internal/fakenet: add a fake network connection

This adds the ability to make a net.Conn based on a
reader/writer pair.
This is primarily useful for pretending stdin/stdout are a network
connection, but can be used for any reader/writer pair.

Change-Id: Ib398e62e0fac423db04aed27cfc48070bda2b93b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/231697
Run-TryBot: Ian Cottrell <iancottrell@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

internal/fakenet/conn.go

@@ -0,0 +1,129 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fakenet
+
+import (
+	"io"
+	"net"
+	"sync"
+	"time"
+)
+
+// NewConn returns a net.Conn built on top of the supplied reader and writer.
+// It decouples the read and write on the conn from the underlying stream
+// to enable Close to abort ones that are in progress.
+// It's primary use is to fake a network connection from stdin and stdout.
+func NewConn(name string, in io.ReadCloser, out io.WriteCloser) net.Conn {
+	c := &fakeConn{
+		name:   name,
+		reader: newFeeder(in.Read),
+		writer: newFeeder(out.Write),
+		in:     in,
+		out:    out,
+	}
+	go c.reader.run()
+	go c.writer.run()
+	return c
+}
+
+type fakeConn struct {
+	name   string
+	reader *connFeeder
+	writer *connFeeder
+	in     io.ReadCloser
+	out    io.WriteCloser
+}
+
+type fakeAddr string
+
+// connFeeder serializes calls to the source function (io.Reader.Read or
+// io.Writer.Write) by delegating them to a channel. This also allows calls to
+// be intercepted when the connection is closed, and cancelled early if the
+// connection is closed while the calls are still outstanding.
+type connFeeder struct {
+	source func([]byte) (int, error)
+	input  chan []byte
+	result chan feedResult
+	mu     sync.Mutex
+	closed bool
+	done   chan struct{}
+}
+
+type feedResult struct {
+	n   int
+	err error
+}
+
+func (c *fakeConn) Close() error {
+	c.reader.close()
+	c.writer.close()
+	c.in.Close()
+	c.out.Close()
+	return nil
+}
+
+func (c *fakeConn) Read(b []byte) (n int, err error)   { return c.reader.do(b) }
+func (c *fakeConn) Write(b []byte) (n int, err error)  { return c.writer.do(b) }
+func (c *fakeConn) LocalAddr() net.Addr                { return fakeAddr(c.name) }
+func (c *fakeConn) RemoteAddr() net.Addr               { return fakeAddr(c.name) }
+func (c *fakeConn) SetDeadline(t time.Time) error      { return nil }
+func (c *fakeConn) SetReadDeadline(t time.Time) error  { return nil }
+func (c *fakeConn) SetWriteDeadline(t time.Time) error { return nil }
+func (a fakeAddr) Network() string                     { return "fake" }
+func (a fakeAddr) String() string                      { return string(a) }
+
+func newFeeder(source func([]byte) (int, error)) *connFeeder {
+	return &connFeeder{
+		source: source,
+		input:  make(chan []byte),
+		result: make(chan feedResult),
+		done:   make(chan struct{}),
+	}
+}
+
+func (f *connFeeder) close() {
+	f.mu.Lock()
+	if !f.closed {
+		f.closed = true
+		close(f.done)
+	}
+	f.mu.Unlock()
+}
+
+func (f *connFeeder) do(b []byte) (n int, err error) {
+	// send the request to the worker
+	select {
+	case f.input <- b:
+	case <-f.done:
+		return 0, io.EOF
+	}
+	// get the result from the worker
+	select {
+	case r := <-f.result:
+		return r.n, r.err
+	case <-f.done:
+		return 0, io.EOF
+	}
+}
+
+func (f *connFeeder) run() {
+	var b []byte
+	for {
+		// wait for an input request
+		select {
+		case b = <-f.input:
+		case <-f.done:
+			return
+		}
+		// invoke the underlying method
+		n, err := f.source(b)
+		// send the result back to the requester
+		select {
+		case f.result <- feedResult{n: n, err: err}:
+		case <-f.done:
+			return
+		}
+	}
+}