From 1ff299bc29222d0b8b43b858124cc24b00fa4cbe Mon Sep 17 00:00:00 2001
From: Florian Weingarten <flo@hackvalue.de>
Date: Fri, 10 May 2013 14:18:00 -0400
Subject: [PATCH] Add support for file truncation in InotifyFileWatcher (cherry
 picked from commit 9de77aad8caca8102e7dd7c936d00ca3f0421ca7)

---
 tail.go  |  2 +-
 watch.go | 22 +++++++++++++++++-----
 2 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/tail.go b/tail.go
index c68d7b2..8e8a71a 100644
--- a/tail.go
+++ b/tail.go
@@ -190,7 +190,7 @@ func (tail *Tail) tailFileSync() {
 						// File got deleted/renamed
 						if tail.ReOpen {
 							// TODO: no logging in a library?
-							log.Printf("Re-opening moved/deleted file %s ...", tail.Filename)
+							log.Printf("Re-opening moved/deleted/truncated file %s ...", tail.Filename)
 							err := tail.reopen()
 							if err != nil {
 								tail.close()
diff --git a/watch.go b/watch.go
index df99e25..cf653d6 100644
--- a/watch.go
+++ b/watch.go
@@ -24,10 +24,11 @@ type FileWatcher interface {
 // InotifyFileWatcher uses inotify to monitor file changes.
 type InotifyFileWatcher struct {
 	Filename string
+	Size     int64
 }
 
 func NewInotifyFileWatcher(filename string) *InotifyFileWatcher {
-	fw := &InotifyFileWatcher{filename}
+	fw := &InotifyFileWatcher{filename, 0}
 	return fw
 }
 
@@ -65,19 +66,29 @@ func (fw *InotifyFileWatcher) ChangeEvents(_ os.FileInfo) chan bool {
 	ch := make(chan bool)
 
 	go func() {
+		defer w.Close()
+		defer w.RemoveWatch(fw.Filename)
+		defer close(ch)
+
 		for {
+			prevSize := fw.Size
+
 			evt := <-w.Event
 			switch {
 			case evt.IsDelete():
 				fallthrough
 
 			case evt.IsRename():
-				close(ch)
-				w.RemoveWatch(fw.Filename)
-				w.Close()
 				return
 
 			case evt.IsModify():
+				fi, _ := os.Stat(fw.Filename)
+				fw.Size = fi.Size()
+
+				if prevSize > 0 && prevSize > fw.Size {
+					return
+				}
+
 				// send only if channel is empty.
 				select {
 				case ch <- true:
@@ -93,10 +104,11 @@ func (fw *InotifyFileWatcher) ChangeEvents(_ os.FileInfo) chan bool {
 // PollingFileWatcher polls the file for changes.
 type PollingFileWatcher struct {
 	Filename string
+	Size     int64
 }
 
 func NewPollingFileWatcher(filename string) *PollingFileWatcher {
-	fw := &PollingFileWatcher{filename}
+	fw := &PollingFileWatcher{filename, 0}
 	return fw
 }