Add a diagnostic for a bad re2 gem

When the system re2 library gets updated, it's often difficult and
time-consuming to determine that a `gem pristine re2` is needed. This
`gdk doctor` diagnostic attempts to reproduce the seg fault that occurs
as a result of mudge/re2#43.

lib/gdk/diagnostic.rb

@@ -10,6 +10,7 @@
 require_relative 'diagnostic/geo'
 require_relative 'diagnostic/git'
 require_relative 'diagnostic/ruby_gems'
+require_relative 'diagnostic/re2'
 
 module GDK
   module Diagnostic
@@ -23,6 +24,7 @@ def self.all
         PendingMigrations
         Geo
         Status
+        Re2
       ]
 
       klasses.map do |const|

lib/gdk/diagnostic/re2.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+require 'open3'
+
+module GDK
+  module Diagnostic
+    class Re2 < Base
+      TITLE = 're2'
+      SCRIPT = "require 're2'; regexp = RE2::Regexp.new('{', log_errors: false); regexp.error unless regexp.ok?"
+
+      def diagnose
+        # When re2 and libre2 are out of sync, a seg fault can occur due
+        # to some memory corruption (https://github.com/mudge/re2/issues/43).
+        # This test doesn't always fail the first time, so repeat the test
+        # several times to be sure.
+        5.times do
+          @stdout, @stderr, @status = Open3.capture3('ruby', '-e', SCRIPT)
+          break unless @status.success?
+        end
+      end
+
+      def success?
+        @status.success? && @stdout.empty? && @stderr.empty?
+      end
+
+      def detail
+        <<~MESSAGE
+          It looks like your system re2 library may have been upgraded, and
+          the re2 gem needs to be rebuilt as a result.
+
+          Please run `gem pristine re2`.
+        MESSAGE
+      end
+    end
+  end
+end

spec/lib/gdk/diagnostic_spec.rb

@@ -17,7 +17,8 @@
         GDK::Diagnostic::Dependencies,
         GDK::Diagnostic::PendingMigrations,
         GDK::Diagnostic::Geo,
-        GDK::Diagnostic::Status
+        GDK::Diagnostic::Status,
+        GDK::Diagnostic::Re2
       ]
 
       expect(described_class.all.map(&:class)).to eq(diagnostic_classes)