Fix and specs for crash in Spawn#limit_io on MRI 1.9.3.

lib/exec_sandbox/spawn.rb

@@ -65,7 +65,18 @@ def self.limit_io(io)
     redirected_fds = Set.new redirects.map(&:first)
     max_fd = LibC.getdtablesize
     0.upto(max_fd) do |fd|
-      LibC.close fd unless redirected_fds.include?(fd)
+      next if redirected_fds.include?(fd)
+
+      # TODO(pwnall): this is slow; consider detecting the Ruby version and
+      #               only running it on buggy MRIs
+      begin
+        # This fails if rb_reserved_fd_p returns 0.
+        # In that case, we shouldn't close the FD, otherwise the VM will crash.
+        IO.new(fd)
+      rescue ArgumentError, Errno::EBADF
+        next
+      end
+      LibC.close fd
     end
   end
 

spec/exec_sandbox/spawn_spec.rb

@@ -13,8 +13,20 @@
       @temp_in.close
       @temp_out = Tempfile.new 'exec_sandbox_rspec'
       @temp_out.close
+
+      # Force-creating a 2nd thread to make MRI 1.9.3 crash without our fix.
+      @lock = Mutex.new
+      @lock.lock
+      Thread.new do
+        loop do
+          sleep 0.1
+          break if @lock.try_lock
+        end
+        @lock.unlock
+      end
     end
     after do
+      @lock.unlock
       @temp_in.unlink
       @temp_out.unlink
     end
@@ -37,7 +49,8 @@
     describe 'with paths' do
       before do
         pid = ExecSandbox::Spawn.spawn bin_fixture(:duplicate),
-            {:in => @temp_in.path, :out => @temp_out.path}
+            {:in => @temp_in.path, :out => @temp_out.path,
+             :err => @temp_out.path}
         @status = ExecSandbox::Wait4.wait4 pid
       end
 

spec/fixtures/fork.rb

@@ -11,7 +11,9 @@
   0.upto(proc_count - 1) do |i|
     pids[i] = fork do
       f.write '+'
+      f.close
       sleep 1
+      exit
     end
   end
   0.upto(proc_count - 1) { |i| Process.waitpid(pids[i]) }

spec/spec_helper.rb

@@ -3,6 +3,9 @@
 require 'rspec'
 require 'exec_sandbox'
 
+# Spawn tests use threads to test against an MRI 1.9.3 crash.
+require 'thread'
+
 # Requires supporting files with custom matchers and macros, etc,
 # in ./support/ and its subdirectories.
 Dir["#{File.dirname(__FILE__)}/support/**/*.rb"].each {|f| require f}