When exec'ing dockerd, look for it in the same directory as the docker binary first, before checking path.

Signed-off-by: Daniel Nephin <dnephin@docker.com>

daemon_unix.go

@@ -5,27 +5,40 @@ package main
 import (
 	"os"
 	"os/exec"
+	"path/filepath"
 	"syscall"
 )
 
 // CmdDaemon execs dockerd with the same flags
-// TODO: add a deprecation warning?
 func (p DaemonProxy) CmdDaemon(args ...string) error {
 	// Use os.Args[1:] so that "global" args are passed to dockerd
 	args = stripDaemonArg(os.Args[1:])
 
-	// TODO: check dirname args[0] first
+	binaryPath, err := findDaemonBinary()
-	binaryAbsPath, err := exec.LookPath(daemonBinary)
 	if err != nil {
 		return err
 	}
 
 	return syscall.Exec(
-		binaryAbsPath,
+		binaryPath,
 		append([]string{daemonBinary}, args...),
 		os.Environ())
 }
 
+// findDaemonBinary looks for the path to the dockerd binary starting with
+// the directory of the current executable (if one exists) and followed by $PATH
+func findDaemonBinary() (string, error) {
+	execDirname := filepath.Dir(os.Args[0])
+	if execDirname != "" {
+		binaryPath := filepath.Join(execDirname, daemonBinary)
+		if _, err := os.Stat(binaryPath); err == nil {
+			return binaryPath, nil
+		}
+	}
+
+	return exec.LookPath(daemonBinary)
+}
+
 // stripDaemonArg removes the `daemon` argument from the list
 func stripDaemonArg(args []string) []string {
 	for i, arg := range args {