Use atomic rename for every HTML report write

In Nix and similar pure-build environments the gem's static assets
ship at mode 0444. The earlier static-asset fix wrote them through a
temp file + rename, but coverage.json and coverage_data.js still went
through plain File.write — which opens the existing path for writing
and so raised EACCES on the second run when those two files were
inherited from a prior run at the same read-only mode. Extract one
atomic_write helper and route all three writes through it. rename is
atomic and overwrite-safe, so the read-only and parallel-worker race
cases collapse into the same clean code path.

Resolves #741.

Author: sferik

lib/simplecov/formatter/html_formatter.rb

@@ -26,8 +26,8 @@ def format(result)
         json = JSON.pretty_generate(JSONFormatter.build_hash(result))
 
         FileUtils.mkdir_p(output_path)
-        File.write(File.join(output_path, JSONFormatter::FILENAME), json)
-        File.write(File.join(output_path, DATA_FILENAME), "window.SIMPLECOV_DATA = #{json};\n", mode: "wb")
+        atomic_write(File.join(output_path, JSONFormatter::FILENAME), json)
+        atomic_write(File.join(output_path, DATA_FILENAME), "window.SIMPLECOV_DATA = #{json};\n")
 
         copy_static_assets
         puts output_message(result) unless @silent
@@ -38,27 +38,33 @@ def format(result)
       def format_from_json(json_path, output_dir)
         FileUtils.mkdir_p(output_dir)
         json = File.read(json_path)
-        File.write(File.join(output_dir, DATA_FILENAME), "window.SIMPLECOV_DATA = #{json};\n", mode: "wb")
+        atomic_write(File.join(output_dir, DATA_FILENAME), "window.SIMPLECOV_DATA = #{json};\n")
         copy_static_assets(output_dir)
       end
 
     private
 
       def copy_static_assets(dest_dir = output_path)
-        # Copy via temp file + atomic rename so parallel test workers writing
-        # to the same coverage directory don't race on the unlink step.
         Dir[File.join(public_dir, "*")].each do |src|
-          dest = File.join(dest_dir, File.basename(src))
-          temp = "#{dest}.#{Process.pid}.#{rand(2**32).to_s(36)}"
-          begin
-            FileUtils.cp(src, temp)
-            File.rename(temp, dest)
-          ensure
-            FileUtils.rm_f(temp)
-          end
+          atomic_write(File.join(dest_dir, File.basename(src)), File.binread(src))
         end
       end
 
+      # Write `content` at `dest` via a uniquely-named temp file in the
+      # same directory, then `File.rename` onto the final path. rename is
+      # atomic and overwrite-safe, so:
+      # - parallel writers can't race on an unlink-then-write window, and
+      # - read-only existing files (e.g. assets shipped at 0444 from
+      #   /nix/store) are replaced cleanly instead of triggering EACCES
+      #   from opening the existing path for writing.
+      def atomic_write(dest, content)
+        temp = "#{dest}.#{Process.pid}.#{rand(2**32).to_s(36)}"
+        File.binwrite(temp, content)
+        File.rename(temp, dest)
+      ensure
+        FileUtils.rm_f(temp)
+      end
+
       def output_message(result)
         "Coverage report generated for #{result.command_name} to #{output_path}. " \
           "#{result.covered_lines} / #{result.total_lines} LOC " \

spec/formatter/html_formatter_spec.rb

@@ -125,6 +125,17 @@ def coverage_data(dir = coverage_dir)
       expect(meta["branch_coverage"]).to be(true).or be(false)
       expect(meta["method_coverage"]).to be(true).or be(false)
     end
+
+    # Regression test for #741: in Nix and similar pure-build environments,
+    # the gem's static assets ship at mode 0444. Earlier code copied them
+    # via `FileUtils.cp_r`, which preserved the read-only mode and then
+    # raised EACCES the next run when it tried to open them for writing.
+    # The temp-file + rename approach bypasses the open-for-write step.
+    it "overwrites read-only assets from prior runs without raising EACCES" do
+      formatter.format(make_result)
+      Dir[File.join(coverage_dir, "*")].each { |f| File.chmod(0o444, f) }
+      expect { formatter.format(make_result) }.not_to raise_error
+    end
   end
 
   describe "#format output behaviour" do