How does golang's net.Conn.Read when to stop reading? - sockets

Question

I'm trying to write a simple sockets based go server. I'm just wondering how does the connection.Read below knows when to stop reading.
(Note: this is not my code, I copied it from Unix Sockets in Go as example)
package main
import (
"log"
"net"
)
func echoServer(c net.Conn) {
for {
buf := make([]byte, 512)
nr, err := c.Read(buf)
if err != nil {
return
}
data := buf[0:nr]
println("Server got:", string(data))
_, err = c.Write(data)
if err != nil {
log.Fatal("Write: ", err)
}
}
}
func main() {
l, err := net.Listen("unix", "/tmp/echo.sock")
if err != nil {
log.Fatal("listen error:", err)
}
for {
fd, err := l.Accept()
if err != nil {
log.Fatal("accept error:", err)
}
go echoServer(fd)
}
}
Is it the EOF character or there's something else?
It would be really helpful if someone can point me to a link official go docs. Thanks.

Answer 1

This is the implementation of the default Read method on net.Conn.Read:
// Read implements the Conn Read method.
func (c *conn) Read(b []byte) (int, error) {
if !c.ok() {
return 0, syscall.EINVAL
}
n, err := c.fd.Read(b)
if err != nil && err != io.EOF {
err = &OpError{Op: "read", Net: c.fd.net, Source: c.fd.laddr, Addr: c.fd.raddr, Err: err}
}
return n, err
}
This is the implementation of the c.fd.Read(b) that is called within the function above:
func (fd *netFD) Read(p []byte) (n int, err error) {
if err := fd.readLock(); err != nil {
return 0, err
}
defer fd.readUnlock()
if len(p) == 0 {
// If the caller wanted a zero byte read, return immediately
// without trying. (But after acquiring the readLock.) Otherwise
// syscall.Read returns 0, nil and eofError turns that into
// io.EOF.
// TODO(bradfitz): make it wait for readability? (Issue 15735)
return 0, nil
}
if err := fd.pd.prepareRead(); err != nil {
return 0, err
}
if fd.isStream && len(p) > 1<<30 {
p = p[:1<<30]
}
for {
n, err = syscall.Read(fd.sysfd, p)
if err != nil {
n = 0
if err == syscall.EAGAIN {
if err = fd.pd.waitRead(); err == nil {
continue
}
}
}
err = fd.eofError(n, err)
break
}
if _, ok := err.(syscall.Errno); ok {
err = os.NewSyscallError("read", err)
}
return
}
So, yes, an EOF will make it stop reading. But so will plenty of other non-nil errors.

Answer 2

It will stop reading when its underlying implementation hits any error.
Error may be an actual I/O error, or it could be the operating system signaling connection closed with io.EOF, or it could be a timeout, so on.