age-plugin: Make arguments to run_state_machine optional

Closes str4d/rage#387.

age-plugin/CHANGELOG.md

@@ -11,6 +11,8 @@ to 1.0.0 are beta releases.
 ## [Unreleased]
 ### Changed
 - MSRV is now 1.65.0.
+- `age_plugin::run_state_machine` now takes optional arguments, to enable the
+  creation of recipient-only or identity-only plugins.
 
 ## [0.4.0] - 2022-10-27
 ### Changed

age-plugin/examples/age-plugin-unencrypted.rs

@@ -152,7 +152,11 @@ fn main() -> io::Result<()> {
     let opts = PluginOptions::parse_args_default_or_exit();
 
     if let Some(state_machine) = opts.age_plugin {
-        run_state_machine(&state_machine, || RecipientPlugin, || IdentityPlugin)
+        run_state_machine(
+            &state_machine,
+            Some(|| RecipientPlugin),
+            Some(|| IdentityPlugin),
+        )
     } else {
         // A real plugin would generate a new identity here.
         print_new_identity(PLUGIN_NAME, &[], &[]);

age-plugin/src/lib.rs

@@ -147,8 +147,8 @@
 //!         // The plugin was started by an age client; run the state machine.
 //!         run_state_machine(
 //!             &state_machine,
-//!             || RecipientPlugin,
-//!             || IdentityPlugin,
+//!             Some(|| RecipientPlugin),
+//!             Some(|| IdentityPlugin),
 //!         )?;
 //!         return Ok(());
 //!     }
@@ -213,14 +213,32 @@ pub fn print_new_identity(plugin_name: &str, identity: &[u8], recipient: &[u8])
 /// argument when starting the plugin.
 pub fn run_state_machine<R: recipient::RecipientPluginV1, I: identity::IdentityPluginV1>(
     state_machine: &str,
-    recipient_v1: impl FnOnce() -> R,
-    identity_v1: impl FnOnce() -> I,
+    recipient_v1: Option<impl FnOnce() -> R>,
+    identity_v1: Option<impl FnOnce() -> I>,
 ) -> io::Result<()> {
     use age_core::plugin::{IDENTITY_V1, RECIPIENT_V1};
 
     match state_machine {
-        RECIPIENT_V1 => recipient::run_v1(recipient_v1()),
-        IDENTITY_V1 => identity::run_v1(identity_v1()),
+        RECIPIENT_V1 => {
+            if let Some(plugin) = recipient_v1 {
+                recipient::run_v1(plugin())
+            } else {
+                Err(io::Error::new(
+                    io::ErrorKind::InvalidInput,
+                    "plugin doesn't support recipient-v1 state machine",
+                ))
+            }
+        }
+        IDENTITY_V1 => {
+            if let Some(plugin) = identity_v1 {
+                identity::run_v1(plugin())
+            } else {
+                Err(io::Error::new(
+                    io::ErrorKind::InvalidInput,
+                    "plugin doesn't support identity-v1 state machine",
+                ))
+            }
+        }
         _ => Err(io::Error::new(
             io::ErrorKind::InvalidInput,
             "unknown plugin state machine",