From 2b75cbbed3660bde9352b1f76ff8315e43013164 Mon Sep 17 00:00:00 2001
From: Patrick Hemmer <phemmer@chewy.com>
Date: Wed, 29 Mar 2017 00:03:35 -0400
Subject: [PATCH] ensure the watcher has started before we read

...otherwise we can run into the scenario where: On start we read the contents of the file, we hit EOF, and start watching for changes. But in between the EOF and start watching, the file is updated. But since the watcher wasn't started, we don't notice.
---
 tail.go | 33 ++++++++++++++++++++++++---------
 1 file changed, 24 insertions(+), 9 deletions(-)

diff --git a/tail.go b/tail.go
index c99cdaa2..5ed15bd4 100644
--- a/tail.go
+++ b/tail.go
@@ -250,6 +250,11 @@ func (tail *Tail) tailFileSync() {
 
 	tail.openReader()
 
+	if err := tail.watchChanges(); err != nil {
+		tail.Killf("Error watching for changes on %s: %s", tail.Filename, err)
+		return
+	}
+
 	var offset int64
 	var err error
 
@@ -331,19 +336,29 @@ func (tail *Tail) tailFileSync() {
 	}
 }
 
-// waitForChanges waits until the file has been appended, deleted,
-// moved or truncated. When moved or deleted - the file will be
-// reopened if ReOpen is true. Truncated files are always reopened.
-func (tail *Tail) waitForChanges() error {
-	if tail.changes == nil {
+// watchChanges ensures the watcher is running.
+func (tail *Tail) watchChanges() error {
+	if tail.changes != nil {
+		return nil
+	}
+	var pos int64
+	var err error
+	if !tail.Pipe {
 		pos, err := tail.file.Seek(0, os.SEEK_CUR)
 		if err != nil {
 			return err
 		}
-		tail.changes, err = tail.watcher.ChangeEvents(&tail.Tomb, pos)
-		if err != nil {
-			return err
-		}
+	}
+	tail.changes, err = tail.watcher.ChangeEvents(&tail.Tomb, pos)
+	return err
+}
+
+// waitForChanges waits until the file has been appended, deleted,
+// moved or truncated. When moved or deleted - the file will be
+// reopened if ReOpen is true. Truncated files are always reopened.
+func (tail *Tail) waitForChanges() error {
+	if err := tail.watchChanges(); err != nil {
+		return err
 	}
 
 	select {