slapi: defend against incomplete server response for bookmarks

Summary:
Sometimes the request:

  api.bookmarks(['master'])

got a HTTP 200 response that decodes to:

  {}

In some code paths, this is then interpreted as `{'master': None}`, meaning the
deletion of the `master` bookmark. That's not ideal. So let's defend against
the error by checking the response length.

Normally, the server reports all bookmark names as requested with values
being either `None` or the commit hash.

Reviewed By: muirdm

Differential Revision: D65855783

fbshipit-source-id: 304ba8e5d72bfe52fd215cf67002a4f11f9005b4

eden/scm/lib/edenapi/src/client.rs

@@ -1410,16 +1410,29 @@ impl SaplingRemoteApi for Client {
         &self,
         bookmarks: Vec<String>,
     ) -> Result<Vec<BookmarkEntry>, SaplingRemoteApiError> {
+        let request_len = bookmarks.len();
         tracing::info!("Requesting {} bookmarks", bookmarks.len());
         let url = self.build_url(paths::BOOKMARKS)?;
         let bookmark_req = BookmarkRequest { bookmarks };
         self.log_request(&bookmark_req, "bookmarks");
+        let bookmarks_wire = bookmark_req.to_wire();
         let req = self
             .configure_request(self.inner.client.post(url))?
-            .cbor(&bookmark_req.to_wire())
+            .cbor(&bookmarks_wire)
             .map_err(SaplingRemoteApiError::RequestSerializationFailed)?;
 
-        self.fetch_vec_with_retry::<BookmarkEntry>(vec![req]).await
+        let response = self
+            .fetch_vec_with_retry::<BookmarkEntry>(vec![req])
+            .await?;
+        if response.len() != request_len {
+            let bookmarks = bookmarks_wire.bookmarks;
+            let message = format!(
+                "Requested bookmarks {:?} but only got {:?}.",
+                bookmarks, &response
+            );
+            return Err(SaplingRemoteApiError::IncompleteResponse(message));
+        }
+        Ok(response)
     }
 
     async fn set_bookmark(

eden/scm/lib/edenapi/trait/src/errors.rs

@@ -48,6 +48,8 @@ pub enum SaplingRemoteApiError {
     NotSupported,
     #[error(transparent)]
     MissingCerts(#[from] auth::MissingCerts),
+    #[error("IncompeteResponse: {0}")]
+    IncompleteResponse(String),
 }
 
 #[derive(Debug, Error)]
@@ -133,6 +135,7 @@ impl SaplingRemoteApiError {
             | HttpError { .. }
             | ServerError(_)
             | NoResponse
+            | IncompleteResponse(_)
             | ParseResponse(_)
             | Other(_) => true,