feat(fswatch): report filewatchers in :checkhealth

runtime/doc/news.txt

@@ -257,6 +257,7 @@ TUI
 
 UI
 
+• |:checkhealth| shows filewatcher info in the Performance section.
 • These builtin "picker" menus delegate to |vim.ui.select()|:
   • :browse oldfiles
   • |:recover|

runtime/lua/vim/_watch.lua

@@ -10,6 +10,28 @@ M.FileChangeType = {
   Deleted = 3,
 }
 
+--- Count of currently-active watchers. Each "watcher" here is one call to vim._watch.watch(),
+--- vim._watch.watchdirs(), or vim._watch.inotify() that hasn't been cancelled yet.
+M.active = { watch = 0, watchdirs = 0, inotify = 0 }
+
+--- Wraps a cancel function so the global counter is decremented exactly once, even if the caller
+--- invokes the cancel function multiple times.
+--- @param backend 'watch'|'watchdirs'|'inotify'
+--- @param cancel fun()
+--- @return fun()
+local function tracked_cancel(backend, cancel)
+  M.active[backend] = M.active[backend] + 1
+  local done = false
+  return function()
+    if done then
+      return
+    end
+    done = true
+    M.active[backend] = M.active[backend] - 1
+    cancel()
+  end
+end
+
 --- @class vim._watch.Opts
 ---
 --- @field debounce? integer ms
@@ -109,7 +131,7 @@ function M.watch(path, opts, callback)
     return function() end
   end
 
-  return function()
+  return tracked_cancel('watch', function()
     local _, stop_err = handle:stop()
     assert(not stop_err, stop_err)
     local is_closing, close_err = handle:is_closing()
@@ -117,7 +139,7 @@ function M.watch(path, opts, callback)
     if not is_closing then
       handle:close()
     end
-  end
+  end)
 end
 
 --- Initializes and starts a |uv_fs_event_t| recursively watching every directory underneath the
@@ -241,7 +263,7 @@ function M.watchdirs(path, opts, callback)
     timer:close()
   end
 
-  return cancel
+  return tracked_cancel('watchdirs', cancel)
 end
 
 --- @param data string
@@ -328,9 +350,9 @@ function M.inotify(path, opts, callback)
     env = { LC_NUMERIC = 'C' },
   })
 
-  return function()
+  return tracked_cancel('inotify', function()
     obj:kill(2)
-  end
+  end)
 end
 
 return M

runtime/lua/vim/health/health.lua

@@ -169,6 +169,42 @@ local function check_config()
   end
 end
 
+-- Note: this is part of check_performance().
+local function check_watchers()
+  health.start('Filewatchers')
+  local a = assert(vim._watch.active)
+  local total = a.watch + a.watchdirs + a.inotify
+  health.info(
+    ('filewatchers (vim._watch): %d (watch=%d, watchdirs=%d, inotify=%d)'):format(
+      total,
+      a.watch,
+      a.watchdirs,
+      a.inotify
+    )
+  )
+
+  -- Walk libuv for an independent view. These counts include handles created outside `vim._watch`
+  -- (e.g. plugins calling `vim.uv.new_fs_event()` directly).
+  local libuv = { fs_event = 0, fs_poll = 0, process = 0, timer = 0 }
+  vim.uv.walk(function(handle)
+    if handle:is_closing() then
+      return
+    end
+    local t = handle:get_type()
+    if libuv[t] ~= nil then
+      libuv[t] = libuv[t] + 1
+    end
+  end)
+  health.info(
+    ('libuv handles: fs_event=%d, fs_poll=%d, process=%d, timer=%d'):format(
+      libuv.fs_event,
+      libuv.fs_poll,
+      libuv.process,
+      libuv.timer
+    )
+  )
+end
+
 local function check_performance()
   health.start('Performance')
 
@@ -199,6 +235,8 @@ local function check_performance()
       'Slow shell invocation (took ' .. vim.fn.printf('%.2f', elapsed_time) .. ' seconds).'
     )
   end
+
+  check_watchers()
 end
 
 -- Load the remote plugin manifest file and check for unregistered plugins

runtime/lua/vim/lsp/health.lua

@@ -112,6 +112,7 @@ local function check_active_clients()
   end
 end
 
+-- See also runtime/lua/vim/health/health.lua:check_watchers()
 local function check_watcher()
   vim.health.start('vim.lsp: File Watcher')