Publish cache files with an atomic rename

lib/fixture_kit/file_cache.rb

@@ -2,6 +2,7 @@
 
 require "json"
 require "fileutils"
+require "active_support/core_ext/file/atomic"
 require "active_support/inflector"
 
 module FixtureKit
@@ -45,7 +46,7 @@ def write(data)
       }
 
       FileUtils.mkdir_p(File.dirname(path))
-      File.write(path, JSON.pretty_generate(content))
+      File.atomic_write(path) { |file| file.write(JSON.pretty_generate(content)) }
     end
 
     private

spec/unit/file_cache_spec.rb

@@ -84,6 +84,25 @@
       expect(File.exist?(nested_path)).to be(true)
     end
 
+    it "leaves nothing but the cache file behind" do
+      file_cache.write(FixtureKit::MemoryCache.new(data: {}, exposed: {}))
+
+      expect(Dir.children(cache_path)).to contain_exactly("test_fixture.json")
+    end
+
+    it "publishes the file with File.atomic_write so a concurrent reader never sees a partial write" do
+      data = FixtureKit::MemoryCache.new(data: {}, exposed: {})
+      FileUtils.mkdir_p(cache_path)
+      file = File.new(file_path, "w")
+      allow(File).to receive(:atomic_write).and_yield(file)
+
+      file_cache.write(data)
+      file.close
+
+      expect(File).to have_received(:atomic_write).with(file_path)
+      expect(JSON.parse(File.read(file_path))).to eq({ "data" => {}, "exposed" => {} })
+    end
+
     it "raises CacheCorruptError when the file contains invalid JSON" do
       FileUtils.mkdir_p(cache_path)
       File.write(file_path, "this is not json")