FIX: under some conditions draft would say it was saving when not

This is a major change to draft internals. Previously there were quite a
few cases where the draft system would say "draft saved", when in fact
we just skipped saving.

This commit ensures the draft system deals with draft ownership handover in
a predictable way.

For example:

- Window 1 editing draft
- Window 2 editing same draft at the same time

Previously we would allow window 1 and 2 to just fight on the same draft
each window overwriting the same draft over an over.

This commit introduces an ownership concept where either window 1 or 2 win
and user is prompted on the loser window to reload screen to correct the issue

This also corrects edge cases where a user could have multiple browser windows
open and posts in 1 window, later to post in the second window. Previously
drafts would break in the second window, this corrects it.

spec/requests/draft_controller_spec.rb

@@ -26,7 +26,6 @@ describe DraftController do
     post = Fabricate(:post, user: user)
 
     post "/draft.json", params: {
-      username: user.username,
       draft_key: "topic",
       sequence: 0,
       data: {
@@ -39,7 +38,6 @@ describe DraftController do
     expect(JSON.parse(response.body)['conflict_user']).to eq(nil)
 
     post "/draft.json", params: {
-      username: user.username,
       draft_key: "topic",
       sequence: 0,
       data: {
@@ -62,4 +60,97 @@ describe DraftController do
     expect(response.status).to eq(200)
     expect(Draft.get(user, 'xxx', 0)).to eq(nil)
   end
+
+  it 'cant trivially resolve conflicts without interaction' do
+
+    user = sign_in(Fabricate(:user))
+
+    DraftSequence.next!(user, "abc")
+
+    post "/draft.json", params: {
+      draft_key: "abc",
+      sequence: 0,
+      data: { a: "test" }.to_json,
+      owner: "abcdefg"
+    }
+
+    expect(response.status).to eq(200)
+    json = JSON.parse(response.body)
+    expect(json["draft_sequence"]).to eq(1)
+  end
+
+  it 'has a clean protocol for ownership handover' do
+    user = sign_in(Fabricate(:user))
+
+    post "/draft.json", params: {
+      draft_key: "abc",
+      sequence: 0,
+      data: { a: "test" }.to_json,
+      owner: "abcdefg"
+    }
+
+    expect(response.status).to eq(200)
+
+    json = JSON.parse(response.body)
+    expect(json["draft_sequence"]).to eq(0)
+
+    post "/draft.json", params: {
+      draft_key: "abc",
+      sequence: 0,
+      data: { b: "test" }.to_json,
+      owner: "hijklmnop"
+    }
+
+    expect(response.status).to eq(200)
+    json = JSON.parse(response.body)
+    expect(json["draft_sequence"]).to eq(1)
+
+    expect(DraftSequence.current(user, "abc")).to eq(1)
+
+    post "/draft.json", params: {
+      draft_key: "abc",
+      sequence: 1,
+      data: { c: "test" }.to_json,
+      owner: "hijklmnop"
+    }
+
+    expect(response.status).to eq(200)
+    json = JSON.parse(response.body)
+    expect(json["draft_sequence"]).to eq(1)
+
+    post "/draft.json", params: {
+      draft_key: "abc",
+      sequence: 1,
+      data: { c: "test" }.to_json,
+      owner: "abc"
+    }
+
+    expect(response.status).to eq(200)
+    json = JSON.parse(response.body)
+    expect(json["draft_sequence"]).to eq(2)
+  end
+
+  it 'raises an error for out-of-sequence draft setting' do
+
+    user = sign_in(Fabricate(:user))
+    seq = DraftSequence.next!(user, "abc")
+    Draft.set(user, "abc", seq, { b: "test" }.to_json)
+
+    post "/draft.json", params: {
+      draft_key: "abc",
+      sequence: seq - 1,
+      data: { a: "test" }.to_json
+    }
+
+    expect(response.status).to eq(409)
+
+    post "/draft.json", params: {
+      draft_key: "abc",
+      sequence: seq + 1,
+      data: { a: "test" }.to_json
+    }
+
+    expect(response.status).to eq(409)
+
+  end
 end