Ensure merge request is created with valid diff object

* Add merge_request_diff validation to merge_request model
* Improve initialize of merge_request_diff object
* Rename some merge_request_diff methods for clarity
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/models/merge_request.rb

@@ -15,7 +15,7 @@ class MergeRequest < ActiveRecord::Base
 
   serialize :merge_params, Hash
 
-  after_create :create_merge_request_diff, unless: :importing?
+  before_validation :ensure_merge_request_diff, on: :create, unless: :importing?
   after_update :update_merge_request_diff
 
   delegate :commits, :real_size, to: :merge_request_diff, prefix: nil
@@ -95,6 +95,7 @@ class MergeRequest < ActiveRecord::Base
   validates :merge_user, presence: true, if: :merge_when_build_succeeds?
   validate :validate_branches, unless: [:allow_broken, :importing?]
   validate :validate_fork
+  validates_associated :merge_request_diff, on: :create, unless: [:allow_broken, :importing?]
 
   scope :by_branch, ->(branch_name) { where("(source_branch LIKE :branch) OR (target_branch LIKE :branch)", branch: branch_name) }
   scope :cared, ->(user) { where('assignee_id = :user OR author_id = :user', user: user.id) }
@@ -286,6 +287,10 @@ class MergeRequest < ActiveRecord::Base
     end
   end
 
+  def ensure_merge_request_diff
+    merge_request_diff || merge_request_diffs.build
+  end
+
   def create_merge_request_diff
     merge_request_diffs.create
   end

app/models/merge_request_diff.rb

@@ -22,28 +22,31 @@ class MergeRequestDiff < ActiveRecord::Base
   serialize :st_commits
   serialize :st_diffs
 
-  validates :head_commit_sha,  presence: true
   validates :start_commit_sha, presence: true
+  validates :head_commit_sha,  presence: true
   validates :base_commit_sha,  presence: true
 
-  after_initialize :ensure_head_commit_sha, if: :persisted?
-  before_create :set_diff_range,   unless: :importing?
-  after_create :reload_content,    unless: :importing?
+  after_initialize :set_diff_range, unless: :importing?
+  after_create :save_git_content,   unless: :importing?
   after_save :keep_around_commits,  unless: :importing?
 
-  def ensure_head_commit_sha
-    self.head_commit_sha ||= last_commit.sha
-  end
-
   def set_diff_range
+    if persisted?
+      # Workaround for old MergeRequestDiff object
+      # that does not have head_commit_sha in the database
+      self.head_commit_sha ||= last_commit.sha
+    else
       self.start_commit_sha ||= merge_request.target_branch_sha
       self.head_commit_sha  ||= merge_request.source_branch_sha
       self.base_commit_sha  ||= find_base_sha
     end
+  end
 
-  def reload_content
-    reload_commits
-    reload_diffs
+  # Collect information about commits and diff from repository
+  # and save it to the database as serialized data
+  def save_git_content
+    save_commits
+    save_diffs
   end
 
   def size
@@ -130,9 +133,9 @@ class MergeRequestDiff < ActiveRecord::Base
     array.map { |hash| Commit.new(Gitlab::Git::Commit.new(hash), merge_request.source_project) }
   end
 
-  # Reload all commits related to current merge request from repo
+  # Load all commits related to current merge request diff from repo
   # and save it as array of hashes in st_commits db field
-  def reload_commits
+  def save_commits
     new_attributes = {}
 
     commits = compare.commits
@@ -165,9 +168,9 @@ class MergeRequestDiff < ActiveRecord::Base
     end
   end
 
-  # Reload diffs between branches related to current merge request from repo
+  # Load diffs between branches related to current merge request diff from repo
   # and save it as array of hashes in st_diffs db field
-  def reload_diffs
+  def save_diffs
     new_attributes = {}
     new_diffs = []