1 files changed, 44 insertions, 0 deletions
diff --git a/config/initializers/enumerator_next_patch.rb b/config/initializers/enumerator_next_patch.rb
new file mode 100644
index 00000000000..e1fc04731ae
--- /dev/null
+++ b/config/initializers/enumerator_next_patch.rb
@@ -0,0 +1,44 @@
+# frozen_string_literal: true
+
+# Monkey patch of Enumerator#next to get better stack traces
+# when an error is raised from within a Fiber.
+# https://bugs.ruby-lang.org/issues/16829
+module EnumeratorNextPatch
+  %w(next next_values peek peek_values).each do |name|
+    define_method(name) do |*args|
+      gitlab_patch_backtrace_marker { super(*args) }
+    rescue Exception => err # rubocop: disable Lint/RescueException
+      err.set_backtrace(err.backtrace + caller) unless
+        has_gitlab_patch_backtrace_marker?(err.backtrace) && backtrace_matches_caller?(err.backtrace)
+
+      raise
+    end
+  end
+
+  private
+
+  def gitlab_patch_backtrace_marker
+    yield
+  end
+
+  # This function tells us whether the exception was generated by #next itself or by something in
+  # the Fiber that it invokes. If it's generated by #next, then the backtrace will have
+  # #gitlab_patch_backtrace_marker as the third item down the trace (since
+  # #gitlab_patch_backtrace_marker calls a block, which in turn calls #next.) If it's generated
+  # by the Fiber that #next invokes, then it won't contain this marker.
+  def has_gitlab_patch_backtrace_marker?(backtrace)
+    match = %r(^(.*):[0-9]+:in `gitlab_patch_backtrace_marker'$).match(backtrace[2])
+
+    !!match && match[1] == __FILE__
+  end
+
+  # This function makes sure that the rest of the stack trace matches in order to avoid missing
+  # an exception that was generated by calling #next on another Enumerator inside the Fiber.
+  # This might miss some *very* contrived scenarios involving recursion, but exceptions don't
+  # provide Fiber information, so it's the best we can do.
+  def backtrace_matches_caller?(backtrace)
+    backtrace[3..] == caller[1..]
+  end
+end
+
+Enumerator.prepend(EnumeratorNextPatch)