Automatically store lock files in /var/lock when possible (issue #110)

This is basically a follow up to issue #101 with the idea that sane
defaults should be provided when possible - most users don't read the
documentation and even when they do they might still not find what
they are looking for (and I don't really blame them, I'm exactly the
same :-).

Issue #101 on GitHub: #101
Issue #110 on GitHub: #110

README.md

@@ -129,10 +129,11 @@ This option controls the location of your session scripts. Its default value is
 
 ### 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:
+The vim-session plug-in uses lock files to prevent double loading of sessions. The default location (directory) of these lock files depends on a couple of factors:
 
-    " Store lock files in a tmpfs that doesn't persist between reboots.
-    let g:session_lock_directory = '/var/lock'
+1. If you have explicitly set the `g:session_lock_directory` option that defines the directory.
+2. If the directory `/var/lock` exists and is writable that is used as a sane default.
+3. As a sane fall back for platforms where `/var/lock` is not available the directory that stores the session scripts themselves is used.
 
 ### The `g:session_default_name` option
 
@@ -263,7 +264,7 @@ 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 September 14, 2014 at 13:07.
+2 Vim scripts on February 13, 2015 at 13:25.
 
 ### Public API for the vim-session plug-in
 
@@ -488,8 +489,8 @@ If you have questions, bug reports, suggestions, etc. the author can be contacte
 
 ## License
 
-This software is licensed under the [MIT license](http://en.wikipedia.org/wiki/MIT_License).  
-© 2014 Peter Odding &lt;<peter@peterodding.com>&gt; and Ingo Karkat.
+This software is licensed under the [MIT license](http://en.wikipedia.org/wiki/MIT_License).
+© 2015 Peter Odding &lt;<peter@peterodding.com>&gt; and Ingo Karkat.
 
 Thanks go out to everyone who has helped to improve the vim-session plug-in (whether through pull requests, bug reports or personal e-mails).
 

autoload/xolox/session.vim

@@ -1,10 +1,10 @@
 " Public API for the vim-session plug-in.
 "
 " Author: Peter Odding
-" Last Change: September 14, 2014
+" Last Change: February 13, 2015
 " URL: http://peterodding.com/code/vim/session/
 
-let g:xolox#session#version = '2.7'
+let g:xolox#session#version = '2.8'
 
 " Public API for session persistence. {{{1
 
@@ -1020,6 +1020,22 @@ endfunction
 
 function! s:lock_file_path(session_path)
   let directory = xolox#misc#option#get('session_lock_directory', '')
+  if empty(directory)
+    " Stale lock files can be really annoying, especially after a reboot
+    " because that just shouldn't happen - it's always a bug. References:
+    "  - https://github.com/xolox/vim-session/issues/97
+    "  - https://github.com/xolox/vim-session/issues/110
+    " One simple way to give a large group of users what they want is to use a
+    " volatile directory that is specifically meant for storing lock files.
+    " I've decided to make this the default when possible. The best reference
+    " I've been able to find on the proper system wide location for lock files
+    " is the following (yes, I know, it's Linux specific, so sue me):
+    " http://www.tldp.org/LDP/Linux-Filesystem-Hierarchy/html/var.html
+    let global_lock_directory = '/var/lock'
+    if filewritable(global_lock_directory) == 2
+      let directory = global_lock_directory
+    endif
+  endif
   if !empty(directory)
     let pathname = xolox#misc#path#merge(directory, xolox#misc#path#encode(a:session_path))
   else

doc/session.txt

@@ -310,13 +310,18 @@ for you. Note that a leading '~' is expanded to your current home directory
 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 default location (directory) of these lock files depends on a couple of
+factors:
+
+1. If you have explicitly set the |g:session_lock_directory| option that
+   defines the directory.
+
+2. If the directory '/var/lock' exists and is writable that is used as a
+   sane default.
+
+3. As a sane fall back for platforms where '/var/lock' is not available the
+   directory that stores the session scripts themselves is used.
+
 -------------------------------------------------------------------------------
 The *g:session_default_name* option
 
@@ -525,7 +530,7 @@ might take a while...)
 Function reference ~
 
 The documentation of the 37 functions below was extracted from 2 Vim scripts on
-September 14, 2014 at 13:07.
+February 13, 2015 at 13:25.
 
 -------------------------------------------------------------------------------
                                            *public-api-for-vim-session-plug-in*
@@ -764,7 +769,7 @@ session. If you like the script please vote for it on Vim Online [19].
                                                               *session-license*
 License ~
 
-This software is licensed under the MIT license [20]. Š 2014 Peter Odding
+This software is licensed under the MIT license [20]. Š 2015 Peter Odding
 <peter@peterodding.com> and Ingo Karkat.
 
 Thanks go out to everyone who has helped to improve the vim-session plug-in