Adds a Docker log source.

When Postfix is running in a Docker container, it's most useful to use
the built-in Docker logging (as with Systemd). Setting
`--docker.enable` allows that.

The log source is using `client.NewEnvClient`, which reads environment
variables to determine which Docker to connect to, and how TLS is
handled.

README.md

@@ -5,7 +5,7 @@ This exporter provides histogram metrics for the size and age of messages stored
 the mail queue. It extracts these metrics from Postfix by connecting to
 a UNIX socket under `/var/spool`. It also counts events by parsing Postfix's
 log entries, using regular expression matching. The log entries are retrieved from
-the systemd journal or from a log file.
+the systemd journal, the Docker logs, or from a log file.
 
 ## Options
 
@@ -18,11 +18,25 @@ These options can be used when starting the `postfix_exporter`
 | `--postfix.showq_path`   | Path at which Postfix places its showq socket        | `/var/spool/postfix/public/showq` |
 | `--postfix.logfile_path` | Path where Postfix writes log entries                | `/var/log/maillog`                |
 | `--log.unsupported`      | Log all unsupported lines                            | `false`                           |
-| `--systemd.enable`       | Read from the systemd journal instead of log         | `false`                           |
+| `--docker.enable`        | Read from the Docker logs instead of a file          | `false`                           |
+| `--docker.container.id`  | The container to read Docker logs from               | `postfix`                         |
+| `--systemd.enable`       | Read from the systemd journal instead of file        | `false`                           |
 | `--systemd.unit`         | Name of the Postfix systemd unit                     | `postfix.service`                 |
 | `--systemd.slice`        | Name of the Postfix systemd slice.                   | `""`                              |
 | `--systemd.journal_path` | Path to the systemd journal                          | `""`                              |
 
+## Events from Docker
+
+Postfix servers running in a [Docker](https://www.docker.com/)
+container can be monitored using the `--docker.enable` flag. The
+default container ID is `postfix`, but can be customized with the
+`--docker.container.id` flag.
+
+The default is to connect to the local Docker, but this can be
+customized using [the `DOCKER_HOST` and
+similar](https://pkg.go.dev/github.com/docker/docker/client?tab=doc#NewEnvClient)
+environment variables.
+
 ## Events from log file
 
 The log file is tailed when processed. Rotating the log files while the exporter

go.mod

@@ -5,8 +5,15 @@ go 1.13
 require (
 	github.com/alecthomas/kingpin v2.2.6+incompatible
 	github.com/coreos/go-systemd/v22 v22.0.0
+	github.com/docker/distribution v2.7.1+incompatible // indirect
+	github.com/docker/docker v1.13.1
+	github.com/docker/go-connections v0.4.0 // indirect
+	github.com/docker/go-units v0.4.0 // indirect
 	github.com/fsnotify/fsnotify v1.4.7 // indirect
 	github.com/hpcloud/tail v1.0.0
+	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
+	github.com/modern-go/reflect2 v1.0.1 // indirect
+	github.com/opencontainers/go-digest v1.0.0 // indirect
 	github.com/prometheus/client_golang v1.4.1
 	github.com/prometheus/client_model v0.2.0
 	github.com/stretchr/testify v1.4.0

