GoLang: Update file that is served by FileServer

Question

I am serving a directory using FileServer as follows:

go func() {
  fs := http.FileServer(http.Dir("./view"))
  err := http.ListenAndServe(":8000", fs)
  if err != nil {
    log.Fatal("ListenAndServe: ", err)
  }
}()

Inside the view directory I have an index.html file, which I am trying to update while the view directory is being served. I observe that the append commands block and update the file only after I stop serving the directory.

Below is the code to modify the file:

func AppendToFile() {
  f, err := os.OpenFile("./view/index.html", os.O_RDWR, 0644)
  if err != nil {
    panic(err)
  }
  defer f.Close()
  // This assumes that the file ends with </body></html>
  f.Seek(-15, 2)
  if _, err = f.WriteString("test test test\n"); err != nil {
    panic(err)
  }
  if _, err = f.WriteString("</body></html>\n"); err != nil {
    panic(err)
  }
}

Is this the expected behavior?

Thank you!

@Marc It is just a simple WriteString at the end of the file. — m0ur
– m0ur, Commented Sep 21, 2021 at 19:27
You don't show how you are calling AppendToFile(). See @sigkilled 's answer - as the function should be able to update the file without issue. — colm.anseo
– colm.anseo, Commented Sep 21, 2021 at 20:33

sigkilled · Accepted Answer · 2021-09-21 20:24:11Z

3

http.FileServer function just returns a handler. So it is not blocking the filesystem. The problem in here might be related with the offset of your file. I've tried in my machine and it worked without any problem.

I've modified your code to this;

package main

import (
    "net/http"
    "os"
    "time"
)

func main() {
    t := time.NewTicker(time.Second)
    defer t.Stop()
    go func() {
        srv := http.FileServer(http.Dir("./test"))
        http.ListenAndServe(":8080", srv)
    }()

    for {
        select {
        case <-t.C:
            appendToFile()
        }
    }
}

func appendToFile() {
    f, err := os.OpenFile("./test/index.html", os.O_RDWR, 0644)
    if err != nil {
        panic(err)
    }
    defer f.Close()
    // This assumes that the file ends with </body></html>
    f.Seek(-16, 2)
    if _, err = f.WriteString("test test test\n"); err != nil {
        panic(err)
    }
    if _, err = f.WriteString("</body></html>\n"); err != nil {
        panic(err)
    }
}

In index.html I put blank document initially,

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>

</body></html>

PS: It is better to check offset of first, then write the string to that position.

answered Sep 21, 2021 at 20:24

sigkilled

2271 silver badge7 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

user4466350 Over a year ago

thanks for taking the time required to reproduce OP code. I felt lazy to deal with the timeouts myself to hit the browser, so integrated the whole thing for simpler testing experience play.golang.org/p/cgzuP2r1Qlx

m0ur Over a year ago

Thank you! It turns out the problem was completely unrelated to this. (I'm relatively new to Go.) I really appreciate your help!

sigkilled Over a year ago

You are welcome.

Collectives™ on Stack Overflow

GoLang: Update file that is served by FileServer

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related