fix(errors)!: forbid interpolated details in wire-facing internal errors

server/crates/arbiter-server/src/utils.rs

@@ -14,3 +14,47 @@ impl<F: FnOnce()> Drop for DeferClosure<F> {
 pub fn defer<F: FnOnce()>(f: F) -> impl Drop + Sized {
     DeferClosure { f: Some(f) }
 }
+
+/// Renders an error together with its full `source` chain as `outer: inner: root`.
+///
+/// Error variants in this crate deliberately keep `Display` terse so that no
+/// internal detail can leak across the gRPC boundary. That same terseness would
+/// hide the cause in the logs, so use this for `tracing` fields, never in a
+/// wire payload.
+pub fn error_chain(err: &dyn core::error::Error) -> String {
+    let mut out = err.to_string();
+    let mut current = err.source();
+    while let Some(source) = current {
+        out.push_str(": ");
+        out.push_str(&source.to_string());
+        current = source.source();
+    }
+    out
+}
+
+#[cfg(test)]
+mod tests {
+    use super::error_chain;
+
+    #[derive(Debug, thiserror::Error)]
+    #[error("root")]
+    struct Root;
+
+    #[derive(Debug, thiserror::Error)]
+    #[error("middle")]
+    struct Middle(#[source] Root);
+
+    #[derive(Debug, thiserror::Error)]
+    #[error("outer")]
+    struct Outer(#[source] Middle);
+
+    #[test]
+    fn walks_the_whole_source_chain() {
+        assert_eq!(error_chain(&Root), "root", "a leaf error renders alone");
+        assert_eq!(
+            error_chain(&Outer(Middle(Root))),
+            "outer: middle: root",
+            "every source link must appear, in order"
+        );
+    }
+}