Add error handling / propagation

- A handler function, instead of returning a Warp reply/rejection, returns a `Result<Reply, ApiError>.` - This is because rejections are meant to say "this filter can't handle this request, but maybe some other can" (see seanmonstar/warp#388 (comment)). - A rejection means Warp will fall through to another filter and ultimately hit a rejection handler, with people reporting rejections take way too long to process with more routes. - In our case, the error in our handler function is final and we also would like to be able to use the ? operator to bail out of the handler if an error exists, which using a Result type handles for us. - ApiError knows how to convert itself to an HTTP response + status code (error-specific), allowing us to implement Reply for ApiError. - We can't implement `Reply` for `Result<Reply, Reply>` (we didn't make the `Result` type), so we have to add a final function `into_response` that converts our `Result` into a Response. We won't need to do this when seanmonstar/warp#909 is merged: ``` .then(my_handler_func) .map(into_response) ```
splitgraph · Aug 17, 2022 · 7492884 · 7492884
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diff --git a/src/frontend/http_utils.rs b/src/frontend/http_utils.rs
@@ -0,0 +1,82 @@
+// Warp error handling and propagation
+// Courtesy of https://github.com/seanmonstar/warp/pull/909#issuecomment-1184854848
+//
+// Usage:
+//
+//   1) A handler function, instead of returning a Warp reply/rejection, returns a
+//   `Result<Reply, ApiError>.`
+//
+//   This is because rejections are meant to say "this filter can't handle this request, but maybe
+//   some other can" (see https://github.com/seanmonstar/warp/issues/388#issuecomment-576453485).
+//   A rejection means Warp will fall through to another filter and ultimately hit a rejection
+//   handler, with people reporting rejections take way too long to process with more routes.
+//
+//   In our case, the error in our handler function is final and we also would like to be able
+//   to use the ? operator to bail out of the handler if an error exists, which using a Result type
+//   handles for us.
+//
+//   2) ApiError knows how to convert itself to an HTTP response + status code (error-specific), allowing
+//   us to implement Reply for ApiError.
+//
+//   3) We can't implement Reply for Result<Reply, Reply> (we don't control Result), so we have to
+//   add a final function `into_response` that converts our Result into a Response. We won't need
+//   to do this when https://github.com/seanmonstar/warp/pull/909 is merged:
+//
+//   ```
+//   .then(my_handler_func)
+//   .map(into_response)
+//   ```
+//
+
+use datafusion::error::DataFusionError;
+
+use warp::hyper::{Body, Response, StatusCode};
+use warp::Reply;
+
+#[derive(Debug)]
+pub enum ApiError {
+    Forbidden,
+    DataFusionError(DataFusionError),
+    HashMismatch(String, String),
+    NotReadOnlyQuery,
+}
+
+// Wrap DataFusion errors so that we can automagically return an
+// `ApiError(DataFusionError)` by using the `?` operator
+impl From<DataFusionError> for ApiError {
+    fn from(err: DataFusionError) -> Self {
+        ApiError::DataFusionError(err)
+    }
+}
+
+impl ApiError {
+    fn status_code_body(self: ApiError) -> (StatusCode, String) {
+        match self {
+            ApiError::Forbidden => (StatusCode::FORBIDDEN, "FORBIDDEN".to_string()),
+            // TODO: figure out which DF errors to propagate, we have ones that are the server's fault
+            // here too (e.g. ResourcesExhaused) and potentially some that leak internal information
+            // (e.g. ObjectStore?)
+            ApiError::DataFusionError(e) => (StatusCode::BAD_REQUEST, e.to_string()),
+            // Mismatched hash
+            ApiError::HashMismatch(expected, got) => (StatusCode::BAD_REQUEST, format!("Invalid hash: expected {0:?}, got {1:?}. Resend your query with {0:?}", expected, got)),
+            ApiError::NotReadOnlyQuery => (StatusCode::METHOD_NOT_ALLOWED, "NOT_READ_ONLY_QUERY".to_string()),
+        }
+    }
+}
+
+impl Reply for ApiError {
+    fn into_response(self) -> Response<Body> {
+        let (status, body) = self.status_code_body();
+        Response::builder()
+            .status(status)
+            .body(body.into())
+            .expect("Could not construct Response")
+    }
+}
+
+pub fn into_response<S: Reply, E: Reply>(reply_res: Result<S, E>) -> Response<Body> {
+    match reply_res {
+        Ok(resp) => resp.into_response(),
+        Err(err) => err.into_response(),
+    }
+}
diff --git a/src/frontend/mod.rs b/src/frontend/mod.rs
@@ -1,3 +1,4 @@
 pub mod http;
+pub mod http_utils;
 #[cfg(feature = "frontend-postgres")]
 pub mod postgres;