test: fsync() codepaths

test/functional/core/fileio_spec.lua

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+local helpers = require('test.functional.helpers')(after_each)
+
+local clear = helpers.clear
+local command = helpers.command
+local eq = helpers.eq
+local eval = helpers.eval
+local feed = helpers.feed
+local funcs = helpers.funcs
+local nvim_prog = helpers.nvim_prog
+local rmdir = helpers.rmdir
+local sleep = helpers.sleep
+
+describe('fileio', function()
+  before_each(function()
+  end)
+  after_each(function()
+    command(':qall!')
+    os.remove('Xtest_startup_shada')
+    os.remove('Xtest_startup_file1')
+    os.remove('Xtest_startup_file2')
+    rmdir('Xtest_startup_swapdir')
+  end)
+
+  it('fsync() codepaths #8304', function()
+    -- This is an "acceptance test" or "smoke test".
+
+    clear({ args={ '-i', 'Xtest_startup_shada',
+                   '--cmd', 'set directory=Xtest_startup_swapdir' } })
+
+    -- These cases ALWAYS force fsync (regardless of 'fsync' option):
+
+    -- 1. Idle (CursorHold) with modified buffers (+ 'swapfile').
+    command('set swapfile')
+    command('set updatetime=1')
+    command('write Xtest_startup_file1')
+    feed('ifoo<esc>h')
+    sleep(2)
+    eq(1, eval('&modified'))
+
+    -- 2. Exit caused by deadly signal (+ 'swapfile').
+    local j = funcs.jobstart({ nvim_prog, '-u', 'NONE', '-i',
+                               'Xtest_startup_shada', '--headless',
+                               '-c', 'set swapfile',
+                               '-c', 'write Xtest_startup_file2',
+                               '-c', 'put =localtime()', })
+    sleep(10)         -- Let Nvim start.
+    funcs.jobstop(j)  -- Send deadly signal.
+
+    -- 3. SIGPWR signal.
+    -- ??
+
+    -- 4. Explicit :preserve command.
+    command('preserve')
+
+    -- 5. Enable 'fsync' option, write file.
+    command('set fsync')
+    feed('ibaz<esc>h')
+    command('write')
+  end)
+end)
+