feat(startup): warn if NVIM_LOG_FILE is inaccessible #38070

Problem:
If NVIM_LOG_FILE, or the default fallback, is inaccessible (e.g.
directory is owned by root), users get confused.

Solution:
Show a warning when $NVIM_LOG_FILE or $XDG_STATE_HOME are inaccessible.

Also fix a latent memory leak: `os_mkdir_recurse` returns a uv error
code (int), but it was stored as `bool`, causing `os_strerror` to
receive an invalid error code and leak memory.

See: https://docs.libuv.org/en/v1.x/errors.html#c.uv_strerror

Co-authored-by: Sean Dewar <6256228+seandewar@users.noreply.github.com>
Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

runtime/lua/vim/_core/defaults.lua

@@ -541,6 +541,18 @@ end
 
 --- Default autocommands. See |default-autocmds|
 do
+  -- Warn if $NVIM_LOG_FILE or $XDG_STATE_HOME are inaccessible. #38039
+  if vim.v.vim_did_enter then
+    require('vim._core.log').check_log_file()
+  else
+    vim.api.nvim_create_autocmd('VimEnter', {
+      once = true,
+      callback = function()
+        require('vim._core.log').check_log_file()
+      end,
+    })
+  end
+
   local nvim_terminal_augroup = vim.api.nvim_create_augroup('nvim.terminal', {})
   vim.api.nvim_create_autocmd('BufReadCmd', {
     pattern = 'term://*',

runtime/lua/vim/_core/log.lua

@@ -0,0 +1,30 @@
+local M = {}
+
+--- Checks that the logfile is accessible.
+function M.check_log_file()
+  if vim.fn.mode() == 'c' then -- Ex mode
+    return
+  end
+
+  local wanted = vim.fn.getenv('__NVIM_LOG_FILE_WANT')
+  if not wanted or wanted == vim.NIL then
+    return
+  end
+
+  local actual = vim.fn.getenv('NVIM_LOG_FILE')
+
+  local msg --[[@type string]]
+  if not actual or actual == vim.NIL or actual == '' then
+    msg = ('log: %q not accessible, logging disabled (stderr)'):format(wanted)
+  elseif actual ~= wanted then
+    msg = ('log: %q not accessible, logging to: %q'):format(wanted, actual)
+  else
+    return
+  end
+
+  vim.defer_fn(function()
+    vim.notify(msg, vim.log.levels.WARN)
+  end, 100)
+end
+
+return M