fix: correctly check group permissions in unix (#428)

* fix: set modifiable when user in group

* feat: add mode caching, fallback to previous, and better checking of permissions

* fix: make is_modifiable check group permissions even if the user is owner of the directory

* refactor: simplify group ID caching

---------

Co-authored-by: Steven Arcangeli <stevearc@stevearc.com>

lua/oil/adapters/files.lua

@@ -412,6 +412,26 @@ M.list = function(url, column_defs, cb)
   end, 10000)
 end
 
+---@type nil|integer[]
+local _group_ids
+---@return integer[]
+local function get_group_ids()
+  if not _group_ids then
+    local output = vim.fn.system({ "id", "-G" })
+    if vim.v.shell_error == 0 then
+      _group_ids = vim.tbl_map(tonumber, vim.split(output, "%s+", { trimempty = true }))
+    else
+      -- If the id command fails, fall back to just using the process group
+      _group_ids = { uv.getgid() }
+      vim.notify(
+        "[oil] missing the `id` command. Some directories may not be modifiable even if you have group access.",
+        vim.log.levels.WARN
+      )
+    end
+  end
+  return _group_ids
+end
+
 ---@param bufnr integer
 ---@return boolean
 M.is_modifiable = function(bufnr)
@@ -433,14 +453,12 @@ M.is_modifiable = function(bufnr)
   end
 
   local uid = uv.getuid()
-  local gid = uv.getgid()
-  local rwx
+  local rwx = stat.mode
   if uid == stat.uid then
-    rwx = bit.rshift(stat.mode, 6)
-  elseif gid == stat.gid then
-    rwx = bit.rshift(stat.mode, 3)
-  else
-    rwx = stat.mode
+    rwx = bit.bor(rwx, bit.rshift(stat.mode, 6))
+  end
+  if vim.tbl_contains(get_group_ids(), stat.gid) then
+    rwx = bit.bor(rwx, bit.rshift(stat.mode, 3))
   end
   return bit.band(rwx, 2) ~= 0
 end