From d5b5ab6aa389a2530f5fbd3c03798cda61419d98 Mon Sep 17 00:00:00 2001
From: Cyril Tovena <cyril.tovena@gmail.com>
Date: Thu, 6 Feb 2020 16:17:44 -0500
Subject: [PATCH 1/9] --dry-run Promtail.

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>
---
 cmd/promtail/main.go               |  3 +-
 go.mod                             |  1 +
 go.sum                             |  2 ++
 pkg/promtail/client/logger.go      | 56 ++++++++++++++++++++++++++++++
 pkg/promtail/client/logger_test.go | 21 +++++++++++
 pkg/promtail/promtail.go           | 20 +++++++----
 pkg/promtail/promtail_test.go      | 26 +++++++++++++-
 7 files changed, 121 insertions(+), 8 deletions(-)
 create mode 100644 pkg/promtail/client/logger.go
 create mode 100644 pkg/promtail/client/logger_test.go

diff --git a/cmd/promtail/main.go b/cmd/promtail/main.go
index 36264575f2e15..2fffe3c0ad9d9 100644
--- a/cmd/promtail/main.go
+++ b/cmd/promtail/main.go
@@ -27,6 +27,7 @@ func init() {
 
 func main() {
 	printVersion := flag.Bool("version", false, "Print this builds version information")
+	dryRun := flag.Bool("dry-run", false, "Start Promtail but print entries instead of sending them to Loki.")
 
 	// Load config, merging config file and CLI flags
 	var config config.Config
@@ -57,7 +58,7 @@ func main() {
 		stages.Debug = true
 	}
 
-	p, err := promtail.New(config)
+	p, err := promtail.New(config, *dryRun)
 	if err != nil {
 		level.Error(util.Logger).Log("msg", "error creating promtail", "error", err)
 		os.Exit(1)
diff --git a/go.mod b/go.mod
index df9ab2d6c86e5..a865b1695a259 100644
--- a/go.mod
+++ b/go.mod
@@ -24,6 +24,7 @@ require (
 	github.com/golang/snappy v0.0.1
 	github.com/gorilla/mux v1.7.1
 	github.com/gorilla/websocket v1.4.0
+	github.com/grafana/tail v0.0.0-20200127140945-4647d4b312f2
 	github.com/grpc-ecosystem/go-grpc-prometheus v1.2.1-0.20191002090509-6af20e3a5340 // indirect
 	github.com/grpc-ecosystem/grpc-opentracing v0.0.0-20180507213350-8e809c8a8645
 	github.com/hashicorp/golang-lru v0.5.3
diff --git a/go.sum b/go.sum
index 69cb52152aec2..fa5018d7d1013 100644
--- a/go.sum
+++ b/go.sum
@@ -395,6 +395,8 @@ github.com/gorilla/websocket v1.4.0 h1:WDFjx/TMzVgy9VdMMQi2K2Emtwi2QcUQsztZ/zLaH
 github.com/gorilla/websocket v1.4.0/go.mod h1:E7qHFY5m1UJ88s3WnNqhKjPHQ0heANvMoAMk2YaljkQ=
 github.com/grafana/tail v0.0.0-20191024143944-0b54ddf21fe7 h1:eeBhshivxpgHEX78QxJkoL251Pjr0B2GL59ZsivnplU=
 github.com/grafana/tail v0.0.0-20191024143944-0b54ddf21fe7/go.mod h1:aS6CMYGLEIABOzX3OL8SqZ3zAZCGN7nmBnqgnyJGxyA=
+github.com/grafana/tail v0.0.0-20200127140945-4647d4b312f2 h1:rleaNROLYWaF/fN7hgk/j+YzD7hPKfJjtwVf3JtgKrA=
+github.com/grafana/tail v0.0.0-20200127140945-4647d4b312f2/go.mod h1:aS6CMYGLEIABOzX3OL8SqZ3zAZCGN7nmBnqgnyJGxyA=
