checkpatch: favor version of the script supplied by projects

Linux kernel is not the only project that uses checkpatch.pl for patch
validation, other projects (for example qemu) use it too, and their
version is different from the one used by the kernel. Also checkpatch.pl
changes from kernel version to kernel version when working with multiple
versions, so requiring a single version covering everything is not
practical.

Additionally running checkpatch.pl on projects that do not use it and do
not use kernel coding style is counter-productive.

To solve this issue make the linter locate and favor the version of
checkpatch.pl belonging to the project that file being linted belongs
to. Also, if checkpatch.pl is not accessible use "true" as the command
so that there are no errors/notifications and no diagnostic.

Author: dtor

lua/lint/linters/checkpatch.lua

@@ -1,7 +1,11 @@
--- The scripts/ directory of the Linux kernel tree needs to be in your PATH or
--- the full path to the checkpatch.pl script can be overriden from user config:
+-- This favors the version of `checkpatch.pl` supplied by the project
+-- to which the file being linted belongs, falling back to a version
+-- accessible via `$PATH`. If a user wishes to use a different version
+-- of the script they can override "cmd" in their config:
 -- require("lint").linters.checkpatch.cmd = '…/checkpatch.pl'
 
+local tool_name = 'checkpatch.pl'
+
 -- path/to/file:line: severity: message
 local pattern = '([^:]+):(%d+): (%a+): (.+)'
 local groups = { 'file', 'lnum', 'severity', 'message' }
@@ -11,8 +15,39 @@ local severity_map = {
   ['CHECK'] = vim.diagnostic.severity.INFO,
 }
 
+local function locate_checkpatch()
+  local current_file = vim.api.nvim_buf_get_name(0)
+  local dir = current_file ~= '' and vim.fs.dirname(current_file) or vim.fn.getcwd()
+
+  -- FIXME: switch to using vim.fs.root() once minimal version is bumped to 0.10.
+  local git_dir = vim.fs.find('.git', {
+    path = dir,
+    upward = true,
+    type = 'directory',
+    limit = 1,
+  })[1]
+
+  if git_dir then
+    local project_root = vim.fs.dirname(git_dir)
+    local tool_path = vim.fs.joinpath(project_root, 'scripts', tool_name)
+    if vim.fn.executable(tool_path) == 1 then
+      return tool_path
+    end
+  end
+
+  if vim.fn.executable(tool_name) == 1 then
+    return tool_name
+  end
+
+  -- If checkpatch executable is not found return "true" which is a valid command
+  -- that will not produce any diagnostic but return success error code. This will
+  -- allow enabling checkpatch linter globally and only return diagnostic if
+  -- a project is actually using it.
+  return "true"
+end
+
 return {
-  cmd = 'checkpatch.pl',
+  cmd = locate_checkpatch,
   stdin = false,
   args = {
     '--strict',