PLT-7407: Back-end plugins (#7409)

* tie back-end plugins together

* fix comment typo

* add tests and a bit of polish

* tests and polish

* add test, don't let backend executable paths escape the plugin directory

plugin/rpcplugin/io_test.go

@@ -0,0 +1,73 @@
+package rpcplugin
+
+import (
+	"io/ioutil"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestNewAsyncReadCloser(t *testing.T) {
+	rf, w, err := os.Pipe()
+	require.NoError(t, err)
+	r := NewAsyncReadCloser(rf)
+	defer r.Close()
+
+	go func() {
+		w.Write([]byte("foo"))
+		w.Close()
+	}()
+
+	foo, err := ioutil.ReadAll(r)
+	require.NoError(t, err)
+	assert.Equal(t, "foo", string(foo))
+}
+
+func TestNewAsyncReadCloser_CloseDuringRead(t *testing.T) {
+	rf, w, err := os.Pipe()
+	require.NoError(t, err)
+	defer w.Close()
+
+	r := NewAsyncReadCloser(rf)
+
+	go func() {
+		time.Sleep(time.Millisecond * 200)
+		r.Close()
+	}()
+	r.Read(make([]byte, 10))
+}
+
+func TestNewAsyncWriteCloser(t *testing.T) {
+	r, wf, err := os.Pipe()
+	require.NoError(t, err)
+	w := NewAsyncWriteCloser(wf)
+	defer w.Close()
+
+	go func() {
+		foo, err := ioutil.ReadAll(r)
+		require.NoError(t, err)
+		assert.Equal(t, "foo", string(foo))
+		r.Close()
+	}()
+
+	n, err := w.Write([]byte("foo"))
+	require.NoError(t, err)
+	assert.Equal(t, 3, n)
+}
+
+func TestNewAsyncWriteCloser_CloseDuringWrite(t *testing.T) {
+	r, wf, err := os.Pipe()
+	require.NoError(t, err)
+	defer r.Close()
+
+	w := NewAsyncWriteCloser(wf)
+
+	go func() {
+		time.Sleep(time.Millisecond * 200)
+		w.Close()
+	}()
+	w.Write(make([]byte, 10))
+}