fix(vim.system): make timeout work properly

Mimic the behaviour of timeout(1) from coreutils.

test/functional/lua/system_spec.lua

@@ -5,27 +5,39 @@ local eq = helpers.eq
 
 local function system_sync(cmd, opts)
   return exec_lua([[
-    return vim.system(...):wait()
+    local obj = vim.system(...)
+    local pid = obj.pid
+    local res = obj:wait()
+
+    -- Check the process is no longer running
+    vim.fn.systemlist({'ps', 'p', tostring(pid)})
+    assert(vim.v.shell_error == 1, 'process still exists')
+
+    return res
   ]], cmd, opts)
 end
 
 local function system_async(cmd, opts)
-  exec_lua([[
+  return exec_lua([[
     local cmd, opts = ...
     _G.done = false
-    vim.system(cmd, opts, function(obj)
+    local obj = vim.system(cmd, opts, function(obj)
       _G.done = true
       _G.ret = obj
     end)
+
+    local done = vim.wait(10000, function()
+      return _G.done
+    end)
+
+    assert(done, 'process did not exit')
+
+    -- Check the process is no longer running
+    vim.fn.systemlist({'ps', 'p', tostring(obj.pid)})
+    assert(vim.v.shell_error == 1, 'process still exists')
+
+    return _G.ret
   ]], cmd, opts)
-
-  while true do
-    if exec_lua[[return _G.done]] then
-      break
-    end
-  end
-
-  return exec_lua[[return _G.ret]]
 end
 
 describe('vim.system', function()
@@ -43,12 +55,12 @@ describe('vim.system', function()
         eq('hellocat', system({ 'cat' }, { stdin = 'hellocat', text = true }).stdout)
       end)
 
-      it ('supports timeout', function()
+      it('supports timeout', function()
         eq({
-          code = 0,
-          signal = 2,
+          code = 124,
+          signal = 15,
           stdout = '',
-          stderr = "Command timed out: 'sleep 10'"
+          stderr = ''
         }, system({ 'sleep', '10' }, { timeout = 1 }))
       end)
     end)