Add check for executable in Ruby threads

Now whenever an update is called a check is made (not just on
installation)

plug.vim

@@ -75,7 +75,7 @@ let s:mac_gui = has('gui_macvim') && has('gui_running')
 let s:is_win = has('win32') || has('win64')
 let s:nvim = exists('##JobActivity') && !s:is_win
 let s:me = resolve(expand('<sfile>:p'))
-let s:base_spec = { 'branch': 'master', 'frozen': 0 }
+let s:base_spec = { 'branch': 'master', 'frozen': 0, 'needs': []}
 let s:TYPE = {
 \   'string':  type(''),
 \   'list':    type([]),
@@ -578,13 +578,6 @@ function! s:assign_name()
   silent! execute 'f' fnameescape(name)
 endfunction
 
-function! s:all_executable(executables)
-  if len(a:executables) == 0
-      return 1
-  endif
-  let are_executable = map(a:executables, 'executable(v:val)')
-  return index(are_executable, 0) == -1
-endfunction
 
 function! s:do(pull, force, todo)
   for [name, spec] in items(a:todo)
@@ -594,15 +587,7 @@ function! s:do(pull, force, todo)
     let installed = has_key(s:update.new, name)
     let updated = installed ? 0 :
       \ (a:pull && !empty(s:system_chomp('git log --pretty=format:"%h" "HEAD...HEAD@{1}"', spec.dir)))
-    " TODO(alexandre): use the function executable on all the parameters in
-    " the 'needs' attribute - if not then don't update.
-    if has_key(spec, 'needs')
-      echom 'Has needs.'
-      let canbuild = s:all_executable(s:to_a(spec.needs))
-    else
-      let canbuild = 1
-    endif
-    if (a:force || installed || updated) && canbuild
+    if a:force || installed || updated 
       execute 'cd' s:esc(spec.dir)
       call append(3, '- Post-update hook for '. name .' ... ')
       let type = type(spec.do)
@@ -672,8 +657,6 @@ function! s:names(...)
   return sort(filter(keys(g:plugs), 'stridx(v:val, a:1) == 0 && s:is_managed(v:val)'))
 endfunction
 
-" TODO (alexandre) - Maybe if you don't have the build commands,
-" you don't pull it at all, (citing the reason).
 function! s:update_impl(pull, force, args) abort
   let args = copy(a:args)
   let threads = (len(args) > 0 && args[-1] =~ '^[1-9][0-9]*$') ?
@@ -894,6 +877,10 @@ function! s:update_vim()
   call s:tick()
 endfunction
 
+function! s:missing_executables(executables)
+  return filter(copy(s:to_a(a:executables)), '!executable(v:val)')
+endfunction
+
 function! s:tick()
 while 1 " Without TCO, Vim stack is bound to explode
   if empty(s:update.todo)
@@ -910,8 +897,14 @@ while 1 " Without TCO, Vim stack is bound to explode
   let new  = !isdirectory(spec.dir)
 
   call s:log(new ? '+' : '*', name, pull ? 'Updating ...' : 'Installing ...')
+  echom 'blah'
   redraw
 
+  let executables = s:missing_executables(spec.needs)
+  if !empty(executables)
+    let msg = 'Please install '. join(executables, ', ') . '.'
+    let s:jobs[name] = { 'running': 0, 'result': msg, 'error': 1 }
+  else
     if !new
       let [valid, msg] = s:git_valid(spec, 0)
       if valid
@@ -932,6 +925,7 @@ while 1 " Without TCO, Vim stack is bound to explode
             \ s:shellesc(spec.branch),
             \ s:shellesc(s:trim(spec.dir))), { 'new': 1 })
     endif
+  endif
 
   if !s:jobs[name].running
     call s:reap(name)
@@ -979,6 +973,17 @@ function! s:update_ruby()
     pids.each { |pid| Process.kill 'TERM', pid.to_i rescue nil }
   end
 
+  def which cmd
+    exts = ENV['PATHEXT'] ? ENV['PATHEXT'].split(';') : ['']
+    ENV['PATH'].split(File::PATH_SEPARATOR).each do |path|
+      exts.each { |ext|
+        exe = File.join(path, "#{cmd}#{ext}")
+        return exe if File.executable?(exe) && !File.directory?(exe)
+      }
+    end
+    return nil
+  end
+
   require 'thread'
   require 'fileutils'
   require 'timeout'
@@ -1106,11 +1111,14 @@ function! s:update_ruby()
       threads << Thread.new {
         while pair = take1.call
           name = pair.first
-          dir, uri, branch = pair.last.values_at *%w[dir uri branch]
+          dir, uri, branch, needs = pair.last.values_at *%w[dir uri branch needs]
           branch = esc branch
           subm = "git submodule update --init --recursive 2>&1"
           exists = File.directory? dir
+          needs = needs.kind_of?(Array) ? needs : [needs]
+          executables = needs.select{ |exe| !which exe }
           ok, result =
+            if executables.empty?
               if exists
                 dir = esc dir
                 ret, data = bt.call "#{cd} #{dir} && git rev-parse --abbrev-ref HEAD 2>&1 && git config remote.origin.url", nil, nil, nil
@@ -1140,6 +1148,9 @@ function! s:update_ruby()
                   FileUtils.rm_rf dir
                 }
               end
+            else
+              [false, "Please install " + executables.join(", ") + " first."]
+            end
           mtx.synchronize { VIM::command("let s:update.new['#{name}'] = 1") } if !exists && ok
           log.call name, result, ok
         end