Add bin/compare

Allows to compare the output of two different prism versions.

It compiles prism twice and forks to allow for different versions in the same script.
It then passes over minimal data to see if anything has changed.

Most bugfixes should impact little to no real code and the test suite is already very extensive.
Running this can give you even more confidence by comparing against real-world-rails or similar.

There are some performance gains to be had here. Basically it is already parallelized because
of `fork` but it can be even better. For simplicity (and because I don't usually
write such code) I leave that as an exercise for the future.
The message on change can also be improved, but right now I'm not certain that's worth the effort.
Just check it manually via the already existing tools.

Author: Earlopain

bin/compare

@@ -0,0 +1,86 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+# Usage: bin/compare main feature-branch .
+
+$:.unshift(File.expand_path("../lib", __dir__))
+
+require "json"
+require 'socket'
+
+def create_prism(ref)
+  parent_socket, child_socket = UNIXSocket.pair
+
+  system("git checkout #{ref}", exception: true)
+  system("bundle exec rake compile", exception: true)
+
+  pid = fork do
+    parent_socket.close
+    require "prism"
+
+    child_socket.puts("Compiling done for #{ref}")
+
+    while(path = child_socket.gets(chomp: true))
+      begin
+        result = Prism.parse(File.read(path), version: "latest")
+        child_socket.puts(serialize_parse_result(result).to_json)
+      rescue Errno::EISDIR
+        # Folder might end with `.rb` and get caught by the glob
+        child_socket.puts("{}")
+      end
+    end
+
+    exit!(0)
+  end
+
+  child_socket.close
+  parent_socket.gets
+  [pid, parent_socket]
+end
+
+def serialize_parse_result(parse_result)
+  {
+    valid: parse_result.success?,
+    errors: parse_result.errors_format.hash,
+    ast: parse_result.value.inspect.hash,
+  }
+end
+
+base_ref = ARGV.shift
+compare_ref = ARGV.shift
+path = ARGV.shift
+
+pid_baseline, socket_baseline = create_prism(base_ref)
+pid_compare, socket_compare = create_prism(compare_ref)
+
+result = +""
+files = Dir.glob(File.join(path, "**/*.rb"))
+files.each_with_index do |source_path, i|
+  puts "#{i}/#{files.size}" if i % 1000 == 0
+
+  socket_baseline.puts(source_path)
+  socket_compare.puts(source_path)
+  
+  baseline = JSON.parse(socket_baseline.gets(chomp: true), symbolize_names: true)
+  compare = JSON.parse(socket_compare.gets(chomp: true), symbolize_names: true)
+
+  if compare[:valid] != baseline[:valid]
+    result << "#{source_path} changed from valid(#{baseline[:valid]}) to valid(#{compare[:valid]})\n"
+  elsif compare[:valid] && baseline[:valid] && compare[:ast] != baseline[:ast]
+    result << "#{source_path} is syntax valid with changed ast}\n"
+  elsif !compare[:valid] && !baseline[:valid] && compare[:errors] != baseline[:errors]
+    result << "#{source_path} is syntax invalid with changed errors\n"
+  end
+end
+
+if result.empty?
+  puts "All good!"
+else
+  puts "Oops:"
+  puts result
+end
+
+socket_baseline.close
+socket_compare.close
+Process.wait(pid_baseline)
+Process.wait(pid_compare)