 github.com/gregjones/httpcache v0.0.0-20170728041850-787624de3eb7/go.mod h1:FecbI9+v66THATjSRHfNgh1IVFe/9kFxbXtjV0ctIMA=
 github.com/grpc-ecosystem/go-grpc-middleware v1.0.1-0.20190118093823-f849b5445de4 h1:z53tR0945TRRQO/fLEVPI6SMv7ZflF0TEaTAoU7tOzg=
 github.com/grpc-ecosystem/go-grpc-middleware v1.0.1-0.20190118093823-f849b5445de4/go.mod h1:FiyG127CGDf3tlThmgyCl78X/SZQqEOJBCDaAfeWzPs=
diff --git a/pkg/promtail/client/logger.go b/pkg/promtail/client/logger.go
new file mode 100644
index 0000000000000..600435c803a9d
--- /dev/null
+++ b/pkg/promtail/client/logger.go
@@ -0,0 +1,56 @@
+package client
+
+import (
+	"fmt"
+	"os"
+	"sync"
+	"text/tabwriter"
+	"time"
+
+	"github.com/cortexproject/cortex/pkg/util"
+	"github.com/fatih/color"
+	"github.com/prometheus/common/model"
+	"gopkg.in/yaml.v2"
+)
+
+type logger struct {
+	*tabwriter.Writer
+	sync.Mutex
+}
+
+// NewLogger creates a new client logger that logs entries instead of sending them.
+func NewLogger(cfgs ...Config) (Client, error) {
+	// make sure the clients config is valid
+	c, err := NewMulti(util.Logger, cfgs...)
+	if err != nil {
+		return nil, err
+	}
+	c.Stop()
+	fmt.Println(color.YellowString("Clients configured:"))
+	for _, cfg := range cfgs {
+		yaml, err := yaml.Marshal(cfg)
+		if err != nil {
+			return nil, err
+		}
+		fmt.Println("----------------------")
+		fmt.Println(string(yaml))
+	}
+	return &logger{
+		Writer: tabwriter.NewWriter(os.Stdout, 0, 4, 0, '\t', 0),
+	}, nil
+}
+
+func (*logger) Stop() {}
+
+func (l *logger) Handle(labels model.LabelSet, time time.Time, entry string) error {
+	l.Lock()
+	defer l.Unlock()
+	fmt.Fprint(l.Writer, color.BlueString(time.Format("2006-01-02T15:04:05")))
+	fmt.Fprint(l.Writer, "\t")
+	fmt.Fprint(l.Writer, color.YellowString(labels.String()))
+	fmt.Fprint(l.Writer, "\t")
+	fmt.Fprint(l.Writer, entry)
+	fmt.Fprint(l.Writer, "\n")
+	l.Flush()
+	return nil
+}
diff --git a/pkg/promtail/client/logger_test.go b/pkg/promtail/client/logger_test.go
new file mode 100644
index 0000000000000..3ff0e948e66aa
--- /dev/null
+++ b/pkg/promtail/client/logger_test.go
@@ -0,0 +1,21 @@
+package client
+
+import (
+	"net/url"
+	"testing"
+	"time"
+
+	"github.com/cortexproject/cortex/pkg/util/flagext"
+	"github.com/prometheus/common/model"
+	"github.com/stretchr/testify/require"
+)
+
+func TestNewLogger(t *testing.T) {
+	_, err := NewLogger([]Config{}...)
+	require.Error(t, err)
+
+	l, err := NewLogger([]Config{{URL: flagext.URLValue{URL: &url.URL{Host: "string"}}}}...)
+	require.NoError(t, err)
+	err = l.Handle(model.LabelSet{"foo": "bar"}, time.Now(), "entry")
+	require.NoError(t, err)
+}
diff --git a/pkg/promtail/promtail.go b/pkg/promtail/promtail.go
index 3fae7132c7083..f256a6233c8c7 100644
--- a/pkg/promtail/promtail.go
+++ b/pkg/promtail/promtail.go
@@ -19,7 +19,7 @@ type Promtail struct {
 }
 
