shada: Fix non-writeable ShaDa directory handling

Before this change,

    nvim -i /etc/shada

segfaults on exit if the file does not exist and user does not have
permissions to create the file at /etc/shada.

Closes #5296
Reported in #5277
https://github.com/neovim/neovim/issues/5277#issuecomment-243937255

src/nvim/shada.c

@@ -2976,7 +2976,7 @@ shada_write_file_nomerge: {}
   if (sd_writer.cookie == NULL) {
     xfree(fname);
     xfree(tempname);
-    if (sd_reader.close != NULL) {
+    if (sd_reader.cookie != NULL) {
       sd_reader.close(&sd_reader);
     }
     return FAIL;

test/functional/shada/helpers.lua

@@ -8,8 +8,8 @@ local mpack = require('mpack')
 local tmpname = helpers.tmpname()
 local additional_cmd = ''
 
-local function nvim_argv()
+local function nvim_argv(shada_file)
-  local argv = {nvim_prog, '-u', 'NONE', '-i', tmpname, '-N',
+  local argv = {nvim_prog, '-u', 'NONE', '-i', shada_file or tmpname, '-N',
                 '--cmd', 'set shortmess+=I background=light noswapfile',
                 '--cmd', additional_cmd,
                 '--embed'}
@@ -20,8 +20,8 @@ local function nvim_argv()
   end
 end
 
-local reset = function()
+local reset = function(shada_file)
-  set_session(spawn(nvim_argv()))
+  set_session(spawn(nvim_argv(shada_file)))
   meths.set_var('tmpname', tmpname)
 end
 

test/functional/shada/shada_spec.lua

@@ -5,6 +5,7 @@ local meths, nvim_command, funcs, eq =
 local write_file, spawn, set_session, nvim_prog, exc_exec =
   helpers.write_file, helpers.spawn, helpers.set_session, helpers.nvim_prog,
   helpers.exc_exec
+
 local lfs = require('lfs')
 local paths = require('test.config.paths')
 
@@ -14,10 +15,14 @@ local shada_helpers = require('test.functional.shada.helpers')
 local reset, clear, get_shada_rw =
   shada_helpers.reset, shada_helpers.clear, shada_helpers.get_shada_rw
 local read_shada_file = shada_helpers.read_shada_file
+local set_additional_cmd = shada_helpers.set_additional_cmd
 
 local wshada, _, shada_fname, clean =
   get_shada_rw('Xtest-functional-shada-shada.shada')
 
+local dirname = 'Xtest-functional-shada-shada.d'
+local dirshada = dirname .. '/main.shada'
+
 if helpers.pending_win32(pending) then return end
 
 describe('ShaDa support code', function()
@@ -25,6 +30,7 @@ describe('ShaDa support code', function()
   after_each(function()
     clear()
     clean()
+    lfs.rmdir(dirname)
   end)
 
   it('preserves `s` item size limit with unknown entries', function()
@@ -232,4 +238,17 @@ describe('ShaDa support code', function()
     eq('', meths.get_option('viminfo'))
     eq('', meths.get_option('shada'))
   end)
+
+  it('does not crash when ShaDa file directory is not writable', function()
+    funcs.mkdir(dirname, '', 0)
+    eq(0, funcs.filewritable(dirname))
+    set_additional_cmd('set shada=')
+    reset(dirshada)
+    meths.set_option('shada', '\'10')
+    eq('Vim(wshada):E886: System error while opening ShaDa file '
+       .. 'Xtest-functional-shada-shada.d/main.shada for reading to merge '
+       .. 'before writing it: permission denied',
+       exc_exec('wshada'))
+    meths.set_option('shada', '')
+  end)
 end)