test: 'nofsync' with deadly signal #26415

Problem:
The test for 'nofsync' swapfile preservation on a deadly signal, does
not actually assert anything.

followup to 1fd29a2884

Solution:
Check that swapfile contents are present after getting SIGTERM.
TODO: this doesn't really verify that 'fsync' was called; it still
passes with this patch:

    diff --git a/src/nvim/main.c b/src/nvim/main.c
    index 216e39f3e81c..7a635520401d 100644
    --- a/src/nvim/main.c
    +++ b/src/nvim/main.c
    @@ -838,7 +838,7 @@ void preserve_exit(const char *errmsg)
           if (errmsg != NULL) {
             os_errmsg("Vim: preserving files...\r\n");
           }
    -      ml_sync_all(false, false, true);  // preserve all swap files
    +      ml_sync_all(false, false, false);  // preserve all swap files
           break;
         }
       }

However it correctly fails with this patch, at least:

    diff --git a/src/nvim/main.c b/src/nvim/main.c
    index 216e39f3e81c..f2306c310ddc 100644
    --- a/src/nvim/main.c
    +++ b/src/nvim/main.c
    @@ -838,7 +838,6 @@ void preserve_exit(const char *errmsg)
           if (errmsg != NULL) {
             os_errmsg("Vim: preserving files...\r\n");
           }
    -      ml_sync_all(false, false, true);  // preserve all swap files
           break;
         }
       }

test/functional/core/fileio_spec.lua

@@ -30,6 +30,8 @@ local feed_command = helpers.feed_command
 local skip = helpers.skip
 local is_os = helpers.is_os
 local is_ci = helpers.is_ci
+local spawn = helpers.spawn
+local set_session = helpers.set_session
 
 describe('fileio', function()
   before_each(function()
@@ -49,6 +51,23 @@ describe('fileio', function()
     rmdir('Xtest_backupdir with spaces')
   end)
 
+  local args = { nvim_prog, '--clean', '--cmd', 'set nofsync directory=Xtest_startup_swapdir', }
+  --- Starts a new nvim session and returns an attached screen.
+  local function startup(extra_args)
+    extra_args = extra_args or {}
+    local argv = vim.tbl_flatten({args, '--embed', extra_args})
+    local screen_nvim = spawn(argv)
+    set_session(screen_nvim)
+    local screen = Screen.new(70, 10)
+    screen:attach()
+    screen:set_default_attr_ids({
+      [1] = {foreground = Screen.colors.NvimDarkGrey4};
+      [2] = {background = Screen.colors.NvimDarkGrey1, foreground = Screen.colors.NvimLightGrey3};
+      [3] = {foreground = Screen.colors.NvimLightCyan};
+    })
+    return screen
+  end
+
   it("fsync() with 'nofsync' #8304", function()
     clear({ args={ '--cmd', 'set nofsync directory=Xtest_startup_swapdir', } })
 
@@ -81,16 +100,24 @@ describe('fileio', function()
     eq('foozubbaz', trim(read_file('Xtest_startup_file1')))
 
     -- 4. Exit caused by deadly signal (+ 'swapfile').
-    local j = funcs.jobstart({ nvim_prog, '-u', 'NONE', '--headless',
-                               '--cmd', 'set nofsync directory=Xtest_startup_swapdir',
-                               '-c', 'set swapfile',
-                               '-c', 'write Xtest_startup_file2',
-                               '-c', 'put =localtime()', })
-    sleep(10)         -- Let Nvim start.
+    local j = funcs.jobstart(vim.tbl_flatten({args, '--embed'}), {rpc=true})
+    funcs.rpcrequest(j, 'nvim_exec2', [[
+      set nofsync directory=Xtest_startup_swapdir
+      edit Xtest_startup_file2
+      write
+      put ='fsyncd text'
+    ]], {})
+    eq('Xtest_startup_swapdir', funcs.rpcrequest(j, 'nvim_eval', '&directory'))
     funcs.jobstop(j)  -- Send deadly signal.
 
+    local screen = startup()
+    feed(':recover Xtest_startup_file2<cr>')
+    screen:expect({any = [[Using swap file "Xtest_startup_swapdir[/\]Xtest_startup_file2%.swp"]]})
+    feed('<cr>')
+    screen:expect({any = 'fsyncd text'})
+
     -- 5. SIGPWR signal.
-    -- ??
+    -- oldtest: Test_signal_PWR()
   end)
 
   it('backup #9709', function()

test/functional/helpers.lua

@@ -438,7 +438,7 @@ function module.connect(file_or_address)
   return Session.new(stream)
 end
 
--- Starts a new global Nvim session.
+-- Starts (and returns) a new global Nvim session.
 --
 -- Parameters are interpreted as startup args, OR a map with these keys:
 --    args:       List: Args appended to the default `nvim_argv` set.
@@ -452,6 +452,7 @@ end
 --    clear{args={'-e'}, args_rm={'-i'}, env={TERM=term}}
 function module.clear(...)
   module.set_session(module.spawn_argv(false, ...))
+  return module.get_session()
 end
 
 -- same params as clear, but does returns the session instead
@@ -943,7 +944,7 @@ function module.add_builddir_to_rtp()
   module.command(string.format([[set rtp+=%s/runtime]], module.test_build_dir))
 end
 
--- Kill process with given pid
+-- Kill (reap) a process by PID.
 function module.os_kill(pid)
   return os.execute((is_os('win')
     and 'taskkill /f /t /pid '..pid..' > nul'

test/functional/ui/screen_basic_spec.lua

@@ -12,7 +12,7 @@ describe('screen', function()
     helpers.nvim_prog,
     '-u', 'NONE',
     '-i', 'NONE',
-    '-N',
+    '-n',
     '--cmd', 'set shortmess+=I background=light noswapfile belloff= noshowcmd noruler',
     '--cmd', 'colorscheme vim',
     '--embed',