Allow customizing the upgrade command line... via the commandline

This allows replacing the einhorn command name (say,
`/usr/local/bin/einhorn`) with a more intricate command
line (e.g. `rbenv exec bundle exec einhorn`). That command line will
be split using shellwords.

To that end, introduce `Einhorn.upgrade_commandline`, a helper method
that can construct the command line used when reloading code.

bin/einhorn

@@ -275,6 +275,10 @@ if true # $0 == __FILE__
       Einhorn::Command.louder(false)
     end
 
+    opts.on('--reexec-as=CMDLINE', 'Substitute CMDLINE for \"einhorn\" when upgrading') do |cmdline|
+      Einhorn::TransientState.re_exec_commandline = Shellwords.shellsplit(cmdline)
+    end
+
     Einhorn.plugins_send(:optparse, opts)
 
     opts.on('--nice MASTER[:WORKER=0][:RENICE_CMD=/usr/bin/renice]', 'Unix nice level at which to run the einhorn processes. If not running as root, make sure to ulimit -e as appopriate.') do |nice|

lib/einhorn.rb

@@ -5,6 +5,7 @@
 require 'socket'
 require 'tmpdir'
 require 'yaml'
+require 'shellwords'
 
 require 'einhorn/third/little-plugger'
 
@@ -329,6 +330,20 @@ def self.socketify!(cmd)
     end
   end
 
+  # Construct and a command and args that can be used to re-exec
+  # Einhorn for upgrades.
+  def self.upgrade_commandline(prefix=[])
+    cmdline = []
+    if Einhorn::TransientState.re_exec_commandline
+      cmdline += Einhorn::TransientState.re_exec_commandline
+    else
+      cmdline << Einhorn::TransientState.script_name
+    end
+    cmdline += prefix
+    cmdline += Einhorn::State.cmd
+    [cmdline[0], cmdline[1..-1]]
+  end
+
   # Perform startup checks to ensure our environment is sane
   def self.sanity_check
     log_info("Running under Ruby #{RUBY_VERSION}")

lib/einhorn/command.rb

@@ -226,11 +226,10 @@ def self.reload
       ENV.update(Einhorn::TransientState.environ)
 
       begin
-        Einhorn::Compat.exec(
-          Einhorn::TransientState.script_name,
-          ['--with-state-fd', read.fileno.to_s, '--'] + Einhorn::State.cmd,
-          :close_others => false
-          )
+        upgrade_cmd, upgrade_args =
+                     Einhorn::Compat.exec(
+                       *Einhorn.upgrade_commandline(['--with-state-fd', read.fileno.to_s, '--']),
+                       :close_others => false)
       rescue SystemCallError => e
         Einhorn.log_error("Could not reload! Attempting to continue. Error was: #{e}")
         Einhorn::State.reloading_for_preload_upgrade = false
@@ -278,7 +277,7 @@ def self.spinup(cmd=nil)
           Einhorn::Event.close_all_for_worker
 
           prepare_child_environment(index)
-          Einhorn::Compat.exec(cmd[0], cmd[1..-1], :close_others => false)
+          Einhorn::Compat.exec(*Einhorn.upgrade_commandline, :close_others => false)
         end
       end