From 2ae838cf7b241ed01503c96464c162d64bd22516 Mon Sep 17 00:00:00 2001
From: Denis Mishankov <mishankov@mail.com>
Date: Sun, 24 Nov 2024 18:37:05 +0300
Subject: [PATCH] tail -f -n X functionality for local files (#68)

---
 go.mod                 |  1 +
 go.sum                 |  4 ++++
 server/sources.go      | 38 ++++++++++++++++++++++++++++++++++++++
 utils/generate_logs.js |  2 +-
 4 files changed, 44 insertions(+), 1 deletion(-)

diff --git a/go.mod b/go.mod
index 4fe8f9e..028a7ea 100644
--- a/go.mod
+++ b/go.mod
@@ -8,6 +8,7 @@ require github.com/go-chi/chi/v5 v5.1.0
 
 require (
 	github.com/gorilla/websocket v1.5.3
+	github.com/icza/backscanner v0.0.0-20240328210400-b40c3a86dec5
 	github.com/mishankov/go-utlz v0.0.3
 	github.com/nxadm/tail v1.4.11
 	golang.org/x/crypto v0.29.0
diff --git a/go.sum b/go.sum
index ff4e893..da56fac 100644
--- a/go.sum
+++ b/go.sum
@@ -6,6 +6,10 @@ github.com/go-chi/chi/v5 v5.1.0 h1:acVI1TYaD+hhedDJ3r54HyA6sExp3HfXq7QWEEY/xMw=
 github.com/go-chi/chi/v5 v5.1.0/go.mod h1:DslCQbL2OYiznFReuXYUmQ2hGd1aDpCnlMNITLSKoi8=
 github.com/gorilla/websocket v1.5.3 h1:saDtZ6Pbx/0u+bgYQ3q96pZgCzfhKXGPqt7kZ72aNNg=
 github.com/gorilla/websocket v1.5.3/go.mod h1:YR8l580nyteQvAITg2hZ9XVh4b55+EU/adAjf1fMHhE=
+github.com/icza/backscanner v0.0.0-20240328210400-b40c3a86dec5 h1:FcxwOojw6pUiPpsf7Q6Fw/pI+7cR6FlapLBEGV/902A=
+github.com/icza/backscanner v0.0.0-20240328210400-b40c3a86dec5/go.mod h1:GYeBD1CF7AqnKZK+UCytLcY3G+UKo0ByXX/3xfdNyqQ=
+github.com/icza/mighty v0.0.0-20180919140131-cfd07d671de6 h1:8UsGZ2rr2ksmEru6lToqnXgA8Mz1DP11X4zSJ159C3k=
+github.com/icza/mighty v0.0.0-20180919140131-cfd07d671de6/go.mod h1:xQig96I1VNBDIWGCdTt54nHt6EeI639SmHycLYL7FkA=
 github.com/mishankov/go-utlz v0.0.3 h1:iSrGlCeJ8La5M9oFuELQ1AIA6W/nWxilfA7bjiP2hoE=
 github.com/mishankov/go-utlz v0.0.3/go.mod h1:ioIgxxn/MMg+7AWli68mApcJXXpaDQ6790Hu0tQFV6E=
 github.com/nxadm/tail v1.4.11 h1:8feyoE3OzPrcshW5/MJ4sGESc5cqmGkGCWlco4l0bqY=
diff --git a/server/sources.go b/server/sources.go
index 2508ed9..1d9183e 100644
--- a/server/sources.go
+++ b/server/sources.go
@@ -7,8 +7,10 @@ import (
 	"iter"
 	"os"
 	"os/exec"
+	"slices"
 	"strconv"
 
+	"github.com/icza/backscanner"
 	"github.com/nxadm/tail"
 	"golang.org/x/crypto/ssh"
 )
@@ -58,6 +60,39 @@ type LocalFile struct {
 }
 
 func (lf LocalFile) Tail() (iter.Seq[string], error) {
+	initialLines := []string{}
+
+	file, err := os.Open(lf.filePath)
+	if err != nil {
+		logger.Error("Error opening local file:", err)
+		return nil, err
+	}
+
+	fileStatus, err := file.Stat()
+	if err != nil {
+		logger.Error("Error getting stat of local file:", err)
+		return nil, err
+	}
+	defer file.Close()
+
+	scanner := backscanner.New(file, int(fileStatus.Size()))
+	for {
+		line, _, err := scanner.LineBytes()
+		if err != nil {
+			if err == io.EOF {
+				break
+			}
+		}
+
+		initialLines = append(initialLines, string(line))
+
+		if len(initialLines) >= lf.initialAmount {
+			break
+		}
+	}
+
+	slices.Reverse(initialLines)
+
 	t, err := tail.TailFile(lf.filePath, tail.Config{Poll: true, Follow: true, ReOpen: true, Location: &tail.SeekInfo{Offset: 0, Whence: io.SeekEnd}})
 	if err != nil {
 		logger.Error("Error setuping tail file:", err)
@@ -65,6 +100,9 @@ func (lf LocalFile) Tail() (iter.Seq[string], error) {
 	}
 
 	return func(yield func(string) bool) {
+		for _, line := range initialLines {
+			yield(line)
+		}
 		for line := range t.Lines {
 			yield(line.Text)
 		}
diff --git a/utils/generate_logs.js b/utils/generate_logs.js
index ada85ae..7132fc4 100644
--- a/utils/generate_logs.js
+++ b/utils/generate_logs.js
@@ -8,7 +8,7 @@ async function run() {
 
     console.log(logLine);
 
-    fs.writeFile("dist/myfile.log", logLine, { flag: "a+" }, (err) => {});
+    fs.writeFile("myfile.log", logLine, { flag: "a+" }, (err) => {});
   }
 }