Add plug#helptags()

plug.vim

@@ -369,13 +369,17 @@ function! s:update(force, ...)
   call s:update_impl(1, a:force, a:000)
 endfunction
 
-function! s:helptags()
+function! plug#helptags()
+  if !exists('g:plugs')
+    return s:err('plug#begin is not called')
+  endif
   for spec in values(g:plugs)
     let docd = join([spec.dir, 'doc'], '/')
     if isdirectory(docd)
       silent! execute 'helptags '. s:esc(docd)
     endif
   endfor
+  return 1
 endfunction
 
 function! s:syntax()
@@ -508,7 +512,7 @@ endfunction
 function! s:finish(pull)
   call append(3, '- Finishing ... ')
   redraw
-  call s:helptags()
+  call plug#helptags()
   call plug#end()
   call setline(4, getline(4) . 'Done!')
   normal! gg

test/run

@@ -30,6 +30,10 @@ EOF
 
 make_dirs xxx/
 make_dirs xxx/after
+mkdir xxx/doc
+cat > xxx/doc/xxx.txt << DOC
+hello *xxx*
+DOC
 
 cat > /tmp/mini-vimrc << VIMRC
 set rtp+=vader.vim

test/workflow.vader

@@ -761,6 +761,16 @@ Execute (Filetype-based on-demand loading):
   setf xxx
   AssertEqual ['/ftdetect', 'after/ftdetect', '/plugin', 'after/plugin', '/ftplugin', 'after/ftplugin', '/indent', 'after/indent', '/syntax', 'after/syntax'], g:xxx
 
+**********************************************************************
+~ plug#helptags()
+**********************************************************************
+
+Execute (plug#helptags):
+  silent! call delete(expand('$PWD/xxx/doc/tags'))
+  Assert !filereadable(expand('$PWD/xxx/doc/tags'))
+  AssertEqual 1, plug#helptags()
+  Assert filereadable(expand('$PWD/xxx/doc/tags'))
+
 Before:
 Execute (Cleanup):
   silent! call system('rm -rf '.temp_plugged)