Reviewers: mp+214150_code.launchpad.net,

Message:
Please take a look.

Description:
Extend the tailer utility.

When looking at the pyjuju debug-log, there was a replay
function that allowed getting the log from the start.
There was no way to tell the tailer to start from the
beginning of the file.

I changed the meaning of NewTailer to be one that doesn't
do any seeking.  The NewTailerBacktrack does seek to the end,
and then goes back a certain number of lines.

There was a bug in the backtrack tailer when zero lines
were passed in.  This is fixed and a test added.

In order to provide a synchronization mechanism for other
tests, the Tailer has a public function param that is called
after the backtracking, and prior to starting tailing.

The number of lines to look back is now a unit, as negative
values make no sense at all.

https://code.launchpad.net/~thumper/juju-core/tailer-tweaks/+merge/214150

(do not edit description out of merge proposal)


Please review this at https://codereview.appspot.com/84290045/

Affected files (+53, -12 lines):
   A [revision details]
   M container/lxc/clonetemplate.go
   M utils/tailer/tailer.go
   M utils/tailer/tailer_test.go


Index: [revision details]
=== added file '[revision details]'
--- [revision details]	2012-01-01 00:00:00 +0000
+++ [revision details]	2012-01-01 00:00:00 +0000
@@ -0,0 +1,2 @@
+Old revision: tarmac-20140404003547-v7dzlndz9bq9h4qd
+New revision: <email address hidden>

Index: container/lxc/clonetemplate.go
=== modified file 'container/lxc/clonetemplate.go'
--- container/lxc/clonetemplate.go	2014-03-12 04:32:02 +0000
+++ container/lxc/clonetemplate.go	2014-04-04 03:33:01 +0000
@@ -179,7 +179,7 @@
  	}

  	tailWriter := &logTail{tick: time.Now()}
-	consoleTailer := tailer.NewTailer(console, tailWriter, 0, nil)
+	consoleTailer := tailer.NewTailer(console, tailWriter, nil)
  	defer consoleTailer.Stop()

  	// We should wait maybe 1 minute between output?


Index: utils/tailer/tailer.go
=== modified file 'utils/tailer/tailer.go'
--- utils/tailer/tailer.go	2013-12-13 11:19:56 +0000
+++ utils/tailer/tailer.go	2014-04-04 03:20:53 +0000
@@ -35,25 +35,37 @@
  	reader      *bufio.Reader
  	writeCloser io.WriteCloser
  	writer      *bufio.Writer
-	lines       int
+	lines       uint
  	filter      TailerFilterFunc
  	bufferSize  int
  	polltime    time.Duration
+	lookBack    bool
+	// StartedListening is purely a hook point for tests. It is a function
+	// that is called after the initial seek prior to reading.
+	StartedTailing func()
  }

-// NewTailer starts a Tailer which reads strings from the passed
+// NewTailerBacktrack starts a Tailer which reads strings from the passed
  // ReadSeeker line by line. If a filter function is specified the read
  // lines are filtered. The matching lines are written to the passed
  // Writer. The reading begins the specified number of matching lines
  // from the end.
-func NewTailer(readSeeker io.ReadSeeker, writer io.Writer, lines int,  
filter TailerFilterFunc) *Tailer {
-	return newTailer(readSeeker, writer, lines, filter, bufferSize, polltime)
+func NewTailerBacktrack(readSeeker io.ReadSeeker, writer io.Writer, lines  
uint, filter TailerFilterFunc) *Tailer {
+	return newTailer(readSeeker, writer, lines, filter, bufferSize, polltime,  
true)
+}
+
+// NewTailer starts a Tailer which reads strings from the passed
+// ReadSeeker line by line. If a filter function is specified the read
+// lines are filtered. The matching lines are written to the passed
+// Writer.
+func NewTailer(readSeeker io.ReadSeeker, writer io.Writer, filter  
TailerFilterFunc) *Tailer {
+	return newTailer(readSeeker, writer, 0, filter, bufferSize, polltime,  
false)
  }

