From 587c5c3f25805df2157af6c216146e304cffed73 Mon Sep 17 00:00:00 2001
From: Aleksandr Razumov <ernado@go-faster.org>
Date: Wed, 13 Sep 2023 20:53:49 +0300
Subject: [PATCH] feat: apply optimizations

---
 .github/workflows/ci.yml |  6 ++---
 go.mod                   |  3 ++-
 go.sum                   |  3 +++
 tail.go                  | 54 ++++++++++++++++++++++++++--------------
 4 files changed, 44 insertions(+), 22 deletions(-)

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 4fed6de..6477e89 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -20,15 +20,15 @@ jobs:
         runner:
           - ubuntu-latest
         go:
-          - 1.18
+          - 1.21.x
         include:
           - arch: 386
             runner: ubuntu-latest
-            go: 1.18
+            go: 1.21.x
           - arch: amd64
             runner: ubuntu-latest
             flags: "-race"
-            go: 1.18
+            go: 1.21.x
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
diff --git a/go.mod b/go.mod
index 8a7defa..100b616 100644
--- a/go.mod
+++ b/go.mod
@@ -1,11 +1,12 @@
 module github.com/go-faster/tail
 
-go 1.18
+go 1.21
 
 require (
 	github.com/fsnotify/fsnotify v1.6.0
 	github.com/go-faster/errors v0.6.1
 	github.com/stretchr/testify v1.8.4
+	go.uber.org/atomic v1.11.0
 	go.uber.org/zap v1.25.0
 	golang.org/x/sync v0.3.0
 )
diff --git a/go.sum b/go.sum
index 9614e55..a6153d7 100644
--- a/go.sum
+++ b/go.sum
@@ -10,7 +10,10 @@ github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZb
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/stretchr/testify v1.8.4 h1:CcVxjf3Q8PM0mHUKJCdn+eZZtm5yQwehR5yeSVQQcUk=
 github.com/stretchr/testify v1.8.4/go.mod h1:sz/lmYIOXD/1dqDmKjjqLyZ2RngseejIcXlSw2iwfAo=
+go.uber.org/atomic v1.11.0 h1:ZvwS0R+56ePWxUNi+Atn9dWONBPp/AUETXlHW0DxSjE=
+go.uber.org/atomic v1.11.0/go.mod h1:LUxbIzbOniOlMKjJjyPfpl4v+PKK2cNJn91OQbhoJI0=
 go.uber.org/goleak v1.2.0 h1:xqgm/S+aQvhWFTtR0XK3Jvg7z8kGV8P4X14IzwN3Eqk=
+go.uber.org/goleak v1.2.0/go.mod h1:XJYK+MuIchqpmGmUSAzotztawfKvYLUIgg7guXrwVUo=
 go.uber.org/multierr v1.10.0 h1:S0h4aNzvfcFsC3dRF1jLoaov7oRaKqRGC/pUEJ2yvPQ=
 go.uber.org/multierr v1.10.0/go.mod h1:20+QtiLqy0Nd6FdQB9TLXag12DsQkrbs3htMFfDN80Y=
 go.uber.org/zap v1.25.0 h1:4Hvk6GtkucQ790dqmj7l1eEnRdKm3k3ZUrUMS2d5+5c=
diff --git a/tail.go b/tail.go
index 57a3277..0225b16 100644
--- a/tail.go
+++ b/tail.go
@@ -11,6 +11,7 @@ import (
 
 	"github.com/fsnotify/fsnotify"
 	"github.com/go-faster/errors"
+	"go.uber.org/atomic"
 	"go.uber.org/zap"
 	"go.uber.org/zap/zapcore"
 )
@@ -101,6 +102,7 @@ type Tailer struct {
 	name    string
 	file    *os.File
 	reader  *bufio.Reader
+	proxy   *offsetProxy
 	watcher *watcher
 	lg      *zap.Logger
 }
@@ -132,17 +134,20 @@ func File(filename string, cfg Config) *Tailer {
 	}
 }
 
-// offset returns the file's current offset and error if any.
-//
-// NB: can be not accurate
-// TODO(ernado): what does it mean exactly?
-func (t *Tailer) offset() (offset int64, err error) {
-	offset, err = t.file.Seek(0, io.SeekCurrent)
-	if err != nil {
-		return offset, errors.Wrap(err, "seek")
-	}
-	offset -= int64(t.reader.Buffered())
-	return offset, nil
+type offsetProxy struct {
+	Reader io.Reader
+	Offset int64
+}
+
+func (o *offsetProxy) Read(p []byte) (n int, err error) {
+	n, err = o.Reader.Read(p)
+	o.Offset += int64(n)
+	return n, err
+}
+
+// offset returns the file's current offset.
+func (t *Tailer) offset() int64 {
+	return t.proxy.Offset - int64(t.reader.Buffered())
 }
 
 func (t *Tailer) closeFile() {
@@ -174,6 +179,14 @@ func (t *Tailer) openFile(ctx context.Context) error {
 			}
 			return errors.Wrap(err, "open")
 		}
+		offset, err := t.file.Seek(0, io.SeekCurrent)
+		if err != nil {
+			return errors.Wrap(err, "seek")
+		}
+		t.proxy = &offsetProxy{
+			Reader: t.file,
+			Offset: offset,
+		}
 		return nil
 	}
 }
@@ -228,17 +241,20 @@ func (t *Tailer) Tail(ctx context.Context, h Handler) error {
 		Data: make([]byte, 0, t.cfg.BufferSize),
 	}
 
+	// Reduce lock contention.
+	var done atomic.Bool
+	go func() {
+		<-ctx.Done()
+		done.Store(true)
+	}()
+
 	for {
-		if err := ctx.Err(); err != nil {
+		if done.Load() {
 			return ctx.Err()
 		}
 
 		// Grab the offset in case we need to back up in the event of a half-line.
-		offset, err := t.offset()
-		if err != nil {
-			return errors.Wrap(err, "offset")
-		}
-
+		offset := t.offset()
 		line.Offset = offset
 		if e := t.lg.Check(zapcore.DebugLevel, "Offset"); e != nil {
 			e.Write(zap.Int64("offset", offset))
@@ -332,4 +348,6 @@ func (t *Tailer) waitForChanges(ctx context.Context, pos int64) error {
 	return nil
 }
 
-func (t *Tailer) resetReader() { t.reader = bufio.NewReaderSize(t.file, t.cfg.BufferSize) }
+func (t *Tailer) resetReader() {
+	t.reader = bufio.NewReaderSize(t.proxy, t.cfg.BufferSize)
+}