Merge pull request rails#55003 from Shopify/amomchilov/report-sets-cause

`ActiveSupport::ErrorReporter#report` assigns a cause to unraised exceptions

Author: byroot

activesupport/lib/active_support/error_reporter.rb

@@ -276,14 +276,12 @@ def report(error, handled: true, severity: handled ? :warning : :error, context:
 
     private
       def ensure_backtrace(error)
-        return if error.frozen? # re-raising won't add a backtrace
+        return if error.frozen? # re-raising won't add a backtrace or set the cause
         return unless error.backtrace.nil?
 
         begin
-          # We could use Exception#set_backtrace, but until Ruby 3.4
-          # it only support setting `Exception#backtrace` and not
-          # `Exception#backtrace_locations`. So raising the exception
-          # is a good way to build a real backtrace.
+          # As of Ruby 3.4, we could use Exception#set_backtrace to set the backtrace,
+          # but there's nothing like Exception#set_cause. Raising+rescuing is the only way to set the cause.
           raise error
         rescue error.class => error
         end

activesupport/test/error_reporter_test.rb

@@ -166,9 +166,21 @@ class ErrorReporterTest < ActiveSupport::TestCase
     assert_nil error.backtrace
     assert_nil error.backtrace_locations
 
-    assert_nil @reporter.report(error)
-    assert_not_predicate error.backtrace, :empty?
-    assert_not_predicate error.backtrace_locations, :empty?
+    @reporter.report(error)
+
+    assert error.backtrace.first.start_with?(__FILE__)
+    assert_equal __FILE__, error.backtrace_locations.first.path
+  end
+
+  test "#report assigns a cause if it's missing" do
+    raise "the original cause"
+  rescue => cause
+    new_error = StandardError.new("A new error that should wrap the StandardError")
+    assert_nil new_error.cause
+
+    @reporter.report(new_error)
+
+    assert_same cause, new_error.cause
   end
 
   test "#record passes through the return value" do