  // newTailer starts a Tailer like NewTailer but allows the setting of
  // the read buffer size and the time between pollings for testing.
-func newTailer(readSeeker io.ReadSeeker, writer io.Writer, lines int,  
filter TailerFilterFunc,
-	bufferSize int, polltime time.Duration) *Tailer {
+func newTailer(readSeeker io.ReadSeeker, writer io.Writer, lines uint,  
filter TailerFilterFunc,
+	bufferSize int, polltime time.Duration, lookBack bool) *Tailer {
  	t := &Tailer{
  		readSeeker: readSeeker,
  		reader:     bufio.NewReaderSize(readSeeker, bufferSize),
@@ -62,6 +74,7 @@
  		filter:     filter,
  		bufferSize: bufferSize,
  		polltime:   polltime,
+		lookBack:   lookBack,
  	}
  	go func() {
  		defer t.tomb.Done()
@@ -98,8 +111,13 @@
  // writer too.
  func (t *Tailer) loop() error {
  	// Position the readSeeker.
-	if err := t.seekLastLines(); err != nil {
-		return err
+	if t.lookBack {
+		if err := t.seekLastLines(); err != nil {
+			return err
+		}
+	}
+	if t.StartedTailing != nil {
+		t.StartedTailing()
  	}
  	// Start polling.
  	// TODO(mue) 2013-12-06
@@ -139,8 +157,12 @@
  	if err != nil {
  		return err
  	}
+	if t.lines == 0 {
+		// We are done, just seeking to the end is sufficient.
+		return nil
+	}
  	seekPos := int64(0)
-	found := 0
+	found := uint(0)
  	buffer := make([]byte, t.bufferSize)
  SeekLoop:
  	for offset > 0 {


Index: utils/tailer/tailer_test.go
=== modified file 'utils/tailer/tailer_test.go'
--- utils/tailer/tailer_test.go	2013-12-13 13:21:44 +0000
+++ utils/tailer/tailer_test.go	2014-04-04 03:20:53 +0000
@@ -59,12 +59,13 @@
  	description           string
  	data                  []string
  	initialLinesWritten   int
-	initialLinesRequested int
+	initialLinesRequested uint
  	bufferSize            int
  	filter                tailer.TailerFilterFunc
  	injector              func(*tailer.Tailer, *readSeeker) func([]string)
  	initialCollectedData  []string
  	appendedCollectedData []string
+	fromStart             bool
  	err                   string
  }{{
  	description: "lines are longer than buffer size",
@@ -182,6 +183,19 @@
  	},
  	appendedCollectedData: alphabetData[5:],
  }, {
+	description:           "ignore current lines",
+	data:                  alphabetData,
+	initialLinesWritten:   5,
+	bufferSize:            5,
+	appendedCollectedData: alphabetData[5:],
+}, {
+	description:           "start from the start",
+	data:                  alphabetData,
+	initialLinesWritten:   5,
+	bufferSize:            5,
+	appendedCollectedData: alphabetData,
+	fromStart:             true,
+}, {
  	description:           "lines are longer than buffer size, less lines  
already written than initially requested",
  	data:                  alphabetData,
  	initialLinesWritten:   3,
@@ -323,7 +337,7 @@
  		reader, writer := io.Pipe()
  		sigc := make(chan struct{}, 1)
  		rs := startReadSeeker(c, test.data, test.initialLinesWritten, sigc)
-		tailer := tailer.NewTestTailer(rs, writer, test.initialLinesRequested,  
test.filter, bufferSize, 2*time.Millisecond)
+		tailer := tailer.NewTestTailer(rs, writer, test.initialLinesRequested,  
test.filter, bufferSize, 2*time.Millisecond, !test.fromStart)
  		linec := startReading(c, tailer, reader, writer)

  		// Collect initial data.
@@ -386,6 +400,9 @@
  // linec is closed due to stopping or an error only the values so far care
  // compared. Checking the reason for termination is done in the test.
  func assertCollected(c *gc.C, linec chan string, compare []string,  
injection func([]string)) {
+	if len(compare) == 0 {
+		return
+	}
  	timeout := time.After(testing.LongWait)
  	lines := []string{}
  	for {