There are several problems with your approach.
Sort-of a preface
The first one is that you do not wait for the server goroutine
to complete.
In Go, once main() exits for whatever reason,
all the other goroutines still running, if any, are simply
teared down forcibly.
You're trying to "synchronize" things using timers,
but this only works in toy situations, and even then it
does so only from time to time.
Hence let's fix your code first:
package main
import (
"fmt"
"log"
"net"
"time"
)
func main() {
addr := "127.0.0.1:8999"
tcpaddr, err := net.ResolveTCPAddr("tcp4", addr)
if err != nil {
log.Fatal(err)
}
listener, err := net.ListenTCP("tcp", tcpaddr)
if err != nil {
log.Fatal(err)
}
// Server
done := make(chan error)
go func(listener net.Listener, done chan<- error) {
for {
conn, err := listener.Accept()
if err != nil {
done <- err
return
}
go func(conn net.Conn) {
var buffer [1024]byte
n, err := conn.Read(buffer[:])
if err != nil {
log.Println(err)
} else {
log.Println(">", string(buffer[0:n]))
}
if err := conn.Close(); err != nil {
log.Println("error closing server conn:", err)
}
}(conn)
}
}(listener, done)
// Client
conn, err := net.Dial("tcp", addr)
if err != nil {
log.Fatal(err)
}
for i := 0; i < 2; i++ {
_, err := conn.Write([]byte("hello"))
if err != nil {
log.Println(err)
err = conn.Close()
if err != nil {
log.Println("error closing client conn:", err)
}
break
}
fmt.Println("ok")
time.Sleep(2 * time.Second)
}
// Shut the server down and wait for it to report back
err = listener.Close()
if err != nil {
log.Fatal("error closing listener:", err)
}
err = <-done
if err != nil {
log.Println("server returned:", err)
}
}
I've spilled a couple of minor fixes
like using log.Fatal (which is
log.Print + os.Exit(1)) instead of panicking,
removed useless else clauses to adhere to the coding standard of keeping the main
flow where it belongs, and lowered the client's timeout.
I have also added checking for possible errors Close on sockets may return.
The interesting part is that we now properly shut the server down by closing the listener and then waiting for the server goroutine to report back (unfortunately Go does not return an error of a custom type from net.Listener.Accept in this case so we can't really check that Accept exited because we've closed the listener).
Anyway, our goroutines are now properly synchronized, and there is
no undefined behaviour, so we can reason about how the code works.
Remaining problems
Some problems still remain.
The more glaring is you making wrong assumption that TCP preserves
message boundaries—that is, if you write "hello" to the client
end of the socket, the server reads back "hello".
This is not true: TCP considers both ends of the connection
as producing and consuming opaque streams of bytes.
This means, when the client writes "hello", the client's
TCP stack is free to deliver "he" and postpone sending "llo",
and the server's stack is free to yield "hell" to the read
call on the socket and only return "o" (and possibly some other
data) in a later read.
So, to make the code "real" you'd need to somehow introduce these
message boundaries into the protocol above TCP.
In this particular case the simplest approach would be either
using "messages" consisting of a fixed-length and agreed-upon
endianness prefix indicating the length of the following
data and then the string data itself.
The server would then use a sequence like
var msg [4100]byte
_, err := io.ReadFull(sock, msg[:4])
if err != nil { ... }
mlen := int(binary.BigEndian.Uint32(msg[:4]))
if mlen < 0 {
// handle error
}
if mlen == 0 {
// empty message; goto 1
}
_, err = io.ReadFull(sock, msg[5:5+mlen])
if err != nil { ... }
s := string(msg[5:5+mlen])
Another approach is to agree on that the messages do not contain
newlines and terminate each message with a newline
(ASCII LF,
, 0x0a).
The server side would then use something like
a usual bufio.Scanner loop to get
full lines from the socket.
The remaining problem with your approach is to not dealing with
what Read on a socket returns: note that io.Reader.Read
(that's what sockets implement, among other things) is allowed
to return an error while having had read some data from the
underlying stream. In your toy example this might rightfully
be unimportant, but suppose that you're writing a wget-like
tool which is able to resume downloading of a file: even if
reading from the server returned some data and an error, you
have to deal with that returned chunk first and only then
handle the error.
Back to the problem at hand
The problem presented in the question, I beleive, happens simply because in your setup you hit some TCP buffering problem due to the tiny length of your messages.
On my box which runs Linux 4.9/amd64 two things reliably "fix"
the problem:
- Sending messages of 4000 bytes in length: the second call
to
Write "sees" the problem immediately.
- Doing more
Write calls.
For the former, try something like
msg := make([]byte, 4000)
for i := range msg {
msg[i] = 'x'
}
for {
_, err := conn.Write(msg)
...
and for the latter—something like
for {
_, err := conn.Write([]byte("hello"))
...
fmt.Println("ok")
time.Sleep(time.Second / 2)
}
(it's sensible to lower the pause between sending stuff in
both cases).
It's interesting to note that the former example hits the
write: connection reset by peer (ECONNRESET in POSIX)
error while the second one hits write: broken pipe
(EPIPE in POSIX).
This is because when we're sending in chunks worth 4k bytes,
some of the packets generated for the stream manage to become
"in flight" before the server's side of the connection manages
to propagate the information on its closure to the client,
and those packets hit an already closed socket and get rejected
with the RST TCP flag set.
In the second example an attempt to send another chunk of data
sees that the client side already knows that the connection
has been teared down and fails the sending without "touching
the wire".
TL;DR, the bottom line
Welcome to the wonderful world of networking. ;-)
I'd recommend buying a copy of "TCP/IP Illustrated",
read it and experiment.
TCP (and IP and other protocols above IP)
sometimes works not like people expect them to by applying
their "common sense".