 // New makes a new Promtail.
-func New(cfg config.Config) (*Promtail, error) {
+func New(cfg config.Config, dryRun bool) (*Promtail, error) {
 	positions, err := positions.New(util.Logger, cfg.PositionsConfig)
 	if err != nil {
 		return nil, err
@@ -30,12 +30,20 @@ func New(cfg config.Config) (*Promtail, error) {
 		cfg.ClientConfigs = append(cfg.ClientConfigs, cfg.ClientConfig)
 	}
 
-	client, err := client.NewMulti(util.Logger, cfg.ClientConfigs...)
-	if err != nil {
-		return nil, err
+	var cl client.Client
+	if dryRun {
+		cl, err = client.NewLogger(cfg.ClientConfigs...)
+		if err != nil {
+			return nil, err
+		}
+	} else {
+		cl, err = client.NewMulti(util.Logger, cfg.ClientConfigs...)
+		if err != nil {
+			return nil, err
+		}
 	}
 
-	tms, err := targets.NewTargetManagers(util.Logger, positions, client, cfg.ScrapeConfig, &cfg.TargetConfig)
+	tms, err := targets.NewTargetManagers(util.Logger, positions, cl, cfg.ScrapeConfig, &cfg.TargetConfig)
 	if err != nil {
 		return nil, err
 	}
@@ -46,7 +54,7 @@ func New(cfg config.Config) (*Promtail, error) {
 	}
 
 	return &Promtail{
-		client:         client,
+		client:         cl,
 		positions:      positions,
 		targetManagers: tms,
 		server:         server,
diff --git a/pkg/promtail/promtail_test.go b/pkg/promtail/promtail_test.go
index c117af245a171..8e8a84f424cd8 100644
--- a/pkg/promtail/promtail_test.go
+++ b/pkg/promtail/promtail_test.go
@@ -7,6 +7,7 @@ import (
 	"math"
 	"math/rand"
 	"net/http"
+	"net/url"
 	"os"
 	"path/filepath"
 	"reflect"
@@ -26,11 +27,14 @@ import (
 	"github.com/prometheus/prometheus/pkg/textparse"
 	"github.com/prometheus/prometheus/promql"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 
 	"github.com/grafana/loki/pkg/logentry/stages"
 	"github.com/grafana/loki/pkg/logproto"
 	"github.com/grafana/loki/pkg/promtail/api"
+	"github.com/grafana/loki/pkg/promtail/client"
 	"github.com/grafana/loki/pkg/promtail/config"
+	"github.com/grafana/loki/pkg/promtail/positions"
 	"github.com/grafana/loki/pkg/promtail/scrape"
 	"github.com/grafana/loki/pkg/promtail/targets"
 )
@@ -84,7 +88,7 @@ func TestPromtail(t *testing.T) {
 
 	// Run.
 
-	p, err := New(buildTestConfig(t, positionsFileName, testDir))
+	p, err := New(buildTestConfig(t, positionsFileName, testDir), false)
 	if err != nil {
 		t.Error("error creating promtail", err)
 		return
@@ -628,3 +632,23 @@ func randName() string {
 	}
 	return string(b)
 }
+
+func Test_DryRun(t *testing.T) {
+
+	f, err := ioutil.TempFile("/tmp", "Test_DryRun")
+	require.NoError(t, err)
+	defer os.Remove(f.Name())
+
+	_, err = New(config.Config{}, true)
+	require.Error(t, err)
+
+	p, err := New(config.Config{
+		ClientConfig: client.Config{URL: flagext.URLValue{URL: &url.URL{Host: "string"}}},
+		PositionsConfig: positions.Config{
+			PositionsFile: f.Name(),
+			SyncPeriod:    time.Second,
+		},
+	}, false)
+	require.NoError(t, err)
+	require.IsType(t, client.MultiClient{}, p.client)
+}

From eea212f7b9008555def5ef35135aa22abec5b141 Mon Sep 17 00:00:00 2001
From: Cyril Tovena <cyril.tovena@gmail.com>
Date: Thu, 6 Feb 2020 16:20:33 -0500
Subject: [PATCH 2/9] Remove changed on gomod

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>
---
 go.mod | 1 -
 go.sum | 2 --
 2 files changed, 3 deletions(-)

diff --git a/go.mod b/go.mod
index a865b1695a259..df9ab2d6c86e5 100644
--- a/go.mod
+++ b/go.mod
@@ -24,7 +24,6 @@ require (
 	github.com/golang/snappy v0.0.1
 	github.com/gorilla/mux v1.7.1
 	github.com/gorilla/websocket v1.4.0
-	github.com/grafana/tail v0.0.0-20200127140945-4647d4b312f2
 	github.com/grpc-ecosystem/go-grpc-prometheus v1.2.1-0.20191002090509-6af20e3a5340 // indirect
 	github.com/grpc-ecosystem/grpc-opentracing v0.0.0-20180507213350-8e809c8a8645
 	github.com/hashicorp/golang-lru v0.5.3
diff --git a/go.sum b/go.sum
index fa5018d7d1013..69cb52152aec2 100644
--- a/go.sum
+++ b/go.sum
@@ -395,8 +395,6 @@ github.com/gorilla/websocket v1.4.0 h1:WDFjx/TMzVgy9VdMMQi2K2Emtwi2QcUQsztZ/zLaH
 github.com/gorilla/websocket v1.4.0/go.mod h1:E7qHFY5m1UJ88s3WnNqhKjPHQ0heANvMoAMk2YaljkQ=
 github.com/grafana/tail v0.0.0-20191024143944-0b54ddf21fe7 h1:eeBhshivxpgHEX78QxJkoL251Pjr0B2GL59ZsivnplU=
 github.com/grafana/tail v0.0.0-20191024143944-0b54ddf21fe7/go.mod h1:aS6CMYGLEIABOzX3OL8SqZ3zAZCGN7nmBnqgnyJGxyA=
-github.com/grafana/tail v0.0.0-20200127140945-4647d4b312f2 h1:rleaNROLYWaF/fN7hgk/j+YzD7hPKfJjtwVf3JtgKrA=
-github.com/grafana/tail v0.0.0-20200127140945-4647d4b312f2/go.mod h1:aS6CMYGLEIABOzX3OL8SqZ3zAZCGN7nmBnqgnyJGxyA=
 github.com/gregjones/httpcache v0.0.0-20170728041850-787624de3eb7/go.mod h1:FecbI9+v66THATjSRHfNgh1IVFe/9kFxbXtjV0ctIMA=
 github.com/grpc-ecosystem/go-grpc-middleware v1.0.1-0.20190118093823-f849b5445de4 h1:z53tR0945TRRQO/fLEVPI6SMv7ZflF0TEaTAoU7tOzg=
 github.com/grpc-ecosystem/go-grpc-middleware v1.0.1-0.20190118093823-f849b5445de4/go.mod h1:FiyG127CGDf3tlThmgyCl78X/SZQqEOJBCDaAfeWzPs=

