Update CE branch whenever EE is ahead of CE branch

This would happen if the corresponding CE branch is behind
CE master, and the CE merge base is ahead of CE branch.

When this happened, we could try to perform a merge to
CE merge base so that we keep up-to-date with the EE branch.

If there's a conflict, the author should resolve it from
the CE branch first.

scripts/ee-specific-lines-check

@@ -7,8 +7,8 @@ git_version
 
 base = find_compare_base
 
-current_numstat = updated_diff_numstat(base.ce_merge_base, base.ee_fetch_base)
-updated_numstat = updated_diff_numstat(base.ce_updated_base, 'HEAD')
+current_numstat = updated_diff_numstat(base.ce_base, base.ee_base)
+updated_numstat = updated_diff_numstat(base.ce_head, base.ee_head)
 
 offenses = updated_numstat.select do |file, updated_delta|
   current_delta = current_numstat[file]

scripts/ee_specific_check/ee_specific_check.rb

@@ -14,7 +14,8 @@ module EESpecificCheck
     'locale/gitlab.pot'
   ].freeze
 
-  CompareBase = Struct.new(:ce_merge_base, :ee_fetch_base, :ce_updated_base)
+  CompareBase = Struct.new(:ce_base, :ee_base, :ce_head, :ee_head)
+  GitStatus = Struct.new(:porcelain, :head)
 
   module_function
 
@@ -30,20 +31,16 @@ module EESpecificCheck
     git_clean
 
     ce_fetch_head = fetch_remote_ce_branch
+    ee_fetch_head = head_commit_sha
     ce_fetch_base = run_git_command("merge-base canonical-ce/master #{ce_fetch_head}")
     ce_merge_base = run_git_command("merge-base canonical-ce/master canonical-ee/master")
     ee_fetch_base = run_git_command("merge-base canonical-ee/master HEAD")
 
-    ce_updated_base =
-      if ce_fetch_head.start_with?('canonical-ce') || # No specific CE branch
-          ce_fetch_base == ce_merge_base # Up-to-date, no rebase needed
-        ce_merge_base
-      else
-        checkout_and_rebase_ce_fetch_head_onto_ce_merge_base(
-          ce_fetch_head, ce_fetch_base, ce_merge_base)
-      end
+    ce_updated_head =
+      find_ce_compare_head(ce_fetch_head, ce_fetch_base, ce_merge_base)
 
-    CompareBase.new(ce_merge_base, ee_fetch_base, ce_updated_base)
+    CompareBase.new(
+      ce_merge_base, ee_fetch_base, ce_updated_head, ee_fetch_head)
   end
 
   def setup_canonical_remotes
@@ -64,15 +61,6 @@ module EESpecificCheck
     "#{remote_to_fetch}/#{branch_to_fetch}"
   end
 
-  def checkout_and_rebase_ce_fetch_head_onto_ce_merge_base(
-    ce_fetch_head, ce_fetch_base, ce_merge_base)
-    # So that we could switch back
-    head = head_commit_sha
-
-    # Use detached HEAD so that we don't update HEAD
-    run_git_command("checkout -f #{ce_fetch_head}")
-    git_clean
-
   # We rebase onto the commit which is the latest commit presented in both
   # CE and EE, i.e. ce_merge_base, cutting off commits aren't merged into
   # EE yet. Here's an example:
@@ -109,13 +97,17 @@ module EESpecificCheck
   # where ce_merge_base..ee_fetch_base is the update-to-date
   # CE/EE difference and ce_fetch_head..HEAD is the changes we made in
   # CE and EE branches.
-    run_git_command("rebase --onto #{ce_merge_base} #{ce_fetch_base}~1 #{ce_fetch_head}")
+  def checkout_and_rebase(new_base, old_base, target_head)
+    with_detached_head(target_head) do
+      run_git_command("rebase --onto #{new_base} #{old_base}~1 #{target_head}")
 
       status = git_status
 
-    if status == ''
-      head_commit_sha
+      if status.porcelain == ''
+        status.head
       else
+        run_git_command("rebase --abort")
+
         say <<~MESSAGE
           💥 Git status not clean! This shouldn't happen, but there are two
           💥 known issues. One can be worked around, and the other can't.
@@ -139,14 +131,23 @@ module EESpecificCheck
 
           ⚠️ Git status:
 
-        #{status}
+          #{status.porcelain}
         MESSAGE
 
-      run_git_command("rebase --abort")
-
         exit(255)
       end
+    end
+  end
 
+  def with_detached_head(target_head)
+    # So that we could switch back
+    head = current_branch
+
+    # Use detached HEAD so that we don't update HEAD
+    run_git_command("checkout -f #{target_head}")
+    git_clean
+
+    yield
   ensure # ensure would still run if we call exit, don't worry
     # Make sure to switch back
     run_git_command("checkout -f #{head}")
@@ -158,7 +159,10 @@ module EESpecificCheck
   end
 
   def git_status
-    run_git_command("status --porcelain")
+    GitStatus.new(
+      run_git_command("status --porcelain"),
+      head_commit_sha
+    )
   end
 
   def git_clean
@@ -171,6 +175,11 @@ module EESpecificCheck
     run_git_command('clean -fd') if ENV['CI']
   end
 
+  def git_ancestor?(ancestor, descendant)
+    run_git_command(
+      "merge-base --is-ancestor #{ancestor} #{descendant} && echo y") == 'y'
+  end
+
   def remove_remotes
     run_git_command(
       "remote remove canonical-ee",
@@ -202,6 +211,50 @@ module EESpecificCheck
     end
   end
 
+  def find_ce_compare_head(ce_fetch_head, ce_fetch_base, ce_merge_base)
+    if git_ancestor?(ce_merge_base, ce_fetch_base) # CE ahead of EE
+      find_backward_ce_head(ce_fetch_head, ce_fetch_base, ce_merge_base)
+    else # EE ahead of CE
+      find_forward_ce_head(ce_merge_base, ce_fetch_head)
+    end
+  end
+
+  def find_backward_ce_head(ce_fetch_head, ce_fetch_base, ce_merge_base)
+    if ce_fetch_head.start_with?('canonical-ce') || # No specific CE branch
+        ce_fetch_base == ce_merge_base # Up-to-date, no rebase needed
+      ce_merge_base
+    else
+      # Rebase CE to remove commits in CE haven't merged into EE
+      checkout_and_rebase(ce_merge_base, ce_fetch_base, ce_fetch_head)
+    end
+  end
+
+  def find_forward_ce_head(ce_merge_base, ce_fetch_head)
+    with_detached_head(ce_fetch_head) do
+      run_git_command("merge #{ce_merge_base}")
+
+      status = git_status
+
+      if status.porcelain == ''
+        status.head
+      else
+        run_git_command("merge --abort")
+
+        say <<~MESSAGE
+          💥 Git status not clean! This means there's a conflict in
+          💥 #{ce_fetch_head} with canonical-ce/master. Please resolve
+          💥 the conflict from CE master and retry this job.
+
+          ⚠️ Git status:
+
+          #{status.porcelain}
+        MESSAGE
+
+        exit(254)
+      end
+    end
+  end
+
   def ce_repo_url
     @ce_repo_url ||= ENV.fetch('CI_REPOSITORY_URL', 'https://gitlab.com/gitlab-org/gitlab-ce.git').sub('gitlab-ee', 'gitlab-ce')
   end