Resolve issue #101: Add support for lock directory configuration

See issue #101 on GitHub:
  #101

README.md

@@ -127,6 +127,13 @@ Note that the vim-session plug-in automatically and unconditionally executes the
 
 This option controls the location of your session scripts. Its default value is `~/.vim/sessions` (on UNIX) or `~\vimfiles\sessions` (on Windows). If you don't mind the default you don't have to do anything; the directory will be created for you. Note that a leading `~` is expanded to your current home directory (`$HOME` on UNIX, `%USERPROFILE%` on Windows).
 
+### The `g:session_lock_directory` option
+
+The vim-session plug-in uses lock files to prevent double loading of sessions. By default the lock files are stored in the same directory as the sessions. If you'd rather store lock files in a dedicated lock file directory you can use this option:
+
+    " Store lock files in a tmpfs that doesn't persist between reboots.
+    let g:session_lock_directory = '/var/lock'
+
 ### The `g:session_default_name` option
 
 The name of the default session without directory or filename extension (you'll never guess what the default is).
@@ -256,7 +263,7 @@ Recently this plug-in switched from reimplementing [:mksession][mksession] to ac
 <!-- Start of generated documentation -->
 
 The documentation of the 37 functions below was extracted from
-2 Vim scripts on July 30, 2014 at 22:23.
+2 Vim scripts on September 14, 2014 at 13:07.
 
 ### Public API for the vim-session plug-in
 

autoload/xolox/session.vim

@@ -1,10 +1,10 @@
 " Public API for the vim-session plug-in.
 "
 " Author: Peter Odding
-" Last Change: July 30, 2014
+" Last Change: September 14, 2014
 " URL: http://peterodding.com/code/vim/session/
 
-let g:xolox#session#version = '2.6.4'
+let g:xolox#session#version = '2.7'
 
 " Public API for session persistence. {{{1
 
@@ -1018,8 +1018,18 @@ function! s:vim_instance_id()
   return id
 endfunction
 
+function! s:lock_file_path(session_path)
+  let directory = xolox#misc#option#get('session_lock_directory', '')
+  if !empty(directory)
+    let pathname = xolox#misc#path#merge(directory, xolox#misc#path#encode(a:session_path))
+  else
+    let pathname = a:session_path
+  endif
+  return pathname . '.lock'
+endfunction
+
 function! s:lock_session(session_path)
-  let lock_file = a:session_path . '.lock'
+  let lock_file = s:lock_file_path(a:session_path)
   if xolox#misc#persist#save(lock_file, s:vim_instance_id())
     if index(s:lock_files, lock_file) == -1
       call add(s:lock_files, lock_file)
@@ -1029,7 +1039,7 @@ function! s:lock_session(session_path)
 endfunction
 
 function! s:unlock_session(session_path)
-  let lock_file = a:session_path . '.lock'
+  let lock_file = s:lock_file_path(a:session_path)
   if delete(lock_file) == 0
     let idx = index(s:lock_files, lock_file)
     if idx >= 0
@@ -1041,7 +1051,7 @@ endfunction
 
 function! s:session_is_locked(session_name, command)
   let session_path = xolox#session#name_to_path(a:session_name)
-  let lock_file = session_path . '.lock'
+  let lock_file = s:lock_file_path(session_path)
   if filereadable(lock_file)
     let this_instance = s:vim_instance_id()
     let other_instance = xolox#misc#persist#load(lock_file)

doc/session.txt

@@ -20,21 +20,22 @@ Contents ~
  4. Options                                                   |session-options|
   1. The |sessionoptions| setting
   2. The |g:session_directory| option
-  3. The |g:session_default_name| option
-  4. The |g:session_default_overwrite| option
-  5. The |g:session_extension| option
-  6. The |g:session_autoload| option
-  7. The |g:session_autosave| option
-  8. The |g:session_autosave_periodic| option
-  9. The |g:session_verbose_messages| option
-  10. The |g:session_default_to_last| option
-  11. The |g:session_persist_colors| option
-  12. The |g:session_persist_globals| option
-  13. The |g:session_restart_environment| option
-  14. The |g:session_command_aliases| option
-  15. The |g:session_menu| option
-  16. The |g:session_name_suggestion_function| option
-  17. The |g:loaded_session| option
+  3. The |g:session_lock_directory| option
+  4. The |g:session_default_name| option
+  5. The |g:session_default_overwrite| option
+  6. The |g:session_extension| option
+  7. The |g:session_autoload| option
+  8. The |g:session_autosave| option
+  9. The |g:session_autosave_periodic| option
+  10. The |g:session_verbose_messages| option
+  11. The |g:session_default_to_last| option
+  12. The |g:session_persist_colors| option
+  13. The |g:session_persist_globals| option
+  14. The |g:session_restart_environment| option
+  15. The |g:session_command_aliases| option
+  16. The |g:session_menu| option
+  17. The |g:session_name_suggestion_function| option
+  18. The |g:loaded_session| option
  5. Compatibility with other plug-ins |session-compatibility-with-other-plug-ins|
  6. Known issues                                         |session-known-issues|
  7. Function reference                             |session-function-reference|
@@ -305,6 +306,17 @@ mind the default you don't have to do anything; the directory will be created
 for you. Note that a leading '~' is expanded to your current home directory
 ('$HOME' on UNIX, '%USERPROFILE%' on Windows).
 
+-------------------------------------------------------------------------------
+The *g:session_lock_directory* option
+
+The vim-session plug-in uses lock files to prevent double loading of sessions.
+By default the lock files are stored in the same directory as the sessions. If
+you'd rather store lock files in a dedicated lock file directory you can use
+this option:
+>
+  " Store lock files in a tmpfs that doesn't persist between reboots.
+  let g:session_lock_directory = '/var/lock'
+<
 -------------------------------------------------------------------------------
 The *g:session_default_name* option
 
@@ -513,7 +525,7 @@ might take a while...)
 Function reference ~
 
 The documentation of the 37 functions below was extracted from 2 Vim scripts on
-July 30, 2014 at 22:23.
+September 14, 2014 at 13:07.
 
 -------------------------------------------------------------------------------
                                            *public-api-for-vim-session-plug-in*