Copy relations between subtasks when copying an issue with its subtasks

When an issue is copied together with its subtasks, relations between the
copied subtasks (precedes/follows, blocks/blocked, relates, etc.) are lost:
Issue#after_create_from_copy only copies the subtasks themselves, not the
relations among them.

This patch keeps a mapping from the original child issues to their copies
and, after all subtasks have been created, replicates every relation whose
both ends were copied. Relations pointing to issues outside the copied
subtree are intentionally left out.

Generated against the 6.1-stable branch (6.1.4). Apply from the Redmine
root directory with:

    patch -p1 < issue_copy_subtask_relations-6.1-stable.patch

Related: #17068

diff --git a/app/models/issue.rb b/app/models/issue.rb
index 3772f60..00547fd 100644
--- a/app/models/issue.rb
+++ b/app/models/issue.rb
@@ -1777,12 +1777,12 @@ class Issue < ApplicationRecord
 
     unless @copied_from.leaf? || @copy_options[:subtasks] == false
       copy_options = (@copy_options || {}).merge(:subtasks => false)
-      copied_issue_ids = {@copied_from.id => self.id}
+      copied_issues = {@copied_from.id => self}
       @copied_from.reload.descendants.reorder("#{Issue.table_name}.lft").each do |child|
         # Do not copy self when copying an issue as a descendant of the copied issue
         next if child == self
         # Do not copy subtasks of issues that were not copied
-        next unless copied_issue_ids[child.parent_id]
+        next unless copied_issues[child.parent_id]
 
         # Do not copy subtasks that are not visible to avoid potential disclosure of private data
         unless child.visible?
@@ -1800,7 +1800,7 @@ class Issue < ApplicationRecord
         end
         copy.author = author
         copy.project = project
-        copy.parent_issue_id = copied_issue_ids[child.parent_id]
+        copy.parent_issue_id = copied_issues[child.parent_id].id
         unless child.fixed_version.present? && child.fixed_version.status == 'open'
           copy.fixed_version_id = nil
         end
@@ -1818,7 +1818,22 @@ class Issue < ApplicationRecord
           end
           next
         end
-        copied_issue_ids[child.id] = copy.id
+        copied_issues[child.id] = copy
+      end
+
+      # Replicate dependencies between copied children
+      @copied_from.reload.descendants.reorder("#{Issue.table_name}.lft").each do |child|
+        copy = copied_issues[child.id]
+        next unless copy
+        child.relations_to.each do |rel|
+          next unless copied_issues.key?(rel.issue_from.id)
+          from = Issue.find_by_id(copied_issues[rel.issue_from.id].id)
+          next unless from
+          relation = IssueRelation.new(:issue_from => from, :issue_to => copy, :relation_type => rel.relation_type)
+          unless relation.save
+            logger.error "Could not create relation while copying ##{child.id} to ##{copy.id} due to validation errors: #{relation.errors.full_messages.join(', ')}" if logger
+          end
+        end
       end
     end
     @after_create_from_copy_handled = true