Validate project path prior to hitting the database.

Closes #45247.

changelogs/unreleased/ab-45247-project-lookups-validation.yml

+---
+title: Validate project path prior to hitting the database.
+merge_request: 18322
+author:
+type: performance

lib/api/helpers.rb

@@ -103,9 +103,9 @@ module API
     end
     def find_project(id)
-      if id =~ /^\d+$/
+      if id.is_a?(Integer) || id =~ /^\d+$/
         Project.find_by(id: id)
-      else
+      elsif id.include?("/")
         Project.find_by_full_path(id)
       end
     end

spec/lib/api/helpers_spec.rb

@@ -3,6 +3,48 @@ require 'spec_helper'
 describe API::Helpers do
   subject { Class.new.include(described_class).new }
+  describe '#find_project' do
+    let(:project) { create(:project) }
+
+    shared_examples 'project finder' do
+      context 'when project exists' do
+        it 'returns requested project' do
+          expect(subject.find_project(existing_id)).to eq(project)
+        end
+
+        it 'returns nil' do
+          expect(subject.find_project(non_existing_id)).to be_nil
+        end
+      end
+    end
+
+    context 'when ID is used as an argument' do
+      let(:existing_id) { project.id }
+      let(:non_existing_id) { (Project.maximum(:id) || 0) + 1 }
+
+      it_behaves_like 'project finder'
+    end
+
+    context 'when PATH is used as an argument' do
+      let(:existing_id) { project.full_path }
+      let(:non_existing_id) { 'something/else' }
+
+      it_behaves_like 'project finder'
+
+      context 'with an invalid PATH' do
+        let(:non_existing_id) { 'undefined' } # path without slash
+
+        it_behaves_like 'project finder'
+
+        it 'does not hit the database' do
+          expect(Project).not_to receive(:find_by_full_path)
+
+          subject.find_project(non_existing_id)
+        end
+      end
+    end
+  end
+
   describe '#find_namespace' do
     let(:namespace) { create(:namespace) }