恐慌：净：不一致fdMutex

Question

I'm trying to implement a simple echo server in Go (go1.8.3 windows/amd64) on Windows 10, but I have this weird panic happening some time after getting a connection from a client (connecting with ncat). I can send and receive messages, but it will crash after a while no matter what I do, and I don't know why.

The code is

package main

import (
    "bufio"
    "log"
    "net"
)

func echo(conn net.Conn, c chan int) {
    c <- 1
    reader := bufio.NewReader(conn)
    msg, err := reader.ReadBytes('
')
    if err != nil {
        log.Printf("error reading from the reader: %v", err)
        return
    }
    conn.Write(msg)
}

func handleConnection(conn net.Conn) {
    log.Print("Got connection from: ", conn.RemoteAddr())
    conn.Write([]byte("Hello, this is a Go echo server
"))
    c := make(chan int)
    for {
        go echo(conn, c)
        <-c
    }
}

func main() {
    listener, err := net.Listen("tcp", "127.0.0.1:3000")
    if err != nil {
        log.Fatalf("couldn't create a tcp server: %v", err)
    }
    for {
        conn, err := listener.Accept()
        defer conn.Close()
        if err != nil {
            log.Printf("couldn't create a connection: %v", err)
            return
        }
        go handleConnection(conn)
    }
}

The error is

panic: net: inconsistent fdMutex

goroutine 1048589 [running]:
net.(*fdMutex).rwlock(0xc0420741c0, 0x1, 0x0)
    C:/Go/src/net/fd_mutex.go:145 +0x1ab
net.(*netFD).readLock(0xc0420741c0, 0x0, 0x0)
    C:/Go/src/net/fd_mutex.go:218 +0x39
net.(*netFD).Read(0xc0420741c0, 0xc365329000, 0x1000, 0x1000, 0x0, 0x0, 0x0)
    C:/Go/src/net/fd_windows.go:444 +0x5d
net.(*conn).Read(0xc04205e030, 0xc365329000, 0x1000, 0x1000, 0x0, 0x0, 0x0)
    C:/Go/src/net/net.go:181 +0x77
bufio.(*Reader).fill(0xc36532bf08)
    C:/Go/src/bufio/bufio.go:97 +0x11e
bufio.(*Reader).ReadSlice(0xc36532bf08, 0xc36532be0a, 0xc36532be00, 0x0,         0x100000000000000, 0x0, 0x1000)
    C:/Go/src/bufio/bufio.go:338 +0xc2
bufio.(*Reader).ReadBytes(0xc36532bf08, 0x100a, 0x1000, 0xc365329000,     0x1000, 0x1000, 0x0)
    C:/Go/src/bufio/bufio.go:416 +0x6d
main.echo(0x574ce0, 0xc04205e030, 0xc042098d80)
    C:/Users/fcfn/goecho/server.go:13 +0x10b
created by main.handleConnection
    C:/Users/fcfn/goecho/server.go:27 +0x271

Answer 1

Thanks to @JimB, I understand the nature of the problem better. Here's what he said:

The actual error is because you're calling Read from an unbounded number of goroutines. The net package assumes there's no reason to ever have more than 1<<20 concurrent operations on a net.Conn. – JimB

---

Some bad code was here, which wasn't quite a fix for the problem at hand. I removed it so it won't mislead anyone, as advised by JimB.

What I learned from JimB:

1. You shouldn't use goroutines unless it is necessary (echo function shouldn't be a goroutine, as reading is a blocking operation, and we can't really do anything with the connection until we understand what the client is sending to us, so no concurrency is needed; handleConnection, on the other hand, is run inside a goroutine because it's supposed to handle multiple connections simultaneously).
2. Errors are precious things, handle them gracefully, do not ignore them.