Split jsonrpc2.go file into multiple files (#65)

This merge request moves some of the contents from the jsonrpc2.go file
into their own designated file. The new files being introduced
(excluding test files) are as follows:

* conn.go
* request.go
* response.go

The motive of this change is to make it easier to navigate the code.
Without this change, the jsonrpc2.go file is 813 lines of code.

conn_test.go

@@ -0,0 +1,103 @@
+package jsonrpc2_test
+
+import (
+	"context"
+	"io"
+	"net"
+	"testing"
+	"time"
+
+	"github.com/sourcegraph/jsonrpc2"
+)
+
+func TestConn_DisconnectNotify(t *testing.T) {
+
+	t.Run("EOF", func(t *testing.T) {
+		connA, connB := net.Pipe()
+		c := jsonrpc2.NewConn(context.Background(), jsonrpc2.NewPlainObjectStream(connB), nil)
+		// By closing connA, connB receives io.EOF
+		if err := connA.Close(); err != nil {
+			t.Error(err)
+		}
+		assertDisconnect(t, c, connB)
+	})
+
+	t.Run("Close", func(t *testing.T) {
+		_, connB := net.Pipe()
+		c := jsonrpc2.NewConn(context.Background(), jsonrpc2.NewPlainObjectStream(connB), nil)
+		if err := c.Close(); err != nil {
+			t.Error(err)
+		}
+		assertDisconnect(t, c, connB)
+	})
+
+	t.Run("Close async", func(t *testing.T) {
+		done := make(chan struct{})
+		_, connB := net.Pipe()
+		c := jsonrpc2.NewConn(context.Background(), jsonrpc2.NewPlainObjectStream(connB), nil)
+		go func() {
+			if err := c.Close(); err != nil && err != jsonrpc2.ErrClosed {
+				t.Error(err)
+			}
+			close(done)
+		}()
+		assertDisconnect(t, c, connB)
+		<-done
+	})
+
+	t.Run("protocol error", func(t *testing.T) {
+		connA, connB := net.Pipe()
+		c := jsonrpc2.NewConn(context.Background(), jsonrpc2.NewPlainObjectStream(connB), nil)
+		connA.Write([]byte("invalid json"))
+		assertDisconnect(t, c, connB)
+	})
+}
+
+func TestConn_Close(t *testing.T) {
+	t.Run("waiting for response", func(t *testing.T) {
+		connA, connB := net.Pipe()
+		nodeA := jsonrpc2.NewConn(
+			context.Background(),
+			jsonrpc2.NewPlainObjectStream(connA), noopHandler{},
+		)
+		defer nodeA.Close()
+		nodeB := jsonrpc2.NewConn(
+			context.Background(),
+			jsonrpc2.NewPlainObjectStream(connB),
+			noopHandler{},
+		)
+		defer nodeB.Close()
+
+		ready := make(chan struct{})
+		done := make(chan struct{})
+		go func() {
+			close(ready)
+			err := nodeB.Call(context.Background(), "m", nil, nil)
+			if err != jsonrpc2.ErrClosed {
+				t.Errorf("got error %v, want %v", err, jsonrpc2.ErrClosed)
+			}
+			close(done)
+		}()
+		// Wait for the request to be sent before we close the connection.
+		<-ready
+		if err := nodeB.Close(); err != nil && err != jsonrpc2.ErrClosed {
+			t.Error(err)
+		}
+		assertDisconnect(t, nodeB, connB)
+		<-done
+	})
+}
+
+func assertDisconnect(t *testing.T, c *jsonrpc2.Conn, conn io.Writer) {
+	select {
+	case <-c.DisconnectNotify():
+	case <-time.After(200 * time.Millisecond):
+		t.Fatal("no disconnect notification")
+	}
+	// Assert that conn is closed by trying to write to it.
+	_, got := conn.Write(nil)
+	want := io.ErrClosedPipe
+	if got != want {
+		t.Fatalf("got %q, want %q", got, want)
+	}
+}