FIX: Avoid errors when updating post and topic count user stats. (#15876)

In ab5361d69a, we rescue from the PG error
but the transaction is already aborted causing any DB query after to
fail. As such, we avoid triggering the error in the first place by
checking that we would not be insertin a negative number into the
counter cache.

Follow-up to ab5361d69a

app/services/user_stat_count_updater.rb

@@ -17,23 +17,24 @@ class UserStatCountUpdater
       return if post.topic.private_message?
       stat = user_stat || post.user.user_stat
 
+      column =
         if post.is_first_post?
-        stat.public_send(action, :topic_count)
+          :topic_count
         elsif post.post_type == Post.types[:regular]
-        stat.public_send(action, :post_count)
+          :post_count
         end
-    rescue ActiveRecord::StatementInvalid => e
-      if e.cause.is_a?(PG::CheckViolation)
+
+      return if column.blank?
+
+      if action == :decrement! && stat.public_send(column) < 1
         # There are still spots in the code base which results in the counter cache going out of sync. However,
-        # we have a job that runs on a daily basis which will correct the count. Therefore, avoid raising an error for now
-        # and log the exception instead.
-        Discourse.warn_exception(
-          e,
-          message: "Attempted to insert negative count into UserStat#post_count or UserStat#topic_count"
-        )
-      else
-        raise
+        # we have a job that runs on a daily basis which will correct the count. Therefore, we always check that we
+        # wouldn't end up with a negative count first before inserting.
+        Rails.logger.warn("Attempted to insert negative count into UserStat##{column}\n#{caller.join('\n')}")
+        return
       end
+
+      stat.public_send(action, column)
     end
   end
 end

spec/services/user_stat_count_updater_spec.rb

@@ -6,6 +6,7 @@ describe UserStatCountUpdater do
   fab!(:user) { Fabricate(:user) }
   fab!(:user_stat) { user.user_stat }
   fab!(:post) { Fabricate(:post) }
+  fab!(:post_2) { Fabricate(:post, topic: post.topic) }
 
   before do
     @orig_logger = Rails.logger
@@ -19,6 +20,10 @@ describe UserStatCountUpdater do
   it 'should log the exception when a negative count is inserted' do
     UserStatCountUpdater.decrement!(post, user_stat: user_stat)
 
-    expect(@fake_logger.warnings.first).to include("PG::CheckViolation")
+    expect(@fake_logger.warnings.last).to match("topic_count")
+
+    UserStatCountUpdater.decrement!(post_2, user_stat: user_stat)
+
+    expect(@fake_logger.warnings.last).to match("post_count")
   end
 end