Save and expose only generic merge error

When an error occurs during merge, the error message is exposed to user
and it is also saved in DB. This error message may be user unfriendly
(as in !41820) and it could also expose a detailed backend information.

Instead of displaying the specific error message, only sanitized generic
message is displayed. This is potentially controversial change because
disadvantage is that user doesn't get specific reason of failure.

Additional changes:
* repository.merge including exceptions is is extracted into a
separate method to make things clearer
* update! is used instead of update so we don't silently ignore
an error

Related to !41857

app/services/merge_requests/merge_service.rb

@@ -50,21 +50,30 @@ module MergeRequests
     end
 
     def commit
-      message = params[:commit_message] || merge_request.merge_commit_message
-
       log_info("Git merge started on JID #{merge_jid}")
-      commit_id = repository.merge(current_user, source, merge_request, message)
+      commit_id = try_merge
+
+      if commit_id
         log_info("Git merge finished on JID #{merge_jid} commit #{commit_id}")
+      else
+        raise MergeError, 'Conflicts detected during merge'
+      end
 
-      raise MergeError, 'Conflicts detected during merge' unless commit_id
+      merge_request.update!(merge_commit_sha: commit_id)
+    end
+
+    def try_merge
+      message = params[:commit_message] || merge_request.merge_commit_message
 
-      merge_request.update(merge_commit_sha: commit_id)
+      repository.merge(current_user, source, merge_request, message)
     rescue Gitlab::Git::HooksService::PreReceiveError => e
-      raise MergeError, e.message
-    rescue StandardError => e
-      raise MergeError, "Something went wrong during merge: #{e.message}"
+      handle_merge_error(log_message: e.message)
+      raise MergeError, 'Something went wrong during merge pre-receive hook'
+    rescue => e
+      handle_merge_error(log_message: e.message)
+      raise MergeError, 'Something went wrong during merge'
     ensure
-      merge_request.update(in_progress_merge_commit_sha: nil)
+      merge_request.update!(in_progress_merge_commit_sha: nil)
     end
 
     def after_merge

changelogs/unreleased/jprovazn-generic-error.yml

+---
+title: Display only generic message on merge error to avoid exposing any potentially
+  sensitive or user unfriendly backend messages.
+merge_request:
+author:
+type: fixed

spec/services/merge_requests/merge_service_spec.rb

@@ -219,7 +219,7 @@ describe MergeRequests::MergeService do
 
         service.execute(merge_request)
 
-        expect(merge_request.merge_error).to include(error_message)
+        expect(merge_request.merge_error).to include('Something went wrong during merge')
         expect(Rails.logger).to have_received(:error).with(a_string_matching(error_message))
       end
 
@@ -231,7 +231,7 @@ describe MergeRequests::MergeService do
 
         service.execute(merge_request)
 
-        expect(merge_request.merge_error).to include(error_message)
+        expect(merge_request.merge_error).to include('Something went wrong during merge pre-receive hook')
         expect(Rails.logger).to have_received(:error).with(a_string_matching(error_message))
       end