Add mock signing backend for tests

lib/src/signing.rs

@@ -111,6 +111,33 @@ pub trait SigningBackend: Debug + Send + Sync {
     fn verify(&self, data: &[u8], signature: &[u8]) -> SignResult<Verification>;
 }
 
+#[derive(Debug)]
+#[allow(missing_docs)]
+pub struct MockSigningBackend {}
+
+impl SigningBackend for MockSigningBackend {
+    fn name(&self) -> &str {
+        "mock"
+    }
+    fn can_read(&self, signature: &[u8]) -> bool {
+        signature.starts_with(b"-----BEGIN MOCK SIGNATURE-----")
+    }
+    fn sign(&self, _: &[u8], _: Option<&str>) -> SignResult<Vec<u8>> {
+        Ok(r#"-----BEGIN MOCK SIGNATURE-----
+mock signature
+-----END MOCK SIGNATURE-----
+"#
+        .into())
+    }
+    fn verify(&self, _: &[u8], signature: &[u8]) -> SignResult<Verification> {
+        if signature.starts_with(b"-----BEGIN MOCK SIGNATURE-----") {
+            Ok(Verification::new(SigStatus::Good, None, None))
+        } else {
+            Ok(Verification::new(SigStatus::Bad, None, None))
+        }
+    }
+}
+
 /// An error type for the signing/verifying operations
 #[derive(Debug, Error)]
 pub enum SignError {
@@ -182,16 +209,23 @@ impl Signer {
             // Box::new(X509Backend::from_settings(settings)?) as Box<dyn SigningBackend>,
         ];
 
-        let main_backend = settings
-            .signing_backend()
-            .map(|backend| {
-                backends
-                    .iter()
-                    .position(|b| b.name() == backend)
-                    .map(|i| backends.remove(i))
-                    .ok_or(SignInitError::UnknownBackend(backend))
-            })
-            .transpose()?;
+        let signing_backend = settings.signing_backend();
+
+        // This mock signing backend is only really intended to be used for testing
+        let main_backend = if Some("mock".into()) == signing_backend {
+            let mock_backend = Box::new(MockSigningBackend {}) as Box<dyn SigningBackend>;
+            Some(mock_backend)
+        } else {
+            signing_backend
+                .map(|backend| {
+                    backends
+                        .iter()
+                        .position(|b| b.name() == backend)
+                        .map(|i| backends.remove(i))
+                        .ok_or(SignInitError::UnknownBackend(backend))
+                })
+                .transpose()?
+        };
 
         let show_signatures = settings
             .config()