Improve error messages with exec, especially if working dir missing (#…

…236)

Fixes #235

internals/daemon/api_exec.go

@@ -64,7 +64,20 @@ func v1PostExec(c *Command, req *http.Request, _ *userState) Response {
 	// Check up-front that the executable exists.
 	_, err := exec.LookPath(payload.Command[0])
 	if err != nil {
-		return statusBadRequest("%v", err)
+		return statusBadRequest("cannot find executable %q", payload.Command[0])
+	}
+
+	// Also check that the working directory exists, to avoid a confusing
+	// error message later that implies the command doesn't exist:
+	//
+	//  fork/exec /usr/local/bin/realcommand: no such file or directory
+	//
+	// Note that this check still doesn't check that the permissions are
+	// correct to use it as a working directory, but this is a good start.
+	if payload.WorkingDir != "" {
+		if !osutil.IsDir(payload.WorkingDir) {
+			return statusBadRequest("cannot find working directory %q", payload.WorkingDir)
+		}
 	}
 
 	// Convert User/UserID and Group/GroupID combinations into raw uid/gid.

internals/daemon/api_exec_test.go

@@ -336,7 +336,7 @@ func (s *execSuite) TestCommandNotFound(c *C) {
 	c.Check(httpResp.StatusCode, Equals, http.StatusBadRequest)
 	c.Check(execResp.StatusCode, Equals, http.StatusBadRequest)
 	c.Check(execResp.Type, Equals, "error")
-	c.Check(execResp.Result["message"], Matches, ".*executable file not found.*")
+	c.Check(execResp.Result["message"], Matches, "cannot find executable .*")
 }
 
 func (s *execSuite) TestUserGroupError(c *C) {