Fix account counters being overwritten by parallel writes (#12045)

spec/models/account_stat_spec.rb

@@ -1,4 +1,57 @@
 require 'rails_helper'
 
 RSpec.describe AccountStat, type: :model do
+  describe '#increment_count!' do
+    it 'increments the count' do
+      account_stat = AccountStat.create(account: Fabricate(:account))
+      expect(account_stat.followers_count).to eq 0
+      account_stat.increment_count!(:followers_count)
+      expect(account_stat.followers_count).to eq 1
+    end
+
+    it 'increments the count in multi-threaded an environment' do
+      account_stat   = AccountStat.create(account: Fabricate(:account), statuses_count: 0)
+      increment_by   = 15
+      wait_for_start = true
+
+      threads = Array.new(increment_by) do
+        Thread.new do
+          true while wait_for_start
+          AccountStat.find(account_stat.id).increment_count!(:statuses_count)
+        end
+      end
+
+      wait_for_start = false
+      threads.each(&:join)
+
+      expect(account_stat.reload.statuses_count).to eq increment_by
+    end
+  end
+
+  describe '#decrement_count!' do
+    it 'decrements the count' do
+      account_stat = AccountStat.create(account: Fabricate(:account), followers_count: 15)
+      expect(account_stat.followers_count).to eq 15
+      account_stat.decrement_count!(:followers_count)
+      expect(account_stat.followers_count).to eq 14
+    end
+
+    it 'decrements the count in multi-threaded an environment' do
+      account_stat   = AccountStat.create(account: Fabricate(:account), statuses_count: 15)
+      decrement_by   = 10
+      wait_for_start = true
+
+      threads = Array.new(decrement_by) do
+        Thread.new do
+          true while wait_for_start
+          AccountStat.find(account_stat.id).decrement_count!(:statuses_count)
+        end
+      end
+
+      wait_for_start = false
+      threads.each(&:join)
+
+      expect(account_stat.reload.statuses_count).to eq 5
+    end
+  end
 end