From fc8248bb5de30eefe308d71f7f1ae47eab4023b8 Mon Sep 17 00:00:00 2001
From: Cyril Tovena <cyril.tovena@gmail.com>
Date: Thu, 6 Feb 2020 17:07:38 -0500
Subject: [PATCH 3/9] Fixes flaky test

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>
---
 pkg/promtail/promtail_test.go | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/pkg/promtail/promtail_test.go b/pkg/promtail/promtail_test.go
index 8e8a84f424cd8..c34477cc7e202 100644
--- a/pkg/promtail/promtail_test.go
+++ b/pkg/promtail/promtail_test.go
@@ -20,6 +20,7 @@ import (
 	"github.com/go-kit/kit/log"
 	"github.com/go-kit/kit/log/level"
 	"github.com/pkg/errors"
+	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/common/model"
 	sd_config "github.com/prometheus/prometheus/discovery/config"
 	"github.com/prometheus/prometheus/discovery/targetgroup"
@@ -642,6 +643,8 @@ func Test_DryRun(t *testing.T) {
 	_, err = New(config.Config{}, true)
 	require.Error(t, err)
 
+	prometheus.DefaultRegisterer = prometheus.NewRegistry() // reset registry, otherwise you can't create 2 weavework server.
+
 	p, err := New(config.Config{
 		ClientConfig: client.Config{URL: flagext.URLValue{URL: &url.URL{Host: "string"}}},
 		PositionsConfig: positions.Config{

From 783efecd67d2a3602f62079246fb370890687750 Mon Sep 17 00:00:00 2001
From: Cyril Tovena <cyril.tovena@gmail.com>
Date: Tue, 11 Feb 2020 13:50:42 -0500
Subject: [PATCH 4/9] Ensure position file is readonly when doing dry-run

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>
---
 pkg/promtail/positions/positions.go          | 72 ++++++++++++--------
 pkg/promtail/positions/positions_test.go     | 44 ++++++++++++
 pkg/promtail/promtail.go                     |  1 +
 pkg/promtail/promtail_test.go                | 10 +++
 pkg/promtail/targets/filetarget.go           |  4 +-
 pkg/promtail/targets/filetargetmanager.go    |  4 +-
 pkg/promtail/targets/journaltargetmanager.go |  2 +-
 pkg/promtail/targets/manager.go              |  2 +-
 pkg/promtail/targets/tailer.go               |  4 +-
 9 files changed, 106 insertions(+), 37 deletions(-)

diff --git a/pkg/promtail/positions/positions.go b/pkg/promtail/positions/positions.go
index 05d777ee23f1e..5f8eac2d59331 100644
--- a/pkg/promtail/positions/positions.go
+++ b/pkg/promtail/positions/positions.go
@@ -23,6 +23,7 @@ type Config struct {
 	SyncPeriod        time.Duration `yaml:"sync_period"`
 	PositionsFile     string        `yaml:"filename"`
 	IgnoreInvalidYaml bool          `yaml:"ignore_invalid_yaml"`
+	ReadOnly          bool          `yaml:"-"`
 }
 
 // RegisterFlags register flags.
@@ -33,7 +34,7 @@ func (cfg *Config) RegisterFlags(flags *flag.FlagSet) {
 }
 
 // Positions tracks how far through each file we've read.
-type Positions struct {
+type positions struct {
 	logger    log.Logger
 	cfg       Config
 	mtx       sync.Mutex
@@ -47,17 +48,40 @@ type File struct {
 	Positions map[string]string `yaml:"positions"`
 }
 
+type Positions interface {
+	// GetString returns how far we've through a file as a string.
+	// JournalTarget writes a journal cursor to the positions file, while
+	// FileTarget writes an integer offset. Use Get to read the integer
+	// offset.
+	GetString(path string) string
+	// Get returns how far we've read through a file. Returns an error
+	// if the value stored for the file is not an integer.
+	Get(path string) (int64, error)
+	// PutString records (asynchronsouly) how far we've read through a file.
+	// Unlike Put, it records a string offset and is only useful for
+	// JournalTargets which doesn't have integer offsets.
+	PutString(path string, pos string)
+	// Put records (asynchronously) how far we've read through a file.
+	Put(path string, pos int64)
+	// Remove removes the position tracking for a filepath
+	Remove(path string)
+	// SyncPeriod returns how often the positions file gets resynced
+	SyncPeriod() time.Duration
+	// Stop the Position tracker.
+	Stop()
+}
+
 // New makes a new Positions.
-func New(logger log.Logger, cfg Config) (*Positions, error) {
-	positions, err := readPositionsFile(cfg, logger)
+func New(logger log.Logger, cfg Config) (*positions, error) {
+	positionData, err := readPositionsFile(cfg, logger)
 	if err != nil {
 		return nil, err
 	}
 
