feat(explorer): add filesystem watchers (#1304)

* feat(explorer): add experimental watchers

This commit introduces watchers to update the tree.
This behavior is introduced behind an "filesystem_watchers" option
which should prevent instabilities.
It will become the default at some point.

Co-authored-by: Alexander Courtis <alex@courtis.org>

lua/nvim-tree/watcher.lua

@@ -0,0 +1,77 @@
+local uv = vim.loop
+
+local log = require "nvim-tree.log"
+local utils = require "nvim-tree.utils"
+
+local M = {}
+local Watcher = {
+  _watchers = {},
+}
+Watcher.__index = Watcher
+
+function Watcher.new(opts)
+  for _, existing in ipairs(Watcher._watchers) do
+    if existing._opts.absolute_path == opts.absolute_path then
+      log.line("watcher", "Watcher:new using existing '%s'", opts.absolute_path)
+      return existing
+    end
+  end
+
+  log.line("watcher", "Watcher:new '%s'", opts.absolute_path)
+
+  local watcher = setmetatable({
+    _opts = opts,
+  }, Watcher)
+
+  watcher = watcher:start()
+
+  table.insert(Watcher._watchers, watcher)
+
+  return watcher
+end
+
+function Watcher:start()
+  log.line("watcher", "Watcher:start '%s'", self._opts.absolute_path)
+
+  local rc, _, name
+
+  self._p, _, name = uv.new_fs_poll()
+  if not self._p then
+    self._p = nil
+    utils.warn(
+      string.format("Could not initialize an fs_poll watcher for path %s : %s", self._opts.absolute_path, name)
+    )
+    return nil
+  end
+
+  local poll_cb = vim.schedule_wrap(function(err)
+    if err then
+      log.line("watcher", "poll_cb for %s fail : %s", self._opts.absolute_path, err)
+    else
+      self._opts.on_event(self._opts.absolute_path)
+    end
+  end)
+
+  rc, _, name = uv.fs_poll_start(self._p, self._opts.absolute_path, self._opts.interval, poll_cb)
+  if rc ~= 0 then
+    utils.warn(string.format("Could not start the fs_poll watcher for path %s : %s", self._opts.absolute_path, name))
+    return nil
+  end
+
+  return self
+end
+
+function Watcher:stop()
+  log.line("watcher", "Watcher:stop  '%s'", self._opts.absolute_path)
+  if self._p then
+    local rc, _, name = uv.fs_poll_stop(self._p)
+    if rc ~= 0 then
+      utils.warn(string.format("Could not stop the fs_poll watcher for path %s : %s", self._opts.absolute_path, name))
+    end
+    self._p = nil
+  end
+end
+
+M.Watcher = Watcher
+
+return M