fix(help): retry once if async parsing fails #36592

Problem:
If the buffer changes during iteration of the query matches,
`get_node_text()` may fail, showing an error to the user.

Solution:
Use `pcall` to not propogate the error to the user. Retry once if an
error occurred.

runtime/ftplugin/help.lua

@@ -67,10 +67,8 @@ vim.keymap.set('n', '[[', function()
 end, { buffer = 0, silent = false, desc = 'Jump to previous section' })
 
 local parser = assert(vim.treesitter.get_parser(0, 'vimdoc', { error = false }))
-local root = parser:parse()[1]:root()
 
--- Add "runnables" for Lua/Vimscript code examples.
-do
+local function runnables()
   ---@type table<integer, { lang: string, code: string }>
   local code_blocks = {}
   local query = vim.treesitter.query.parse(
@@ -85,6 +83,7 @@ do
   ]]
   )
 
+  local root = parser:parse()[1]:root()
   for _, match, metadata in query:iter_matches(root, 0, 0, -1) do
     for id, nodes in pairs(match) do
       local name = query.captures[id]
@@ -115,10 +114,17 @@ do
   end, { buffer = true })
 end
 
-local url_ns = vim.api.nvim_create_namespace('nvim.help.urls')
-local filepath = vim.fs.normalize(vim.api.nvim_buf_get_name(0))
+-- Retry once if the buffer has changed during the iteration of the code
+-- blocks. See #36574.
+if not pcall(runnables) then
+  runnables()
+end
 
-if vim.fs.relpath(vim.env.VIMRUNTIME, filepath) ~= nil then
+local url_ns = vim.api.nvim_create_namespace('nvim.help.urls')
+local function urls()
+  local filepath = vim.fs.normalize(vim.api.nvim_buf_get_name(0))
+
+  if vim.fs.relpath(vim.env.VIMRUNTIME, filepath) ~= nil then
     local base = 'https://neovim.io/doc/user/helptag.html?tag='
     local query = vim.treesitter.query.parse(
       'vimdoc',
@@ -131,6 +137,8 @@ if vim.fs.relpath(vim.env.VIMRUNTIME, filepath) ~= nil then
     ]]
     )
 
+    vim.api.nvim_buf_clear_namespace(0, url_ns, 0, -1)
+    local root = parser:parse()[1]:root()
     for _, match, _ in query:iter_matches(root, 0, 0, -1) do
       for id, nodes in pairs(match) do
         if query.captures[id] == 'helplink' then
@@ -146,6 +154,13 @@ if vim.fs.relpath(vim.env.VIMRUNTIME, filepath) ~= nil then
         end
       end
     end
+  end
+end
+
+-- Retry once if the buffer has changed during the iteration of the code
+-- blocks. See #36574.
+if not pcall(urls) then
+  urls()
 end
 
 vim.b.undo_ftplugin = (vim.b.undo_ftplugin or '')