go.sum

@@ -17,6 +17,14 @@ github.com/coreos/go-systemd/v22 v22.0.0/go.mod h1:xO0FLkIi5MaZafQlIrOotqXZ90ih+
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/docker/distribution v2.7.1+incompatible h1:a5mlkVzth6W5A4fOsS3D2EO5BUmsJpcB+cRlLU7cSug=
+github.com/docker/distribution v2.7.1+incompatible/go.mod h1:J2gT2udsDAN96Uj4KfcMRqY0/ypR+oyYUYmja8H+y+w=
+github.com/docker/docker v1.13.1 h1:IkZjBSIc8hBjLpqeAbeE5mca5mNgeatLHBy3GO78BWo=
+github.com/docker/docker v1.13.1/go.mod h1:eEKB0N0r5NX/I1kEveEz05bcu8tLC/8azJZsviup8Sk=
+github.com/docker/go-connections v0.4.0 h1:El9xVISelRB7BuFusrZozjnkIM5YnzCViNKohAFqRJQ=
+github.com/docker/go-connections v0.4.0/go.mod h1:Gbd7IOopHjR8Iph03tsViu4nIes5XhDvyHbTtUxmeec=
+github.com/docker/go-units v0.4.0 h1:3uh0PgVws3nIA0Q+MwDC8yjEPf9zjRfZZWXZYDct3Tw=
+github.com/docker/go-units v0.4.0/go.mod h1:fgPhTUdO+D/Jk86RDLlptpiXQzgHJF7gydDDbaIK4Dk=
 github.com/fsnotify/fsnotify v1.4.7 h1:IXs+QLmnXW2CcXuY+8Mzv/fWEsPGWxqefPtCP5CnV9I=
 github.com/fsnotify/fsnotify v1.4.7/go.mod h1:jwhsz4b93w/PPRr/qN1Yymfu8t87LnFCMoQvtojpjFo=
 github.com/go-kit/kit v0.8.0/go.mod h1:xBxKIO96dXMWWy0MnWVtmwkA9/13aqxPnvrjFYMA2as=
@@ -53,7 +61,10 @@ github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd/go.mod h1:6dJ
 github.com/modern-go/reflect2 v0.0.0-20180701023420-4b7aa43c6742/go.mod h1:bx2lNnkwVCuqBIxFjflWJWanXIb3RllmbCylyMrvgv0=
 github.com/modern-go/reflect2 v1.0.1/go.mod h1:bx2lNnkwVCuqBIxFjflWJWanXIb3RllmbCylyMrvgv0=
 github.com/mwitkow/go-conntrack v0.0.0-20161129095857-cc309e4a2223/go.mod h1:qRWi+5nqEBWmkhHvq77mSJWrCKwh8bxhgT7d/eI7P4U=
+github.com/opencontainers/go-digest v1.0.0 h1:apOUWs51W5PlhuyGyz9FCeeBIOUDA/6nW8Oi/yOhh5U=
+github.com/opencontainers/go-digest v1.0.0/go.mod h1:0JzlMkj0TRzQZfJkVvzbP0HBR3IKzErnv2BNG4W4MAM=
 github.com/pkg/errors v0.8.0/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
+github.com/pkg/errors v0.8.1 h1:iURUrRGxPUNPdy5/HRSm+Yj6okJ6UtLINN0Q9M4+h3I=
 github.com/pkg/errors v0.8.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
@@ -83,6 +94,7 @@ github.com/stretchr/testify v1.4.0/go.mod h1:j7eGeouHqKxXV5pUuKE4zz7dFj8WfuZ+81P
 golang.org/x/crypto v0.0.0-20180904163835-0709b304e793/go.mod h1:6SG95UA2DQfeDnfUPMdvaQW0Q7yPrPDi9nlGo2tz2b4=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
 golang.org/x/net v0.0.0-20181114220301-adae6a3d119a/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
+golang.org/x/net v0.0.0-20190613194153-d28f0bde5980 h1:dfGZHvZk057jK2MCeWus/TowKpJ8y4AmooUzdBSR9GU=
 golang.org/x/net v0.0.0-20190613194153-d28f0bde5980/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
 golang.org/x/sync v0.0.0-20181108010431-42b317875d0f/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sync v0.0.0-20181221193216-37e7f081c4d4/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=

logsource.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"context"
 	"fmt"
 	"io"
 
@@ -17,7 +18,7 @@ type LogSourceFactory interface {
 	// 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)
+	New(context.Context) (LogSourceCloser, error)
 }
 
 type LogSourceCloser interface {
@@ -48,9 +49,9 @@ func InitLogSourceFactories(app *kingpin.Application) {
 // NewLogSourceFromFactories iterates through the factories and
 // attempts to instantiate a log source. The first factory to return
 // success wins.
-func NewLogSourceFromFactories() (LogSourceCloser, error) {
+func NewLogSourceFromFactories(ctx context.Context) (LogSourceCloser, error) {
 	for _, f := range logSourceFactories {
-		src, err := f.New()
+		src, err := f.New(ctx)
 		if err != nil {
 			return nil, err
 		}

logsource_docker.go

@@ -0,0 +1,96 @@
+// +build !nodocker
+
+package main
+
+import (
+	"bufio"
+	"context"
+	"io"
+	"log"
+	"strings"
+
+	"github.com/alecthomas/kingpin"
+	"github.com/docker/docker/api/types"
+	"github.com/docker/docker/client"
+)
+
+// A DockerLogSource reads log records from the given Docker
+// journal.
+type DockerLogSource struct {
+	client      DockerClient
+	containerID string
+	reader      *bufio.Reader
+}
+
+// A DockerClient is the client interface that client.Client
+// provides. See https://pkg.go.dev/github.com/docker/docker/client
+type DockerClient interface {
+	io.Closer
+	ContainerLogs(context.Context, string, types.ContainerLogsOptions) (io.ReadCloser, error)
+}
+
+// NewDockerLogSource returns a log source for reading Docker logs.
+func NewDockerLogSource(ctx context.Context, c DockerClient, containerID string) (*DockerLogSource, error) {
+	r, err := c.ContainerLogs(ctx, containerID, types.ContainerLogsOptions{
+		ShowStdout: true,
+		ShowStderr: true,
+		Follow:     true,
+		Tail:       "0",
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	logSrc := &DockerLogSource{
+		client:      c,
+		containerID: containerID,
+		reader:      bufio.NewReader(r),
+	}
+
+	return logSrc, nil
+}
+
+func (s *DockerLogSource) Close() error {
+	return s.client.Close()
+}
+
+func (s *DockerLogSource) Path() string {
+	return "docker:" + s.containerID
+}
+
+func (s *DockerLogSource) Read(ctx context.Context) (string, error) {
+	line, err := s.reader.ReadString('\n')
+	if err != nil {
+		return "", err
+	}
+	return strings.TrimSpace(line), nil
+}
+
+// A dockerLogSourceFactory is a factory that can create
+// DockerLogSources from command line flags.
+type dockerLogSourceFactory struct {
+	enable      bool
+	containerID string
+}
+
+func (f *dockerLogSourceFactory) Init(app *kingpin.Application) {
+	app.Flag("docker.enable", "Read from Docker logs. Environment variable DOCKER_HOST can be used to change the address. See https://pkg.go.dev/github.com/docker/docker/client?tab=doc#NewEnvClient for more information.").Default("false").BoolVar(&f.enable)
+	app.Flag("docker.container.id", "ID/name of the Postfix Docker container.").Default("postfix").StringVar(&f.containerID)
+}
+
+func (f *dockerLogSourceFactory) New(ctx context.Context) (LogSourceCloser, error) {
+	if !f.enable {
+		return nil, nil
+	}
+
+	log.Println("Reading log events from Docker")
+	c, err := client.NewEnvClient()
+	if err != nil {
+		return nil, err
+	}
+	return NewDockerLogSource(ctx, c, f.containerID)
+}
+
+func init() {
+	RegisterLogSourceFactory(&dockerLogSourceFactory{})
+}

