FEATURE: experimental hidden setting for draft backups

Under exceptional situations the automatic draft feature can fail.

This new **hidden, default off** site setting
`backup_drafts_to_pm_length` will automatically backup any draft that is
saved by the system to a dedicated PM (originating from self)

The body of that PM will contain the text of the reply.

We can enable this feature strategically on sites exhibiting issues to
diagnose issues with the draft system and offer a recourse to users who
appear to lose drafts. We automatically checkpoint these drafts every 5
minutes forcing a new revision each 5 minutes so you can revert to old
content.

Longer term we are considering automatically enabling this kind of feature
for extremely long drafts where the risk is really high one could lose
days of writing.

spec/models/draft_spec.rb

@@ -3,37 +3,70 @@
 require 'rails_helper'
 
 describe Draft do
-  before do
-    @user = Fabricate(:user)
+
+  fab!(:user) do
+    Fabricate(:user)
   end
+
+  context 'backup_drafts_to_pm_length' do
+    it "correctly backs up drafts to a personal message" do
+      SiteSetting.backup_drafts_to_pm_length = 1
+
+      draft = {
+        reply: "this is a reply",
+        random_key: "random"
+      }
+
+      Draft.set(user, "new_private_message", 0, draft.to_json)
+      draft["reply"] = "test" * 100
+      Draft.set(user, "new_private_message", 77, draft.to_json)
+
+      draft_post = BackupDraftPost.find_by(user_id: user.id, key: "new_private_message").post
+
+      expect(draft_post.revisions.count).to eq(0)
+
+      freeze_time 10.minutes.from_now
+
+      # this should trigger a post revision as 10 minutes have passed
+      draft["reply"] = "hello"
+      Draft.set(user, "new_private_message", 77, draft.to_json)
+
+      draft_topic = BackupDraftTopic.find_by(user_id: user.id)
+      expect(draft_topic.topic.posts_count).to eq(2)
+
+      draft_post.reload
+      expect(draft_post.revisions.count).to eq(1)
+    end
+  end
+
   it "can get a draft by user" do
-    Draft.set(@user, "test", 0, "data")
-    expect(Draft.get(@user, "test", 0)).to eq "data"
+    Draft.set(user, "test", 0, "data")
+    expect(Draft.get(user, "test", 0)).to eq "data"
   end
 
   it "uses the user id and key correctly" do
-    Draft.set(@user, "test", 0, "data")
+    Draft.set(user, "test", 0, "data")
     expect(Draft.get(Fabricate.build(:coding_horror), "test", 0)).to eq nil
   end
 
   it "should overwrite draft data correctly" do
-    Draft.set(@user, "test", 0, "data")
-    Draft.set(@user, "test", 0, "new data")
-    expect(Draft.get(@user, "test", 0)).to eq "new data"
+    Draft.set(user, "test", 0, "data")
+    Draft.set(user, "test", 0, "new data")
+    expect(Draft.get(user, "test", 0)).to eq "new data"
   end
 
   it "should clear drafts on request" do
-    Draft.set(@user, "test", 0, "data")
-    Draft.clear(@user, "test", 0)
-    expect(Draft.get(@user, "test", 0)).to eq nil
+    Draft.set(user, "test", 0, "data")
+    Draft.clear(user, "test", 0)
+    expect(Draft.get(user, "test", 0)).to eq nil
   end
 
   it "should disregard old draft if sequence decreases" do
-    Draft.set(@user, "test", 0, "data")
-    Draft.set(@user, "test", 1, "hello")
-    Draft.set(@user, "test", 0, "foo")
-    expect(Draft.get(@user, "test", 0)).to eq nil
-    expect(Draft.get(@user, "test", 1)).to eq "hello"
+    Draft.set(user, "test", 0, "data")
+    Draft.set(user, "test", 1, "hello")
+    Draft.set(user, "test", 0, "foo")
+    expect(Draft.get(user, "test", 0)).to eq nil
+    expect(Draft.get(user, "test", 1)).to eq "hello"
   end
 
   it 'can cleanup old drafts' do
@@ -71,25 +104,25 @@ describe Draft do
     let(:public_topic) { public_post.topic }
 
     let(:stream) do
-      Draft.stream(user: @user)
+      Draft.stream(user: user)
     end
 
     it "should include the correct number of drafts in the stream" do
-      Draft.set(@user, "test", 0, '{"reply":"hey.","action":"createTopic","title":"Hey"}')
-      Draft.set(@user, "test2", 0, '{"reply":"howdy"}')
+      Draft.set(user, "test", 0, '{"reply":"hey.","action":"createTopic","title":"Hey"}')
+      Draft.set(user, "test2", 0, '{"reply":"howdy"}')
       expect(stream.count).to eq(2)
     end
 
     it "should include the right topic id in a draft reply in the stream" do
-      Draft.set(@user, "topic_#{public_topic.id}", 0, '{"reply":"hi"}')
+      Draft.set(user, "topic_#{public_topic.id}", 0, '{"reply":"hi"}')
       draft_row = stream.first
       expect(draft_row.topic_id).to eq(public_topic.id)
     end
 
     it "should include the right draft username in the stream" do
-      Draft.set(@user, "topic_#{public_topic.id}", 0, '{"reply":"hey"}')
+      Draft.set(user, "topic_#{public_topic.id}", 0, '{"reply":"hey"}')
       draft_row = stream.first
-      expect(draft_row.draft_username).to eq(@user.username)
+      expect(draft_row.draft_username).to eq(user.username)
     end
 
   end