Use proper user-facing error message with unexpected Zulip inputs

Author: Urgau

src/interactions.rs

@@ -24,15 +24,20 @@ impl<'a> ErrorComment<'a> {
         }
     }
 
-    pub async fn post(&self, client: &GithubClient) -> anyhow::Result<Comment> {
+    pub fn markdown(message: &str) -> anyhow::Result<String> {
         let mut body = String::new();
-        writeln!(body, "**Error**: {}", self.message)?;
+        writeln!(body, "**Error**: {message}")?;
         writeln!(body)?;
         writeln!(
             body,
             "Please file an issue on GitHub at [triagebot](https://github.com/rust-lang/triagebot) if there's \
             a problem with this bot, or reach out on [#t-infra](https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra) on Zulip."
         )?;
+        Ok(body)
+    }
+
+    pub async fn post(&self, client: &GithubClient) -> anyhow::Result<Comment> {
+        let body = Self::markdown(&self.message)?;
         self.issue.post_comment(client, &body).await
     }
 }

src/zulip.rs

@@ -12,6 +12,7 @@ use crate::handlers::Context;
 use crate::handlers::docs_update::docs_update;
 use crate::handlers::pr_tracking::get_assigned_prs;
 use crate::handlers::project_goals::{self, ping_project_goals_owners};
+use crate::interactions::ErrorComment;
 use crate::utils::pluralize;
 use crate::zulip::api::{MessageApiResponse, Recipient};
 use crate::zulip::client::ZulipClient;
@@ -101,7 +102,13 @@ pub async fn webhook(
         Ok(req) => req,
         Err(rejection) => {
             tracing::error!(?rejection);
-            return rejection.into_response();
+            return Json(Response {
+                content: ErrorComment::markdown(
+                    "unable to handle this Zulip request: invalid JSON input",
+                )
+                .expect("creating a error message without fail"),
+            })
+            .into_response();
         }
     };