logsource_docker_test.go

@@ -0,0 +1,79 @@
+// +build !nodocker
+
+package main
+
+import (
+	"context"
+	"io"
+	"io/ioutil"
+	"strings"
+	"testing"
+
+	"github.com/docker/docker/api/types"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNewDockerLogSource(t *testing.T) {
+	ctx := context.Background()
+	c := &fakeDockerClient{}
+	src, err := NewDockerLogSource(ctx, c, "acontainer")
+	if err != nil {
+		t.Fatalf("NewDockerLogSource failed: %v", err)
+	}
+
+	assert.Equal(t, []string{"acontainer"}, c.containerLogsCalls, "A call to ContainerLogs should be made.")
+
+	if err := src.Close(); err != nil {
+		t.Fatalf("Close failed: %v", err)
+	}
+
+	assert.Equal(t, 1, c.closeCalls, "A call to Close should be made.")
+}
+
+func TestDockerLogSource_Path(t *testing.T) {
+	ctx := context.Background()
+	c := &fakeDockerClient{}
+	src, err := NewDockerLogSource(ctx, c, "acontainer")
+	if err != nil {
+		t.Fatalf("NewDockerLogSource failed: %v", err)
+	}
+	defer src.Close()
+
+	assert.Equal(t, "docker:acontainer", src.Path(), "Path should be set by New.")
+}
+
+func TestDockerLogSource_Read(t *testing.T) {
+	ctx := context.Background()
+
+	c := &fakeDockerClient{
+		logsReader: ioutil.NopCloser(strings.NewReader("Feb 13 23:31:30 ahost anid[123]: aline\n")),
+	}
+	src, err := NewDockerLogSource(ctx, c, "acontainer")
+	if err != nil {
+		t.Fatalf("NewDockerLogSource 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.")
+}
+
+type fakeDockerClient struct {
+	logsReader io.ReadCloser
+
+	containerLogsCalls []string
+	closeCalls         int
+}
+
+func (c *fakeDockerClient) ContainerLogs(ctx context.Context, containerID string, opts types.ContainerLogsOptions) (io.ReadCloser, error) {
+	c.containerLogsCalls = append(c.containerLogsCalls, containerID)
+	return c.logsReader, nil
+}
+
+func (c *fakeDockerClient) Close() error {
+	c.closeCalls++
+	return nil
+}

logsource_file.go

@@ -69,7 +69,7 @@ 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) {
+func (f *fileLogSourceFactory) New(ctx context.Context) (LogSourceCloser, error) {
 	if f.path == "" {
 		return nil, nil
 	}

logsource_systemd.go

@@ -112,7 +112,7 @@ func (f *systemdLogSourceFactory) Init(app *kingpin.Application) {
 	app.Flag("systemd.journal_path", "Path to the systemd journal").Default("").StringVar(&f.path)
 }
 
-func (f *systemdLogSourceFactory) New() (LogSourceCloser, error) {
+func (f *systemdLogSourceFactory) New(ctx context.Context) (LogSourceCloser, error) {
 	if !f.enable {
 		return nil, nil
 	}

main.go

@@ -13,6 +13,7 @@ import (
 
 func main() {
 	var (
+		ctx                 = context.Background()
 		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()
@@ -23,7 +24,7 @@ func main() {
 	InitLogSourceFactories(app)
 	kingpin.MustParse(app.Parse(os.Args[1:]))
 
-	logSrc, err := NewLogSourceFromFactories()
+	logSrc, err := NewLogSourceFromFactories(ctx)
 	if err != nil {
 		log.Fatalf("Error opening log source: %s", err)
 	}
@@ -53,7 +54,7 @@ func main() {
 			panic(err)
 		}
 	})
-	ctx, cancelFunc := context.WithCancel(context.Background())
+	ctx, cancelFunc := context.WithCancel(ctx)
 	defer cancelFunc()
 	go exporter.StartMetricCollection(ctx)
 	log.Print("Listening on ", *listenAddress)