Reject core cache body ranges where the end precedes the start (#1182)

Co-authored-by: Sy Brand <[email protected]>

Author: cceckman-at-fastly

runtime/fastly/builtins/cache-core.cpp

@@ -612,6 +612,14 @@ bool CacheEntry::body(JSContext *cx, unsigned argc, JS::Value *vp) {
       }
       options.end = JS::ToUint64(end);
     }
+
+    // Reject cases where the start is greater than the end.
+    // Ideally this would be a host-side check... but we didn't do it there to begin with,
+    // so we couple it to an SDK/runtime upgrade.
+    if (!start_val.isUndefined() && !end_val.isUndefined() && options.end > options.start) {
+      JS_ReportErrorASCII(cx, "end field is before the start field");
+      return false;
+    }
   }
 
   auto res = handle.get_body(options);