backend: web: Refactor error handling

Trying to make errors more ergonomic and idiomatic.  Requiring the
handler functions to return a two-tuple with the HTTP status and the
`Json` guard would have gotten cumbersome very quickly.  To make it
simpler, there are two new error enumerations:

* `Error` is a general-purpose type that can be returned from the
  service layer.  There is a variant for each type of problem that can
  occur.
* `ApiError` is specifically for the API layer, and implements the
  Rocket `Responder` trait.  This is where mapping errors to HTTP status
  codes takes place.

The `From` trait is implemented for `ApiError`, allowing `Error` values
returned from the service layer to be passed directly back from request
handlers using the `?` operator.  Additionally, the `Error` enum
implements `From` for JSON-RPC errors, essentially mapping any unhandled
error from the RPC layer to HTTP 503 Service Unavailable, indicating a
problem communicating with the daemon.

backend/src/server/context.rs

@@ -1,9 +1,7 @@
 use super::config::Config;
 use crate::rpc::gen_client::Client;
-use crate::server::error::{ApiError, ErrorResponse};
+use crate::server::error::Error;
 use jsonrpc_core_client::transports::ipc::connect;
-use rocket::http::Status as HttpStatus;
-use rocket::serde::json::Json;
 
 pub struct Context {
     config: Config,
@@ -18,16 +16,10 @@ impl Context {
         Self { config }
     }
 
-    pub async fn client(&self) -> Result<Client, ApiError> {
+    pub async fn client(&self) -> Result<Client, Error> {
         match connect(&self.config.socket).await {
             Ok(client) => Ok(client),
-            Err(e) => Err((
-                HttpStatus::ServiceUnavailable,
-                Json(ErrorResponse::new(format!(
-                    "Cannot connect to daemon: {}",
-                    e
-                ))),
-            )),
+            Err(e) => Err(format!("Cannot connect to daemon: {}", e)),
         }
     }
 

backend/src/server/error.rs

@@ -1,14 +1,13 @@
-use rocket::http::Status as HttpStatus;
+use jsonrpc_core_client::RpcError;
+use rocket::response::Responder;
 use rocket::serde::json::Json;
 use rocket::serde::Serialize;
 
-#[derive(Serialize)]
+#[derive(Debug, Serialize)]
 pub struct ErrorResponse {
     pub error: String,
 }
 
-pub type ApiError = (HttpStatus, Json<ErrorResponse>);
-
 impl ErrorResponse {
     pub fn new<S: Into<String>>(message: S) -> Self {
         Self {
@@ -17,6 +16,35 @@ impl ErrorResponse {
     }
 }
 
-pub fn error<S: Into<String>>(status: HttpStatus, message: S) -> ApiError {
-    (status, Json(ErrorResponse::new(message)))
+#[derive(Debug, Responder)]
+pub enum ApiError {
+    #[response(status = 503, content_type = "json")]
+    ServiceUnavailable(Json<ErrorResponse>),
+
+    #[response(status = 401, content_type = "json")]
+    AuthError(Json<ErrorResponse>),
+}
+
+pub enum Error {
+    ServiceUnavailable(String),
+    AuthError(String),
+}
+
+impl From<Error> for ApiError {
+    fn from(error: Error) -> Self {
+        match error {
+            Error::ServiceUnavailable(e) => {
+                Self::ServiceUnavailable(Json(ErrorResponse::new(e)))
+            }
+            Error::AuthError(e) => {
+                Self::AuthError(Json(ErrorResponse::new(e)))
+            }
+        }
+    }
+}
+
+impl From<RpcError> for Error {
+    fn from(error: RpcError) -> Self {
+        Self::ServiceUnavailable(error.to_string())
+    }
 }

backend/src/server/routes/auth.rs

@@ -1,8 +1,8 @@
 use super::super::context::Context;
-use super::super::error::{error, ApiError};
+use super::super::error::{ApiError, Error};
 use crate::models::auth::LoginResponse;
 use rocket::form::{Form, FromForm};
-use rocket::http::{Cookie, CookieJar, Status};
+use rocket::http::{Cookie, CookieJar};
 use rocket::serde::json::Json;
 use rocket::serde::Serialize;
 use rocket::State;
@@ -31,8 +31,8 @@ pub async fn login(
         .await
     {
         Ok(_) => {}
-        Err(e) => return Err(error(Status::Unauthorized, e.to_string())),
-    }
+        Err(e) => return Err(Error::AuthError(e.to_string()).into()),
+    };
     let cookie = AuthCookie {
         username: form.username.into(),
     };