Use job API for system() calls in GUI Vim to avoid overhead (#1325)

In GUI Vim (gvim, MacVim), each system() call incurs significant
overhead due to command prompt window creation, making operations
like PlugDiff ~5x slower than in terminal Vim or Neovim.

Add s:system_job() which uses job_start with file-based output to
run commands without that overhead, and use it in s:system() when
running in GUI Vim with job support.

Since v:shell_error is read-only in Vim 9, introduce s:shell_error
which is set by both code paths and used at all call sites.

Fixes #1312

plug.vim

@@ -68,6 +68,7 @@ let s:mac_gui = has('gui_macvim') && has('gui_running')
 let s:is_win = has('win32')
 let s:nvim = has('nvim-0.2') || (has('nvim') && exists('*jobwait') && !s:is_win)
 let s:vim8 = has('patch-8.0.0039') && exists('*job_start')
+let s:shell_error = 0
 if s:is_win && &shellslash
   set noshellslash
   let s:me = resolve(expand('<sfile>:p'))
@@ -170,7 +171,7 @@ function! s:git_origin_branch(spec)
 
   " The command may not return the name of a branch in detached HEAD state
   let result = s:lines(s:system('git symbolic-ref --short HEAD', a:spec.dir))
-  return v:shell_error ? '' : result[-1]
+  return s:shell_error ? '' : result[-1]
 endfunction
 
 if s:is_win
@@ -1097,7 +1098,7 @@ function! s:checkout(spec)
     let credential_helper = s:disable_credential_helper() ? '-c credential.helper= ' : ''
     let output = s:system(
           \ 'git '.credential_helper.'fetch --depth 999999 && git checkout '.plug#shellescape(sha).' --', a:spec.dir)
-    let error = v:shell_error
+    let error = s:shell_error
   endif
   return [output, error]
 endfunction
@@ -1303,7 +1304,7 @@ function! s:update_finish()
         let tag = spec.tag
         if tag =~ '\*'
           let tags = s:lines(s:system('git tag --list '.plug#shellescape(tag).' --sort -version:refname 2>&1', spec.dir))
-          if !v:shell_error && !empty(tags)
+          if !s:shell_error && !empty(tags)
             let tag = tags[0]
             call s:log4(name, printf('Latest tag for %s -> %s', spec.tag, tag))
             call append(3, '')
@@ -1311,7 +1312,7 @@ function! s:update_finish()
         endif
         call s:log4(name, 'Checking out '.tag)
         let out = s:system('git checkout -q '.plug#shellescape(tag).' -- 2>&1', spec.dir)
-        let error = v:shell_error
+        let error = s:shell_error
       endif
       if !error && filereadable(spec.dir.'/.gitmodules') &&
             \ (s:update.force || has_key(s:update.new, name) || s:is_updated(spec.dir))
@@ -1319,7 +1320,7 @@ function! s:update_finish()
         let out .= s:bang('git submodule update --init --recursive'.s:submodule_opt.' 2>&1', spec.dir)
         let error = v:shell_error
       endif
-      let msg = s:format_message(v:shell_error ? 'x': '-', name, out)
+      let msg = s:format_message(error ? 'x': '-', name, out)
       if error
         call add(s:update.errors, name)
         call s:regress_bar()
@@ -1480,7 +1481,7 @@ function! s:spawn(name, spec, queue, opts)
     endif
   else
     let job.lines = s:lines(call('s:system', has_key(a:opts, 'dir') ? [argv, a:opts.dir] : [argv]))
-    let job.error = v:shell_error != 0
+    let job.error = s:shell_error != 0
     let job.running = 0
   endif
 endfunction
@@ -2334,6 +2335,22 @@ function! s:with_cd(cmd, dir, ...)
   return printf('%s %s %s %s', cd, plug#shellescape(a:dir, {'script': script, 'shell': &shell}), sep, a:cmd)
 endfunction
 
+function! s:system_job(cmd) abort
+  let tmp = tempname()
+  let job = job_start(['/bin/sh', '-c', a:cmd], {
+  \ 'out_io': 'file',
+  \ 'out_name': tmp,
+  \ 'err_io': 'out',
+  \})
+  while job_status(job) ==# 'run'
+    sleep 1m
+  endwhile
+  let s:shell_error = job_info(job).exitval
+  let result = filereadable(tmp) ? join(readfile(tmp, 'b'), "\n") : ''
+  silent! call delete(tmp)
+  return result
+endfunction
+
 function! s:system(cmd, ...)
   let batchfile = ''
   try
@@ -2343,7 +2360,9 @@ function! s:system(cmd, ...)
       " but it cannot set the working directory for the command.
       " Assume that the command does not rely on the shell.
       if has('nvim') && a:0 == 0
-        return system(a:cmd)
+        let ret = system(a:cmd)
+        let s:shell_error = v:shell_error
+        return ret
       endif
       let cmd = join(map(copy(a:cmd), 'plug#shellescape(v:val, {"shell": &shell, "script": 0})'))
       if s:is_powershell(&shell)
@@ -2358,7 +2377,12 @@ function! s:system(cmd, ...)
     if s:is_win && type(a:cmd) != s:TYPE.list
       let [batchfile, cmd] = s:batchfile(cmd)
     endif
-    return system(cmd)
+    if s:vim8 && has('gui_running') && !s:is_win
+      return s:system_job(cmd)
+    endif
+    let ret = system(cmd)
+    let s:shell_error = v:shell_error
+    return ret
   finally
     let [&shell, &shellcmdflag, &shellredir] = [sh, shellcmdflag, shrd]
     if s:is_win && filereadable(batchfile)
@@ -2369,7 +2393,7 @@ endfunction
 
 function! s:system_chomp(...)
   let ret = call('s:system', a:000)
-  return v:shell_error ? '' : substitute(ret, '\n$', '', '')
+  return s:shell_error ? '' : substitute(ret, '\n$', '', '')
 endfunction
 
 function! s:git_validate(spec, check_branch)
@@ -2412,7 +2436,7 @@ function! s:git_validate(spec, check_branch)
           \ 'git', 'rev-list', '--count', '--left-right',
           \ printf('HEAD...origin/%s', origin_branch)
           \ ], a:spec.dir)), '\t')
-        if v:shell_error || len(ahead_behind) != 2
+        if s:shell_error || len(ahead_behind) != 2
           let err = "Failed to compare with the origin. The default branch might have changed.\nPlugClean required."
         else
           let [ahead, behind] = ahead_behind
@@ -2562,7 +2586,7 @@ function! s:upgrade()
 
   try
     let out = s:system(['git', 'clone', '--depth', '1', s:plug_src, tmp])
-    if v:shell_error
+    if s:shell_error
       return s:err('Error upgrading vim-plug: '. out)
     endif