-	p := &Positions{
+	p := &positions{
 		logger:    logger,
 		cfg:       cfg,
-		positions: positions,
+		positions: positionData,
 		quit:      make(chan struct{}),
 		done:      make(chan struct{}),
 	}
@@ -66,39 +90,28 @@ func New(logger log.Logger, cfg Config) (*Positions, error) {
 	return p, nil
 }
 
-// Stop the Position tracker.
-func (p *Positions) Stop() {
+func (p *positions) Stop() {
 	close(p.quit)
 	<-p.done
 }
 
-// PutString records (asynchronsouly) how far we've read through a file.
-// Unlike Put, it records a string offset and is only useful for
-// JournalTargets which doesn't have integer offsets.
-func (p *Positions) PutString(path string, pos string) {
+func (p *positions) PutString(path string, pos string) {
 	p.mtx.Lock()
 	defer p.mtx.Unlock()
 	p.positions[path] = pos
 }
 
-// Put records (asynchronously) how far we've read through a file.
-func (p *Positions) Put(path string, pos int64) {
+func (p *positions) Put(path string, pos int64) {
 	p.PutString(path, strconv.FormatInt(pos, 10))
 }
 
-// GetString returns how far we've through a file as a string.
-// JournalTarget writes a journal cursor to the positions file, while
-// FileTarget writes an integer offset. Use Get to read the integer
-// offset.
-func (p *Positions) GetString(path string) string {
+func (p *positions) GetString(path string) string {
 	p.mtx.Lock()
 	defer p.mtx.Unlock()
 	return p.positions[path]
 }
 
-// Get returns how far we've read through a file. Returns an error
-// if the value stored for the file is not an integer.
-func (p *Positions) Get(path string) (int64, error) {
+func (p *positions) Get(path string) (int64, error) {
 	p.mtx.Lock()
 	defer p.mtx.Unlock()
 	pos, ok := p.positions[path]
@@ -108,23 +121,21 @@ func (p *Positions) Get(path string) (int64, error) {
 	return strconv.ParseInt(pos, 10, 64)
 }
 
-// Remove removes the position tracking for a filepath
-func (p *Positions) Remove(path string) {
+func (p *positions) Remove(path string) {
 	p.mtx.Lock()
 	defer p.mtx.Unlock()
 	p.remove(path)
 }
 
-func (p *Positions) remove(path string) {
+func (p *positions) remove(path string) {
 	delete(p.positions, path)
 }
 
-// SyncPeriod returns how often the positions file gets resynced
-func (p *Positions) SyncPeriod() time.Duration {
+func (p *positions) SyncPeriod() time.Duration {
 	return p.cfg.SyncPeriod
 }
 
-func (p *Positions) run() {
+func (p *positions) run() {
 	defer func() {
 		p.save()
 		level.Debug(p.logger).Log("msg", "positions saved")
@@ -143,7 +154,10 @@ func (p *Positions) run() {
 	}
 }
 
-func (p *Positions) save() {
+func (p *positions) save() {
+	if p.cfg.ReadOnly {
+		return
+	}
 	p.mtx.Lock()
 	positions := make(map[string]string, len(p.positions))
 	for k, v := range p.positions {
@@ -156,7 +170,7 @@ func (p *Positions) save() {
 	}
 }
 
-func (p *Positions) cleanup() {
+func (p *positions) cleanup() {
 	p.mtx.Lock()
 	defer p.mtx.Unlock()
 	toRemove := []string{}
diff --git a/pkg/promtail/positions/positions_test.go b/pkg/promtail/positions/positions_test.go
index c391aa2348af2..294ba84258380 100644
--- a/pkg/promtail/positions/positions_test.go
+++ b/pkg/promtail/positions/positions_test.go
@@ -5,7 +5,9 @@ import (
 	"os"
 	"strings"
 	"testing"
+	"time"
 
