Add helpers for collecting simplecov data during a prspec run

Closes !9

Author: bagedevimo

CHANGES.md

@@ -1,5 +1,9 @@
 # Changelog
 
+## Unreleased
+
+* Add support for simplecov reporting and collation
+
 ## 3.0.0
 
 * Fix reporting of `aggregate_failures` errors.

README.md

@@ -82,6 +82,36 @@ You might also want to detect whether your spec suite is running under ParallelR
   do_something unless ParallelRSpec.running?
 ```
 
+## SimpleCov
+
+SimpleCov doesn't track coverage across forked processes by default. ParallelRSpec ships a helper that handles the wiring. For example, in `spec_helper.rb` add something like:
+
+```ruby
+  require 'simplecov'
+  require 'parallel_rspec/simplecov'
+
+  ParallelRSpec.simplecov('rails') do
+    enable_coverage :branch
+    add_filter '/spec/'
+  end
+```
+
+Per-worker output is intentionally always written as a `.resultset.json` so you can use any SimpleCov formatter without it correctly supporting parallel processes etc. Pass a `formatters:` array to control the final format.
+
+```ruby
+  require 'simplecov-cobertura'
+
+  ParallelRSpec.simplecov('rails',
+    formatters: [
+      SimpleCov::Formatter::HTMLFormatter,
+      SimpleCov::Formatter::CoberturaFormatter,
+    ]
+  ) do
+    enable_coverage :branch
+    add_filter '/spec/'
+  end
+```
+
 ## Limitations
 
 ParallelRSpec can't parallelize the specs in groups that use `before(:context)` or `after(:context)` (AKA `before(:all)` or `after(:all)`) hooks, since these may mean that there is state shared between examples.

lib/parallel_rspec/runner.rb

@@ -66,6 +66,10 @@ def run_specs(example_groups)
 
           persist_example_statuses
 
+          if defined?(ParallelRSpec) && ParallelRSpec.respond_to?(:simplecov_enabled?) && ParallelRSpec.simplecov_enabled?
+            ParallelRSpec.collate_simplecov!
+          end
+
           success ? 0 : @configuration.failure_exit_code
         end
       end

lib/parallel_rspec/simplecov.rb

@@ -0,0 +1,80 @@
+require "simplecov"
+require "parallel_rspec/config"
+
+module ParallelRSpec
+  @simplecov_enabled = false
+  @simplecov_base_dir = nil
+  @simplecov_formatters = nil
+
+  def self.simplecov_enabled?
+    @simplecov_enabled
+  end
+
+  def self.simplecov(profile = nil, formatters: nil, &block)
+    ::SimpleCov.enable_for_subprocesses true
+
+    base_dir = ::SimpleCov.coverage_dir
+
+    force_worker_settings = lambda do
+      # Prevent forked processes races generating the HTML report; each
+      # process just dumps a .resultset.json that the runner collates into
+      # the user's chosen formatter(s) at the end of the run.
+      ::SimpleCov.print_error_status = false
+      ::SimpleCov.formatter ::SimpleCov::Formatter::SimpleFormatter
+      ::SimpleCov.minimum_coverage 0
+    end
+
+    start_simplecov = lambda do |command_name, dir|
+      ::SimpleCov.command_name command_name
+      ::SimpleCov.coverage_dir dir
+      ::SimpleCov.start(profile, &block)
+    end
+
+    ParallelRSpec::Config.after_fork do |worker_number|
+      force_worker_settings.call
+      start_simplecov.call(
+        "RSpec Worker #{worker_number}",
+        File.join(base_dir, "rspec_#{worker_number}"),
+      )
+    end
+
+    force_worker_settings.call
+    start_simplecov.call("RSpec Server", File.join(base_dir, "rspec_server"))
+
+    @simplecov_base_dir = base_dir
+    @simplecov_formatters = formatters
+    @simplecov_enabled = true
+  end
+
+  def self.collate_simplecov!
+    base_dir = @simplecov_base_dir || ::SimpleCov.coverage_dir
+
+    # Force the server process's resultset to disk before we glob. SimpleCov's
+    # own at_exit handler will harmlessly re-dump it after collation finishes.
+    ::SimpleCov.result.format!
+
+    pattern = File.join(base_dir, "rspec_*", ".resultset.json")
+    resultsets = Dir.glob(pattern)
+
+    if resultsets.empty?
+      warn "[parallel_rspec] no SimpleCov resultsets found under #{pattern}; skipping collation"
+      return
+    end
+
+    collate_formatter =
+      case (@simplecov_formatters || []).length
+      when 0 then nil
+      when 1 then @simplecov_formatters.first
+      else ::SimpleCov::Formatter::MultiFormatter.new(@simplecov_formatters)
+      end
+
+    puts "Collating #{resultsets.size} SimpleCov resultset(s)..."
+
+    ::SimpleCov.collate(resultsets) do
+      coverage_dir base_dir
+      formatter collate_formatter if collate_formatter
+    end
+  rescue => e
+    warn "[parallel_rspec] simplecov collation failed: #{e.class}: #{e.message}"
+  end
+end