Use spawn() instead of open3 and avoid use of Dir.chdir

Changing the current directory affects the whole process, so without
eliminating the use of Dir.chdir we cannot safely run threaded workers
or dry-run agents on a threaded rack application server, etc.

app/models/agents/shell_command_agent.rb

@@ -1,5 +1,3 @@
-require 'open3'
-
 module Agents
   class ShellCommandAgent < Agent
     default_schedule "never"
@@ -88,23 +86,25 @@ module Agents
     end
 
     def run_command(path, command)
-      result = nil
-      errors = nil
-      exit_status = nil
-
-      Dir.chdir(path){
-        begin
-          stdin, stdout, stderr, wait_thr = Open3.popen3(command)
-          exit_status = wait_thr.value.to_i
-          result = stdout.gets(nil)
-          errors = stderr.gets(nil)
-        rescue Exception => e
-          errors = e.to_s
-        end
-      }
+      begin
+        rout, wout = IO.pipe
+        rerr, werr = IO.pipe
+
+        pid = spawn(command, chdir: path, out: wout, err: werr)
 
-      result = result.to_s.strip
-      errors = errors.to_s.strip
+        wout.close
+        werr.close
+
+        (result = rout.read).strip!
+        (errors = rerr.read).strip!
+
+        _, status = Process.wait2(pid)
+        exit_status = status.exitstatus
+      rescue Exception => e
+        errors = e.to_s
+        result = ''.freeze
+        exit_status = nil
+      end
 
       [result, errors, exit_status]
     end