *Journal) NextMessage() (s string, c uint64, err error) {
-	var e *sdjournal.JournalEntry
-
-	// Read to next
-	c, err = j.Next()
-	if err != nil {
-		return
-	}
-	// Return when on the end of journal
-	if c == 0 {
-		return
-	}
-
-	// Get entry
-	e, err = j.GetEntry()
-	if err != nil {
-		return
-	}
-	ts := time.Unix(0, int64(e.RealtimeTimestamp)*int64(time.Microsecond))
-
-	// Format entry
-	s = fmt.Sprintf(
-		"%s %s %s[%s]: %s",
-		ts.Format(time.Stamp),
-		e.Fields["_HOSTNAME"],
-		e.Fields["SYSLOG_IDENTIFIER"],
-		e.Fields["_PID"],
-		e.Fields["MESSAGE"],
-	)
-
-	return
-}
-
-// systemdFlags sets the flags for use with systemd
-func systemdFlags(enable *bool, unit, slice, path *string, app *kingpin.Application) {
-	app.Flag("systemd.enable", "Read from the systemd journal instead of log").Default("false").BoolVar(enable)
-	app.Flag("systemd.unit", "Name of the Postfix systemd unit.").Default("postfix.service").StringVar(unit)
-	app.Flag("systemd.slice", "Name of the Postfix systemd slice. Overrides the systemd unit.").Default("").StringVar(slice)
-	app.Flag("systemd.journal_path", "Path to the systemd journal").Default("").StringVar(path)
-}
-
-// CollectLogfileFromJournal Collects entries from the systemd journal.
-func (e *PostfixExporter) CollectLogfileFromJournal() error {
-	e.journal.Lock()
-	defer e.journal.Unlock()
-
-	r := e.journal.Wait(time.Duration(1) * time.Second)
-	if r < 0 {
-		log.Print("error while waiting for journal!")
-	}
-	for {
-		m, c, err := e.journal.NextMessage()
-		if err != nil {
-			return err
-		}
-		if c == 0 {
-			break
-		}
-		e.CollectFromLogLine(m)
-	}
-
-	return nil
-}