Merge branch 'fix-migration-helper-race-conditions' into 'master'

Fix migration helper race conditions

## What does this MR do?

This MR fixes two problems with the migration helpers:

1. An error in `change_column_null` would not drop the previously created column
2. `update_column_in_batches` would rely on the number of rows in a table to determine how many to update. This meant that newly inserted rows (after the `COUNT`) would not be taken into account.

This MR also removes an outdated comment for `update_column_in_batches`.

## Are there points in the code the reviewer needs to double check?

No.

## Why was this MR needed?

See above.

## What are the relevant issue numbers?

Fixes #18483

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] ~~API support added~~
- [ ] Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4618

lib/gitlab/database/migration_helpers.rb

@@ -31,8 +31,6 @@ module Gitlab
       # Any data inserted while running this method (or after it has finished
       # running) is _not_ updated automatically.
       #
-      # This method _only_ updates rows where the column's value is set to NULL.
-      #
       # table - The name of the table.
       # column - The name of the column to update.
       # value - The value for the column.
@@ -55,10 +53,10 @@ module Gitlab
           first['count'].
           to_i
 
-        # Update in batches of 5%
+        # Update in batches of 5% until we run out of any rows to update.
         batch_size = ((total / 100.0) * 5.0).ceil
 
-        while processed < total
+        loop do
           start_row = exec_query(%Q{
             SELECT id
             FROM #{quoted_table}
@@ -66,6 +64,9 @@ module Gitlab
             LIMIT 1 OFFSET #{processed}
           }).to_hash.first
 
+          # There are no more rows to process
+          break unless start_row
+
           stop_row = exec_query(%Q{
             SELECT id
             FROM #{quoted_table}
@@ -126,6 +127,8 @@ module Gitlab
         begin
           transaction do
             update_column_in_batches(table, column, default)
+
+            change_column_null(table, column, false) unless allow_null
           end
         # We want to rescue _all_ exceptions here, even those that don't inherit
         # from StandardError.
@@ -134,8 +137,6 @@ module Gitlab
 
           raise error
         end
-
-        change_column_null(table, column, false) unless allow_null
       end
     end
   end

spec/lib/gitlab/database/migration_helpers_spec.rb

@@ -120,6 +120,19 @@ describe Gitlab::Database::MigrationHelpers, lib: true do
           model.add_column_with_default(:projects, :foo, :integer, default: 10)
         end.to raise_error(RuntimeError)
       end
+
+      it 'removes the added column whenever changing a column NULL constraint fails' do
+        expect(model).to receive(:change_column_null).
+          with(:projects, :foo, false).
+          and_raise(RuntimeError)
+
+        expect(model).to receive(:remove_column).
+          with(:projects, :foo)
+
+        expect do
+          model.add_column_with_default(:projects, :foo, :integer, default: 10)
+        end.to raise_error(RuntimeError)
+      end
     end
 
     context 'inside a transaction' do