Golang TCP File Transfer Gets Stuck In the Middle

I'm having some file transfer issue over TCP in go. The file transfer works sometimes and sometimes it gets stuck in the middle. When it gets stuck, it looks like it is expecting data in the communication channel but there is no data and no error as well. Hence it gets stuck indefinitely. To make thing confusing it shows this behavior for same file i.e for same file it works sometimes and sometimes it doesn't work.

This is how my program works. It'll listen for incoming requests. The requests are in JSON format. Based on request type it'll do different operation. I'm posting the code segment related to file transfer.

server.go

package main
import (
"bufio"
"encoding/json"
"fmt"
_"io"
"net"
"os"
)
const (
COMMAND_RECEIVE_FILE    = "TRANSFER_FILE"
COMMAND_EXIT            = "EXIT"
CONNECTION_TYPE = "tcp"
CONNECTION_PORT = "3645"
CONNECTION_HOST = ""
BUFFER_SIZE     = 1024
)
type Command struct {
Identifier string `json:"identifier"`
Name       string `json:"name"`
Size       int64  `json:"size"`
}
type Result struct {
Message     string        `json:"message"`
}
func receiveFile(connection net.Conn, fileName string, fileSize int64) Result {
fmt.Println("Receiving file")
result := Result{Message: ""}
file, err := os.Create(fileName)
if err != nil {
fmt.Println(err)
result.Message = "Error opening file: " + fileName
return result
}
defer file.Close()
fileBuffer := make([]byte, BUFFER_SIZE)
bytesRead := int64(0)
count := 0
for {
if fileSize-bytesRead < int64(BUFFER_SIZE) {
fileBuffer = make([]byte, fileSize-bytesRead)
}
fmt.Println("Reading ", BUFFER_SIZE, " bytes of data")
n, err := connection.Read(fileBuffer)
count++
fmt.Println("Completed reading", n, " bytes of data, count=", count)
file.Write(fileBuffer[0:n])
bytesRead += int64(n)
if err != nil {
result.Message = "File transfer incomplete"
break
}
if bytesRead >= fileSize {
result.Message = "File transfer complete"
break
}
}
file.Chmod(0777)
return result
}
func main() {
ln, err := net.Listen(CONNECTION_TYPE, CONNECTION_HOST + ":"+CONNECTION_PORT)
if err != nil {
fmt.Println("error opening a tcp connection")
}
for {
fmt.Println("waiting for new connection")
conn, err := ln.Accept()
if err != nil {
} else {
var commandStr string
reader := bufio.NewReader(conn)
var exitStatus = 1
for exitStatus == 1 {
fmt.Println("Waiting for new command: ")
line,_,err := reader.ReadLine()
if err != nil {
conn.Close()
exitStatus = 0
break
} else {
fmt.Println("Size read :", len(line))
}
commandStr = string(line)
fmt.Println("CommandStr: ", commandStr)
var msg Command
err = json.Unmarshal([]byte(commandStr), &msg)
if err != nil {
fmt.Println("Error")
conn.Close()
break
}
result := Result{}
fmt.Println("Received new command: ", msg.Identifier)
switch msg.Identifier {
case COMMAND_RECEIVE_FILE:
result = receiveFile(conn, msg.Name, msg.Size)
case COMMAND_EXIT:
exitStatus = 0
conn.Close()
default:
result = Result{Message: "Unrecognized command"}
}
out, _ := json.Marshal(result)
fmt.Fprint(conn, string(out)+"\n")
}
}
}
}

test.go

package main
import (
"bufio"
"encoding/json"
"fmt"
"io"
"log"
"net"
"os"
"strings"
_"time"
)
const (
COMMAND_TRANSFER_FILE   = "TRANSFER_FILE"
COMMAND_EXIT            = "EXIT"
CONNECTION_TYPE = "tcp"
CONNECTION_PORT = "3645"
CONNECTION_HOST = ""
)
type Command struct {
Identifier string `json:"identifier"`
Name       string `json:"name"`
Size       int64  `json:"size"`
}
type Result struct {
Message     string        `json:"message"`
}
func main() {
conn, _ := net.Dial(CONNECTION_TYPE, CONNECTION_HOST + ":" + CONNECTION_PORT)
decoder := json.NewDecoder(conn)
com := Command{}
sourceFileName := ""
destinationFileName := ""
for {
com = Command{}
reader := bufio.NewReader(os.Stdin)
identifier, _ := reader.ReadString('\n')
com.Identifier = strings.TrimSpace(identifier)
switch com.Identifier {
case COMMAND_TRANSFER_FILE:
fmt.Print("Source file name:")
sourceFileName, _ = reader.ReadString('\n')
sourceFileName = strings.TrimSpace(sourceFileName)
fmt.Print("Destination file name:")
destinationFileName, _ = reader.ReadString('\n')
com.Name = strings.TrimSpace(destinationFileName)
file, err := os.Open(sourceFileName)
if err != nil {
log.Fatal(err)
}
defer file.Close()
fileInfo, err := file.Stat()
fileSize := fileInfo.Size()
com.Size = fileSize
case COMMAND_EXIT:
conn.Close()
os.Exit(0)
}
out, _ := json.Marshal(com)
conn.Write([]byte(string(out) + "\n"))
if strings.Compare(com.Identifier, COMMAND_TRANSFER_FILE) == 0 {
file, err := os.Open(sourceFileName)
if err != nil {
log.Fatal(err)
}
defer file.Close()
n, err := io.Copy(conn, file)
if err != nil {
log.Fatal(err)
}
fmt.Println(n, "bytes sent")
}
var msg Result
err := decoder.Decode(&msg)
if err != nil {
fmt.Println(err)
}
fmt.Println(msg)
}
}

I tested it on both Linux and Windows and it shows same behavior on both system. The only thing I can think of is that the sender is faster than the receiver even though I'm running it on the same machine. If that is the case, what will be a best practice to solve it other than the handshaking mechanism.

You can't wrap the net.Conn in a bufio.Reader, then continue to use the net.Conn. The reason your function is blocked is because you left data buffered in the reader, so you won't ever reach the desired message size.

You need to pass the reader to the receiveFile function in order to not lose the buffered data.

You are also ignoring the isPrefix return value from ReadLine. I would follow the documentation and use ReadBytes instead if you're not going to handle all cases from that method.