+	"github.com/cortexproject/cortex/pkg/util"
 	"github.com/go-kit/kit/log"
 	"github.com/stretchr/testify/require"
 )
@@ -136,3 +138,45 @@ func TestReadPositionsFromBadYamlIgnoreCorruption(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, map[string]string{}, out)
 }
+
+func Test_ReadOnly(t *testing.T) {
+	temp := tempFilename(t)
+	defer func() {
+		_ = os.Remove(temp)
+	}()
+	yaml := []byte(`positions:
+  /tmp/random.log: "17623"
+`)
+	err := ioutil.WriteFile(temp, yaml, 0644)
+	if err != nil {
+		t.Fatal(err)
+	}
+	p, err := New(util.Logger, Config{
+		SyncPeriod:    20 * time.Nanosecond,
+		PositionsFile: temp,
+		ReadOnly:      true,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer p.Stop()
+	p.Put("/foo/bar/f", 12132132)
+	p.PutString("/foo/f", "100")
+	pos, err := p.Get("/tmp/random.log")
+	if err != nil {
+		t.Fatal(err)
+	}
+	require.Equal(t, int64(17623), pos)
+	p.save()
+	out, err := readPositionsFile(Config{
+		PositionsFile:     temp,
+		IgnoreInvalidYaml: true,
+		ReadOnly:          true,
+	}, log.NewNopLogger())
+
+	require.NoError(t, err)
+	require.Equal(t, map[string]string{
+		"/tmp/random.log": "17623",
+	}, out)
+
+}
diff --git a/pkg/promtail/promtail.go b/pkg/promtail/promtail.go
index e5767dec825a3..50dd4ccefe150 100644
--- a/pkg/promtail/promtail.go
+++ b/pkg/promtail/promtail.go
@@ -36,6 +36,7 @@ func New(cfg config.Config, dryRun bool) (*Promtail, error) {
 		if err != nil {
 			return nil, err
 		}
+		cfg.PositionsConfig.ReadOnly = true
 	} else {
 		cl, err = client.NewMulti(util.Logger, cfg.ClientConfigs...)
 		if err != nil {
diff --git a/pkg/promtail/promtail_test.go b/pkg/promtail/promtail_test.go
index c34477cc7e202..f4c76548adb3a 100644
--- a/pkg/promtail/promtail_test.go
+++ b/pkg/promtail/promtail_test.go
@@ -644,6 +644,16 @@ func Test_DryRun(t *testing.T) {
 	require.Error(t, err)
 
 	prometheus.DefaultRegisterer = prometheus.NewRegistry() // reset registry, otherwise you can't create 2 weavework server.
+	_, err = New(config.Config{
+		ClientConfig: client.Config{URL: flagext.URLValue{URL: &url.URL{Host: "string"}}},
+		PositionsConfig: positions.Config{
+			PositionsFile: f.Name(),
+			SyncPeriod:    time.Second,
+		},
+	}, true)
+	require.NoError(t, err)
+
+	prometheus.DefaultRegisterer = prometheus.NewRegistry()
 
 	p, err := New(config.Config{
 		ClientConfig: client.Config{URL: flagext.URLValue{URL: &url.URL{Host: "string"}}},
diff --git a/pkg/promtail/targets/filetarget.go b/pkg/promtail/targets/filetarget.go
index 2a70dd1e36161..4bb02b494c960 100644
--- a/pkg/promtail/targets/filetarget.go
+++ b/pkg/promtail/targets/filetarget.go
@@ -68,7 +68,7 @@ type FileTarget struct {
 	logger log.Logger
 
 	handler          api.EntryHandler
-	positions        *positions.Positions
+	positions        positions.Positions
 	labels           model.LabelSet
 	discoveredLabels model.LabelSet
 
@@ -84,7 +84,7 @@ type FileTarget struct {
 }
 
 // NewFileTarget create a new FileTarget.
-func NewFileTarget(logger log.Logger, handler api.EntryHandler, positions *positions.Positions, path string, labels model.LabelSet, discoveredLabels model.LabelSet, targetConfig *Config) (*FileTarget, error) {
+func NewFileTarget(logger log.Logger, handler api.EntryHandler, positions positions.Positions, path string, labels model.LabelSet, discoveredLabels model.LabelSet, targetConfig *Config) (*FileTarget, error) {
 
 	watcher, err := fsnotify.NewWatcher()
 	if err != nil {
diff --git a/pkg/promtail/targets/filetargetmanager.go b/pkg/promtail/targets/filetargetmanager.go
index a096c7192c948..4ce42593913b4 100644
--- a/pkg/promtail/targets/filetargetmanager.go
+++ b/pkg/promtail/targets/filetargetmanager.go
@@ -54,7 +54,7 @@ type FileTargetManager struct {
 // NewFileTargetManager creates a new TargetManager.
 func NewFileTargetManager(
 	logger log.Logger,
-	positions *positions.Positions,
+	positions positions.Positions,
 	client api.EntryHandler,
 	scrapeConfigs []scrape.Config,
 	targetConfig *Config,
@@ -178,7 +178,7 @@ func (tm *FileTargetManager) AllTargets() map[string][]Target {
 // targetSyncer sync targets based on service discovery changes.
 type targetSyncer struct {
 	log          log.Logger
-	positions    *positions.Positions
+	positions    positions.Positions
 	entryHandler api.EntryHandler
 	hostname     string
 
