bug: return 0 instead of -1 when error occurred on a write

[daynobug]

fix panic in case error not nil. m gets -1 when writes to a closed connection

[panjf2000]

I appreciate your effort here, but I don't think returning -1 is allowed, see io.WriterTo and io.Writer. If a Writer doesn't comply with the interface contract and returns -1, I'd rather raise a panic for it. That's also what the Go std libraries do currently, check this out.

[daynobug]

The -1 is actually returning by gnet itself.
I am writing a proxy by using gnet.Run and gnet.NewClient. Part of my code is kind of like that:

type ProxyContext struct {
	sync.Mutex
	frontend gnet.Conn
	backend  gnet.Conn
}

func (f *Frontend) OnTraffic(c gnet.Conn) gnet.Action {
	ctx := c.Context()
	proxyContext, o := ctx.(*ProxyContext)
	if !o || proxyContext == nil {
	}

	proxyContext.Lock()
	n, err := c.WriteTo(proxyContext.backend)
	if err != nil {
		log.Println(err)
		return gnet.Close
	}
	proxyContext.Unlock()
	log.Println("frontend write", n)

	return gnet.None
}

The frontend is using gnet.Conn::WriteTo to write data to the backend server, but the backend server has already closed. And then It just raised panic.
So I found gnet.Conn::Write is using gnet.Conn::write, and the gnet.Conn::write returns -1 and an error when the connection has closed.

gnet/connection_unix.go

Line 152 in f5e5ef9

return -1, os.NewSyscallError("write", err)

gnet/connection_unix.go

Line 364 in f5e5ef9

return c.write(p)

panic here because of the m is -1. panic: runtime error: slice bounds out of range [-1:]

gnet/connection_unix.go

Line 387 in f5e5ef9

c.buffer = c.buffer[m:]

At the same time, I found gnet.Conn::writev returns -1 too.

gnet/connection_unix.go

Line 186 in f5e5ef9

return -1, os.NewSyscallError("writev", err)

Maybe I should change both of the -1 to 0.

[panjf2000]

Returning -1 from a Write-like method is definitely a mistake and we should fix that kind of case.

[daynobug]

done

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 79.08%. Comparing base (f5e5ef9) to head (cb1207e).

Files	Patch %	Lines
connection_unix.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #569   +/-   ##
=======================================
  Coverage   79.08%   79.08%           
=======================================
  Files          25       25           
  Lines        2109     2109           
=======================================
  Hits         1668     1668           
  Misses        305      305           
  Partials      136      136           

Flag	Coverage Δ
unittests	79.08% <0.00%> (ø)

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[panjf2000]

Thanks!