Fix long commit message error

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/321862

**Problem**

`Marshal.dump` returns `TypeError (singleton can't be dumped)` for
commit hashes with long commit  messages.

**Solution**

Introduce a 1 KiB limit for commit message on project details page.

changelogs/unreleased/321862_fix_long_commit_messages_error.yml

+---
+title: Fix 500 error for long commit messages
+merge_request: 55320
+author:
+type: fixed

lib/gitlab/tree_summary.rb

@@ -104,12 +104,12 @@ module Gitlab
     end
 
     def fetch_last_cached_commits_list
-      cache_key = ['projects', project.id, 'last_commits_list', commit.id, ensured_path, offset, limit]
+      cache_key = ['projects', project.id, 'last_commits', commit.id, ensured_path, offset, limit]
 
       commits = Rails.cache.fetch(cache_key, expires_in: CACHE_EXPIRE_IN) do
         repository
           .list_last_commits_for_tree(commit.id, ensured_path, offset: offset, limit: limit, literal_pathspec: true)
-          .transform_values!(&:to_hash)
+          .transform_values! { |commit| commit_to_hash(commit) }
       end
 
       commits.transform_values! { |value| Commit.from_hash(value, project) }
@@ -121,6 +121,12 @@ module Gitlab
       resolved_commits[commit.id] ||= commit
     end
 
+    def commit_to_hash(commit)
+      commit.to_hash.tap do |hash|
+        hash[:message] = hash[:message].to_s.truncate_bytes(1.kilobyte, omission: '...')
+      end
+    end
+
     def commit_path(commit)
       Gitlab::Routing.url_helpers.project_commit_path(project, commit)
     end

spec/lib/gitlab/tree_summary_spec.rb

@@ -57,14 +57,12 @@ RSpec.describe Gitlab::TreeSummary do
     context 'with caching', :use_clean_rails_memory_store_caching do
       subject { Rails.cache.fetch(key) }
 
-      before do
-        summarized
-      end
-
       context 'Repository tree cache' do
         let(:key) { ['projects', project.id, 'content', commit.id, path] }
 
         it 'creates a cache for repository content' do
+          summarized
+
           is_expected.to eq([{ file_name: 'a.txt', type: :blob }])
         end
       end
@@ -72,11 +70,34 @@ RSpec.describe Gitlab::TreeSummary do
       context 'Commits list cache' do
         let(:offset) { 0 }
         let(:limit) { 25 }
-        let(:key) { ['projects', project.id, 'last_commits_list', commit.id, path, offset, limit] }
+        let(:key) { ['projects', project.id, 'last_commits', commit.id, path, offset, limit] }
 
         it 'creates a cache for commits list' do
+          summarized
+
           is_expected.to eq('a.txt' => commit.to_hash)
         end
+
+        context 'when commit has a very long message' do
+          before do
+            repo.create_file(
+              project.creator,
+              'long.txt',
+              '',
+              message: message,
+              branch_name: project.default_branch_or_master
+            )
+          end
+
+          let(:message) { 'a' * 1025 }
+          let(:expected_message) { message[0...1021] + '...' }
+
+          it 'truncates commit message to 1 kilobyte' do
+            summarized
+
+            is_expected.to include('long.txt' => a_hash_including(message: expected_message))
+          end
+        end
       end
     end
   end