diff --git a/pkg/promtail/targets/journaltargetmanager.go b/pkg/promtail/targets/journaltargetmanager.go
index 46034e7de2f93..b6f3641ce8cf3 100644
--- a/pkg/promtail/targets/journaltargetmanager.go
+++ b/pkg/promtail/targets/journaltargetmanager.go
@@ -17,7 +17,7 @@ type JournalTargetManager struct{}
 // on this platform.
 func NewJournalTargetManager(
 	logger log.Logger,
-	positions *positions.Positions,
+	positions positions.Positions,
 	client api.EntryHandler,
 	scrapeConfigs []scrape.Config,
 ) (*JournalTargetManager, error) {
diff --git a/pkg/promtail/targets/manager.go b/pkg/promtail/targets/manager.go
index 87020a2f70908..eaf58c9ab0767 100644
--- a/pkg/promtail/targets/manager.go
+++ b/pkg/promtail/targets/manager.go
@@ -20,7 +20,7 @@ type targetManager interface {
 // TargetManagers manages a list of target managers.
 type TargetManagers struct {
 	targetManagers []targetManager
-	positions      *positions.Positions
+	positions      positions.Positions
 }
 
 // NewTargetManagers makes a new TargetManagers
diff --git a/pkg/promtail/targets/tailer.go b/pkg/promtail/targets/tailer.go
index ed44867406d2d..c15ce32b98d88 100644
--- a/pkg/promtail/targets/tailer.go
+++ b/pkg/promtail/targets/tailer.go
@@ -18,7 +18,7 @@ import (
 type tailer struct {
 	logger    log.Logger
 	handler   api.EntryHandler
-	positions *positions.Positions
+	positions positions.Positions
 
 	path string
 	tail *tail.Tail
@@ -29,7 +29,7 @@ type tailer struct {
 	done chan struct{}
 }
 
-func newTailer(logger log.Logger, handler api.EntryHandler, positions *positions.Positions, path string) (*tailer, error) {
+func newTailer(logger log.Logger, handler api.EntryHandler, positions positions.Positions, path string) (*tailer, error) {
 	// Simple check to make sure the file we are tailing doesn't
 	// have a position already saved which is past the end of the file.
 	fi, err := os.Stat(path)

From d09bc1ee023caec4f1d354c23bbdc5d43b44a5c9 Mon Sep 17 00:00:00 2001
From: Cyril Tovena <cyril.tovena@gmail.com>
Date: Tue, 11 Feb 2020 16:09:00 -0500
Subject: [PATCH 5/9] Add information about dry run mode in the troubleshooting
 section.

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>
---
 docs/clients/promtail/troubleshooting.md | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/docs/clients/promtail/troubleshooting.md b/docs/clients/promtail/troubleshooting.md
index 58c98e2272698..ab0fbc79848b8 100644
--- a/docs/clients/promtail/troubleshooting.md
+++ b/docs/clients/promtail/troubleshooting.md
@@ -3,6 +3,19 @@
 This document describes known failure modes of `promtail` on edge cases and the
 adopted trade-offs.
 
+## Dry running
+
+Promtail can be configured to print log stream entries instead of sending them to Loki.
+This can be used in combination with [piping data](#pipe-data-to-promtail) to debug or troubleshoot promtail log parsing.
+
+In dry run mode, Promtail still support reading from a [positions](configuration.md#position_config) file however no update will be made to the targeted file, this is to ensure you can easily retry the same set of lines.
+
+To start Promtail in dry run mode use the flag `--dry-run` as shown in the example below:
+
+```bash
+cat my.log | promtail --dry-run --client.url http://127.0.0.1:3100/loki/api/v1/push
+```
+
 ## Pipe data to Promtail
 
 Promtail supports piping data for sending logs to Loki. This is a very useful way to troubleshooting your configuration.

From 9f1e4cddbe09e4780bc8240d7dfe1af374dce9e5 Mon Sep 17 00:00:00 2001
From: Cyril Tovena <cyril.tovena@gmail.com>
Date: Tue, 11 Feb 2020 16:34:20 -0500
Subject: [PATCH 6/9] Add more spaces.

