vim: de-quote 'shell' option

De-quote only if first and last character of &shell are double-quotes.
Avoid string escape issues when passing escaped '&shell' values.

Author: janlazo

after/ftplugin/ps1.vim

@@ -21,24 +21,18 @@ if !exists('*s:help')
     \ ? 'powershell.exe'
     \ : 'pwsh'
     let pager = has('win32') ? 'more' : 'less'
-    if !executable(newshell)
-      echomsg newshell 'is not in PATH'
-      return
-    endif
-
     let shell = &shell
-    " de-quote
-    if shell[0] ==# '"'
-      let shell = shell[1:-2]
+    if !executable(newshell) || !SetShell(newshell)
+      echoerr newshell 'cannot be set'
+      return
     endif
 
     try
-      execute 'SetShell' newshell
       let help_fmt = 'Get-Help %s | ' . pager
       let cmd = printf(help_fmt, dotvim8#shellescape(expand('<cword>')))
       call dotvim8#bang(cmd)
     finally
-      execute 'SetShell' shell
+      call SetShell(shell)
     endtry
   endfunction
 endif

autoload/dotvim8.vim

@@ -48,11 +48,7 @@ function! dotvim8#shellescape(arg, ...)
   endif
 
   let opts = get(a:000, 0, {})
-  let shell = fnamemodify(get(opts, 'shell', &shell), ':t')
-  " de-quote
-  if shell[0] ==# '"'
-    let shell = shell[1:-2]
-  endif
+  let shell = fnamemodify(UnescapeShell(get(opts, 'shell', &shell)), ':t')
   let script = get(opts, 'script', 0)
   let arg = get(opts, 'escape_argv', 1) && (has('win32') || has('win32unix')) ?
             \ escape(a:arg, '"\') : a:arg
@@ -81,11 +77,7 @@ function! dotvim8#bang(cmd)
     echomsg 'Command is empty string'
     return
   endif
-  let shell = &shell
-  " de-quote
-  if shell[0] ==# '"'
-    let shell = shell[1:-2]
-  endif
+  let shell = UnescapeShell(&shell)
 
   if s:has_term
     if has('nvim')

shared.vim

@@ -162,12 +162,38 @@ endif
 if has('modify_fname')
   let s:base_dir = expand('<sfile>:p:h')
 
-  function! s:set_shell(shell)
-    let shell = a:shell
-    " de-quote
-    if shell[0] ==# '"'
-      let shell = shell[1:-2]
+  " Unescape 'shell' option in case that the user escaped it.
+  function! UnescapeShell(shell)
+    if (has('nvim') || has('win32')) && a:shell =~# '^".\+"$'
+      " de-quote
+      return a:shell[1:-2]
     endif
+    if !has('nvim') && has('unix') && a:shell =~# '\'
+      " strip backslashes
+      return substitute(a:shell, '\', '', 'g')
+    endif
+    return a:shell
+  endfunction
+
+  " Escape 'shell' option depending on the editor.
+  function! EscapeShell(shell)
+    if a:shell =~# ' '
+      if has('nvim') || (has('win32') && has('patch-8.1.2115'))
+        " quote
+        return '"' . a:shell . '"'
+      endif
+      " TODO: Investigate the patch that added support for escaped 'shell'.
+      if !has('nvim') && has('unix') && has('patch-8.0.1176')
+        " add backslashes
+        return escape(a:shell, ' ')
+      endif
+    endif
+    return a:shell
+  endfunction
+
+  " If shell is supported, return 1. Else, return 0.
+  function! SetShell(shell)
+    let shell = UnescapeShell(a:shell)
     let tail = fnamemodify(shell, ':t')
 
     if tail =~# '^cmd\.exe'
@@ -214,16 +240,19 @@ if has('modify_fname')
       let &shellxquote = (!has('nvim') && has('win32')) ? '"' : ''
     else
       echoerr a:shell 'is not supported in Vim' v:version
-      return
+      return 0
     endif
-    let &shell = match(shell, ' ') == -1 ? shell : '"' . shell . '"'
+    let &shell = EscapeShell(shell)
+    return 1
   endfunction
 
-  if has('win32')
-    call s:set_shell(has('nvim') || !executable($COMSPEC) ? 'cmd.exe' : $COMSPEC)
-  elseif has('unix')
-    call s:set_shell(filter([$SHELL, 'sh'], 'executable(v:val)')[0])
-  endif
+  let s:shells = filter(has('win32') ? [$COMSPEC, 'cmd.exe'] : [$SHELL, 'sh'], 'executable(v:val)')
+  for s:shell in s:shells
+    if SetShell(s:shell)
+      break
+    endif
+  endfor
+  unlet s:shell s:shells
 endif
 
 " Moved from normal to tiny version since 8.1.1901
@@ -429,7 +458,7 @@ if has('user_commands')
   endif
 
   if has('modify_fname')
-    command! -nargs=1 -complete=shellcmd SetShell call s:set_shell(<f-args>)
+    command! -nargs=1 -complete=shellcmd SetShell call SetShell(<f-args>)
   endif
 
   if has('syntax')