Introduce support for user-provided extensions

This change introduces support for discovering and executing
user-provided extensions to the program. Extensions are useful for
allowing users to provide additional functionality on top of the
nitrocli proper. Implementation wise we stick to an approach similar to
git or cargo subcommands in nature: we search the directories listed in
the PATH environment variable for a file that starts with "nitrocli-",
followed by the extension name. This file is then executed. It is
assumed that the extension recognizes (or at least not prohibits) the
following arguments: --nitrocli (providing the path to the nitrocli
binary), --model (with the model passed to the main program), and
--verbosity (the verbosity level).

1 files changed, 4 insertions, 0 deletions

diff --git a/src/args.rs b/src/args.rs
index 6cf37c8..a8f43e1 100644
--- a/src/args.rs
+++ b/src/args.rs
@@ -4,6 +4,7 @@
 // SPDX-License-Identifier: GPL-3.0-or-later
 
 use std::convert;
+use std::ffi;
 
 /// Provides access to a Nitrokey device
 #[derive(Debug, structopt::StructOpt)]
@@ -107,6 +108,9 @@ Command! {
     Status => crate::commands::status,
     /// Interacts with the device's unencrypted volume
     Unencrypted(UnencryptedArgs) => |ctx, args: UnencryptedArgs| args.subcmd.execute(ctx),
+    /// An extension and its arguments.
+    #[structopt(external_subcommand)]
+    Extension(Vec<ffi::OsString>) => crate::commands::extension,
   ]
 }