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>
---
 pkg/promtail/client/logger.go | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pkg/promtail/client/logger.go b/pkg/promtail/client/logger.go
index 600435c803a9d..5b8f3c412bf0d 100644
--- a/pkg/promtail/client/logger.go
+++ b/pkg/promtail/client/logger.go
@@ -36,7 +36,7 @@ func NewLogger(cfgs ...Config) (Client, error) {
 		fmt.Println(string(yaml))
 	}
 	return &logger{
-		Writer: tabwriter.NewWriter(os.Stdout, 0, 4, 0, '\t', 0),
+		Writer: tabwriter.NewWriter(os.Stdout, 0, 8, 0, '\t', 0),
 	}, nil
 }
 

From 7c9b0ad7a1f1d435bda566a2aaac564471ab271c Mon Sep 17 00:00:00 2001
From: Cyril Tovena <cyril.tovena@gmail.com>
Date: Tue, 11 Feb 2020 16:37:46 -0500
Subject: [PATCH 7/9] Fixes journaltarget refactor.

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>
---
 pkg/promtail/targets/journaltarget.go | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/pkg/promtail/targets/journaltarget.go b/pkg/promtail/targets/journaltarget.go
index 10c6a5cba3184..905ff94bc6430 100644
--- a/pkg/promtail/targets/journaltarget.go
+++ b/pkg/promtail/targets/journaltarget.go
@@ -78,7 +78,7 @@ var defaultJournalEntryFunc = func(c sdjournal.JournalReaderConfig, cursor strin
 type JournalTarget struct {
 	logger        log.Logger
 	handler       api.EntryHandler
-	positions     *positions.Positions
+	positions     positions.Positions
 	positionPath  string
 	relabelConfig []*relabel.Config
 	config        *scrape.JournalTargetConfig
@@ -92,7 +92,7 @@ type JournalTarget struct {
 func NewJournalTarget(
 	logger log.Logger,
 	handler api.EntryHandler,
-	positions *positions.Positions,
+	positions positions.Positions,
 	jobName string,
 	relabelConfig []*relabel.Config,
 	targetConfig *scrape.JournalTargetConfig,
@@ -113,7 +113,7 @@ func NewJournalTarget(
 func journalTargetWithReader(
 	logger log.Logger,
 	handler api.EntryHandler,
-	positions *positions.Positions,
+	positions positions.Positions,
 	jobName string,
 	relabelConfig []*relabel.Config,
 	targetConfig *scrape.JournalTargetConfig,

From b2e59759cd3a5d3f653b8c3247902c02e0b576b3 Mon Sep 17 00:00:00 2001
From: Cyril Tovena <cyril.tovena@gmail.com>
Date: Tue, 11 Feb 2020 16:46:11 -0500
Subject: [PATCH 8/9] Fixes journaltarget refactor.

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>
---
 pkg/promtail/targets/journaltargetmanager_linux.go | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pkg/promtail/targets/journaltargetmanager_linux.go b/pkg/promtail/targets/journaltargetmanager_linux.go
index d538a3e7ac0ca..f4a8e45ee36bd 100644
--- a/pkg/promtail/targets/journaltargetmanager_linux.go
+++ b/pkg/promtail/targets/journaltargetmanager_linux.go
@@ -21,7 +21,7 @@ type JournalTargetManager struct {
 // NewJournalTargetManager creates a new JournalTargetManager.
 func NewJournalTargetManager(
 	logger log.Logger,
-	positions *positions.Positions,
+	positions positions.Positions,
 	client api.EntryHandler,
 	scrapeConfigs []scrape.Config,
 ) (*JournalTargetManager, error) {

From 35e4a8ef1616cdeff1c1d19fc883cd9f35dcb27d Mon Sep 17 00:00:00 2001
From: Cyril Tovena <cyril.tovena@gmail.com>
Date: Tue, 11 Feb 2020 17:08:08 -0500
Subject: [PATCH 9/9] Fixes journaltarget refactor.

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>
---
 pkg/promtail/positions/positions.go      | 2 +-
 pkg/promtail/positions/positions_test.go | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/pkg/promtail/positions/positions.go b/pkg/promtail/positions/positions.go
index 5f8eac2d59331..202ba65b533f3 100644
--- a/pkg/promtail/positions/positions.go
+++ b/pkg/promtail/positions/positions.go
@@ -72,7 +72,7 @@ type Positions interface {
 }
 
 // New makes a new Positions.
-func New(logger log.Logger, cfg Config) (*positions, error) {
+func New(logger log.Logger, cfg Config) (Positions, error) {
 	positionData, err := readPositionsFile(cfg, logger)
 	if err != nil {
 		return nil, err
diff --git a/pkg/promtail/positions/positions_test.go b/pkg/promtail/positions/positions_test.go
index 294ba84258380..0919c62cf9122 100644
--- a/pkg/promtail/positions/positions_test.go
+++ b/pkg/promtail/positions/positions_test.go
@@ -167,7 +167,7 @@ func Test_ReadOnly(t *testing.T) {
 		t.Fatal(err)
 	}
 	require.Equal(t, int64(17623), pos)
-	p.save()
+	p.(*positions).save()
 	out, err := readPositionsFile(Config{
 		PositionsFile:     temp,
 		IgnoreInvalidYaml: true,