Add release-grade logic for finding legacy Vagrant

After lots of experimentation I have landed on this as my proposal for
how we have our Go binary find its Ruby counterpart: just have it grab
it from the $PATH! @evanphx showed me this neat trick where by borrowing
a couple of helper methods from `exec` and tweaking them we can get
logic that will do a $PATH lookup that excludes "ourself". This allows
us to have both `vagrant` executables on the path... and means that
switching between Gogo-by-default or Legacy-by-default is just a matter
of tweaking $PATH order.

It _also_ means that we don't need any different lookup logic for
"release mode" vs "development mode" which is what I was looking at
before this solution.

In order to continue to facilitate development, I've generated a binstub
for vagrant using `bundle binstubs vagrant --standalone --path
./binstubs`, and I've updated the Nix development setup to prepend this
directory to the $PATH.

NOTE: Non-Nix users will need to modify their $PATH in the same way to
get the same behavior in development.

binstubs/vagrant

@@ -0,0 +1,14 @@
+#!/usr/bin/env ruby
+#
+# This file was generated by Bundler.
+#
+# The application 'vagrant' is installed as part of a gem, and
+# this file is here to facilitate running it.
+#
+
+require "pathname"
+path = Pathname.new(__FILE__)
+$:.unshift File.expand_path "../..", path.realpath
+
+require "bundler/setup"
+load File.expand_path "../../bin/vagrant", path.realpath

internal/client/server.go

@@ -2,11 +2,12 @@ package client
 
 import (
 	"context"
+	"io/fs"
 	"net"
 	"os"
 	"os/exec"
 	"path/filepath"
-	"runtime"
+	"strings"
 	"time"
 
 	"github.com/golang/protobuf/ptypes/empty"
@@ -180,22 +181,20 @@ func (c *Client) initLocalServer(ctx context.Context) (_ *grpc.ClientConn, err e
 	return client.Conn(), nil
 }
 
+// initVagrantRubyRuntime launches legacy vagrant as a gRPC server using the
+// "serve" command.
+//
+// NOTE: We are assuming that the first executable we find in $PATH that is not
+// _us_ is the legacy vagrant executable. It's up the packaging to ensure
+// that is how things are set up.
 func (c *Client) initVagrantRubyRuntime() (rubyRuntime plugin.ClientProtocol, err error) {
-	// TODO: Update for actual release usage. This is dev only now.
-	_, this_dir, _, _ := runtime.Caller(0)
-	cmd := exec.Command(
-		"bundle", "exec", "vagrant", "serve",
-	)
-	level := os.Getenv("VAGRANT_LOG")
-	cmd.Env = []string{
-		"BUNDLE_GEMFILE=" + filepath.Join(this_dir, "../../..", "Gemfile"),
-		"VAGRANT_I_KNOW_WHAT_IM_DOING_PLEASE_BE_QUIET=true",
-		"VAGRANT_LOG=" + level,
-		"VAGRANT_LOG_FILE=/tmp/vagrant.log",
+	var vagrantPath string
+	vagrantPath, err = lookPathSkippingSelf("vagrant")
+	if err != nil {
+		return
 	}
-
 	config := serverclient.RubyVagrantPluginConfig(c.logger)
-	config.Cmd = cmd
+	config.Cmd = exec.Command(vagrantPath, "serve")
 	rc := plugin.NewClient(config)
 	if _, err = rc.Start(); err != nil {
 		return
@@ -235,3 +234,51 @@ func (c *Client) negotiateApiVersion(ctx context.Context) error {
 	c.logger.Info("negotiated api version", "version", vsn)
 	return nil
 }
+
+// lookPathSkippingSelf is a copy of exec.LookPath modified to skip the
+// currently running executable.
+func lookPathSkippingSelf(file string) (string, error) {
+	myselfPath, err := os.Executable()
+	if err != nil {
+		return "", err
+	}
+	myself, err := os.Stat(myselfPath)
+	if err != nil {
+		return "", err
+	}
+	if strings.Contains(file, "/") {
+		err := findExecutable(file, myself)
+		if err == nil {
+			return file, nil
+		}
+		return "", &exec.Error{Name: file, Err: err}
+	}
+	path := os.Getenv("PATH")
+	for _, dir := range filepath.SplitList(path) {
+		if dir == "" {
+			// Unix shell semantics: path element "" means "."
+			dir = "."
+		}
+		path := filepath.Join(dir, file)
+		if err := findExecutable(path, myself); err == nil {
+			return path, nil
+		}
+	}
+	return "", &exec.Error{Name: file, Err: exec.ErrNotFound}
+}
+
+// findExecutableSkippingSelf is a copy of exec.findExecutable modified to skip
+// the provided FileInfo. It's used to power lookPathSkippingSelf.
+func findExecutable(file string, skip os.FileInfo) error {
+	d, err := os.Stat(file)
+	if err != nil {
+		return err
+	}
+	if os.SameFile(d, skip) {
+		return fs.ErrPermission
+	}
+	if m := d.Mode(); !m.IsDir() && m&0111 != 0 {
+		return nil
+	}
+	return fs.ErrPermission
+}

nix/vagrant.nix

@@ -61,9 +61,13 @@ mkShell rec {
     zlib
   ];
 
-  # workaround for npm/gulp dep compilation
-  # https://github.com/imagemin/optipng-bin/issues/108
   shellHook = ''
+    # workaround for npm/gulp dep compilation
+    # https://github.com/imagemin/optipng-bin/issues/108
     LD=$CC
+
+    # Prepend binstubs to PATH for development, which causes Vagrant-agogo
+    # to use the legacy Vagrant in this repo. See client.initVagrantRubyRuntime
+    PATH=$PWD/binstubs:$PATH
   '';
 }