Merge branch '4020-rebase-message' into 'master'

Resolve 'Rebase fails with "something went wrong" and huge ugly conflict error.'

Closes #41820

See merge request gitlab-org/gitlab-ce!16326

app/services/merge_requests/rebase_service.rb

 module MergeRequests
   class RebaseService < MergeRequests::WorkingCopyBaseService
+    REBASE_ERROR = 'Rebase failed. Please rebase locally'.freeze
+
     def execute(merge_request)
       @merge_request = merge_request
 
       if rebase
         success
       else
-        error('Failed to rebase. Should be done manually')
+        error(REBASE_ERROR)
       end
     end
 
@@ -22,8 +24,8 @@ module MergeRequests
 
       true
     rescue => e
-      log_error('Failed to rebase branch:')
-      log_error(e.message, save_message_on_model: true)
+      log_error(REBASE_ERROR, save_message_on_model: true)
+      log_error(e.message)
       false
     end
   end

changelogs/unreleased/4020-rebase-message.yml

+---
+title: Display user friendly error message if rebase fails.
+merge_request:
+author:
+type: fixed

spec/services/merge_requests/rebase_service_spec.rb

@@ -32,7 +32,7 @@ describe MergeRequests::RebaseService do
 
       it 'returns an error' do
         expect(service.execute(merge_request)).to match(status: :error,
-                                                        message: 'Failed to rebase. Should be done manually')
+                                                        message: described_class::REBASE_ERROR)
       end
     end
 
@@ -41,15 +41,15 @@ describe MergeRequests::RebaseService do
         allow(repository).to receive(:run_git!).and_raise('Something went wrong')
       end
 
-      it 'saves the error message' do
+      it 'saves a generic error message' do
         subject.execute(merge_request)
 
-        expect(merge_request.reload.merge_error).to eq 'Something went wrong'
+        expect(merge_request.reload.merge_error).to eq described_class::REBASE_ERROR
       end
 
       it 'returns an error' do
         expect(service.execute(merge_request)).to match(status: :error,
-                                                        message: 'Failed to rebase. Should be done manually')
+                                                        message: described_class::REBASE_ERROR)
       end
     end
 
@@ -58,15 +58,15 @@ describe MergeRequests::RebaseService do
         allow(repository).to receive(:run_git!).and_raise(Gitlab::Git::Repository::GitError, 'Something went wrong')
       end
 
-      it 'saves the error message' do
+      it 'saves a generic error message' do
         subject.execute(merge_request)
 
-        expect(merge_request.reload.merge_error).to eq 'Something went wrong'
+        expect(merge_request.reload.merge_error).to eq described_class::REBASE_ERROR
       end
 
       it 'returns an error' do
         expect(service.execute(merge_request)).to match(status: :error,
-                                                        message: 'Failed to rebase. Should be done manually')
+                                                        message: described_class::REBASE_ERROR)
       end
     end