Merge SimpleCov sessions across rake spec:ci buckets

rake spec:ci runs ~9 sequential rspec processes via spec_organizer. With
SimpleCov.command_name set to a static 'rspec' in every process, each
bucket overwrote the previous one's session in coverage/.resultset.json
and only the last process's coverage survived. On Evergreen the last
bucket is connection-pool-stress, all of whose examples are pending, so
the resultset ended up with 0% coverage on every tracked file.

Make command_name unique per PID so each bucket stores a separate
session, and merge line hits across all sessions in CoverageGate.

Author: comandeo-mongo

.evergreen/lib/coverage_gate.rb

@@ -68,17 +68,30 @@ def load_current
   end
 
   def parse_resultset(data)
-    _, run = data.first
-    coverage = run.fetch('coverage')
-    coverage.each_with_object({}) do |(abs_path, file_data), out|
+    merged = data.each_value.with_object({}) do |run, acc|
+      run.fetch('coverage').each do |abs_path, file_data|
+        lines = file_data.is_a?(Hash) ? file_data['lines'] : file_data
+        existing = acc[abs_path]
+        acc[abs_path] = existing ? merge_lines(existing, lines) : lines.dup
+      end
+    end
+
+    merged.each_with_object({}) do |(abs_path, lines), out|
       rel = relative_path(abs_path)
       next unless rel
 
-      lines = file_data.is_a?(Hash) ? file_data['lines'] : file_data
       out[rel] = count_lines(lines)
     end
   end
 
+  def merge_lines(a, b)
+    a.zip(b).map do |x, y|
+      next nil if x.nil? && y.nil?
+
+      (x.is_a?(Integer) ? x : 0) + (y.is_a?(Integer) ? y : 0)
+    end
+  end
+
   def load_baseline
     return { 'files' => {} } unless File.exist?(@baseline_path)
 

spec/lite_spec_helper.rb

@@ -4,7 +4,9 @@
   require 'simplecov'
   require 'simplecov-json'
 
-  SimpleCov.command_name 'rspec'
+  # Unique per process so concurrent buckets in rake spec:ci do not overwrite
+  # each other's session in coverage/.resultset.json.
+  SimpleCov.command_name "rspec-#{Process.pid}"
   SimpleCov.formatter = SimpleCov::Formatter::MultiFormatter.new(
     [
       SimpleCov::Formatter::HTMLFormatter,

spec/mongo/coverage_gate_spec.rb

@@ -36,6 +36,15 @@ def write_resultset(files)
     File.write(resultset_path, JSON.dump('rspec' => { 'coverage' => coverage }))
   end
 
+  def write_multi_session_resultset(sessions)
+    payload = sessions.each_with_index.with_object({}) do |(files, idx), out|
+      coverage = files.transform_keys { |rel| File.join(project_root, rel) }
+                      .transform_values { |lines| { 'lines' => lines } }
+      out["rspec-#{idx}"] = { 'coverage' => coverage }
+    end
+    File.write(resultset_path, JSON.dump(payload))
+  end
+
   def write_baseline(files)
     File.write(baseline_path, JSON.pretty_generate('files' => files))
   end
@@ -112,6 +121,21 @@ def write_baseline(files)
         expect(output.string).to include('missing')
       end
     end
+
+    context 'when the resultset has multiple sessions (parallel buckets)' do
+      it 'merges line hits across sessions before comparing' do
+        write_multi_session_resultset(
+          [
+            # bucket A covers lines 1 and 2
+            { 'lib/mongo/foo.rb' => [ nil, 1, 1, 0, nil ] },
+            # bucket B covers line 3 (but not 1 or 2)
+            { 'lib/mongo/foo.rb' => [ nil, 0, 0, 1, nil ] },
+          ]
+        )
+        write_baseline('lib/mongo/foo.rb' => { 'covered' => 3, 'total' => 3 })
+        expect(gate.check).to eq(0)
+      end
+    end
   end
 
   describe '#update_baseline' do