PLT-7407: Back-end plugin mechanism (#7177)

* begin backend plugin wip

* flesh out rpcplugin. everything done except for minor supervisor stubs

* done with basic plugin infrastructure

* simplify tests

* remove unused test lines

plugin/rpcplugin/ipc.go

@@ -0,0 +1,28 @@
+package rpcplugin
+
+import (
+	"io"
+	"os"
+)
+
+// Returns a new IPC for the parent process and a set of files to pass on to the child.
+//
+// The returned files must be closed after the child process is started.
+func NewIPC() (io.ReadWriteCloser, []*os.File, error) {
+	parentReader, childWriter, err := os.Pipe()
+	if err != nil {
+		return nil, nil, err
+	}
+	childReader, parentWriter, err := os.Pipe()
+	if err != nil {
+		parentReader.Close()
+		childWriter.Close()
+		return nil, nil, err
+	}
+	return NewReadWriteCloser(parentReader, parentWriter), []*os.File{childReader, childWriter}, nil
+}
+
+// Returns the IPC instance inherited by the process from its parent.
+func InheritedIPC(fd0, fd1 uintptr) (io.ReadWriteCloser, error) {
+	return NewReadWriteCloser(os.NewFile(fd0, ""), os.NewFile(fd1, "")), nil
+}