keymgr: Create a KeyStoreError trait for keystore errors.

The new `BoxedError` type will replace `tor_keymgr::Error`.

Part of #901

Cargo.lock

@@ -4522,6 +4522,7 @@ name = "tor-keymgr"
 version = "0.1.0"
 dependencies = [
  "derive_more",
+ "dyn-clone",
  "fs-mistrust",
  "ssh-key",
  "thiserror",

crates/tor-keymgr/Cargo.toml

@@ -23,6 +23,7 @@ __is_experimental = []
 
 [dependencies]
 derive_more = "0.99.3"
+dyn-clone = "1.0.11"
 fs-mistrust = { path = "../fs-mistrust", version = "0.7.1", features = ["serde", "walkdir"] }
 ssh-key = { version = "0.5.1", features = ["std"] }
 thiserror = "1"

crates/tor-keymgr/src/err.rs

@@ -4,11 +4,44 @@ use tor_error::{ErrorKind, HasKind};
 #[cfg(feature = "keymgr")]
 use {crate::key_type::ssh::SshKeyAlgorithm, crate::KeyType};
 
+use dyn_clone::DynClone;
 use thiserror::Error;
 
 #[cfg(feature = "keymgr")]
 use {std::io, std::path::PathBuf, std::sync::Arc};
 
+use std::error::Error as StdError;
+use std::fmt;
+
+/// An Error type for this crate.
+// TODO hs: replace Error with BoxedError
+#[allow(unreachable_pub)]
+pub type BoxedError = Box<dyn KeystoreError>;
+
+/// An error returned by a [`KeyStore`](crate::KeyStore).
+// TODO hs: replace Error with KeyStoreError and create an `ArtiNativeKeyStoreError: KeyStoreError`
+// type for ArtiNativeKeyStore.
+pub trait KeystoreError:
+    HasKind + AsRef<dyn StdError> + DynClone + fmt::Debug + fmt::Display + Send + Sync + 'static
+{
+}
+
+// Generate a Clone impl for Box<dyn KeystoreError>
+dyn_clone::clone_trait_object!(KeystoreError);
+
+impl<K: KeystoreError + Send + Sync> From<K> for BoxedError {
+    fn from(k: K) -> Self {
+        Box::new(k)
+    }
+}
+
+impl StdError for BoxedError {
+    fn source(&self) -> Option<&(dyn StdError + 'static)> {
+        let e: &dyn StdError = self;
+        e.source()
+    }
+}
+
 /// A key store error.
 //
 // TODO hs: refactor this error type.

crates/tor-keymgr/src/lib.rs

@@ -53,7 +53,7 @@ mod mgr;
 #[cfg(not(feature = "keymgr"))]
 mod dummy;
 
-pub use err::Error;
+pub use err::{Error, KeystoreError};
 pub use key_specifier::{ArtiPath, CTorPath, KeySpecifier};
 
 #[cfg(feature = "keymgr")]