Make :TSInstall work in Nix by adding a second module installation target (#473)

* Ignore tags file in project root

* Make :TSInstall work with Nix

This commit adds logic to determine where to install parsers, meaning
the *.so files. Until now the package path of the nvim-treesitter plugin
was used. But when installed with Nix, the plugin lands in "/nix/store",
which is read-only.

With this commit $XDG_DATA_HOME/nvim/site/parser/*.go will be used as
the parser installation path. The directory will be created if it
doesn't exist.

* Add generate_join function

The generate_join function is used to create two other functions, one
to join path segments, the other to join strings with a space for error
messages.

lua/nvim-treesitter/utils.lua

@@ -17,6 +17,26 @@ function M.setup_commands(mod, commands)
   end
 end
 
+function M.get_path_sep()
+  return fn.has('win32') == 1 and '\\' or '/'
+end
+
+-- Returns a function that joins the given arguments with separator. Arguments
+-- can't be nil. Example:
+--[[
+print(M.generate_join(" ")("foo", "bar"))
+--]]
+-- prints "foo bar"
+function M.generate_join(separator)
+  return function (...)
+    return table.concat({...}, separator)
+  end
+end
+
+local join_path = M.generate_join(M.get_path_sep())
+
+local join_space = M.generate_join(" ")
+
 function M.get_package_path()
   -- Path to this source file, removing the leading '@'
   local source = string.sub(debug.getinfo(1, 'S').source, 2)
@@ -34,7 +54,51 @@ function M.get_cache_dir()
     return '/tmp'
   end
 
-  return nil, 'Invalid cache rights, '..fn.stdpath('data')..' or /tmp should be read/write'
+  return nil, join_space('Invalid cache rights,', fn.stdpath('data'), 'or /tmp should be read/write')
+end
+
+-- Returns $XDG_DATA_HOME/nvim/site, but could use any directory that is in
+-- runtimepath
+function M.get_site_dir()
+  local path_sep = M.get_path_sep()
+  return join_path(fn.stdpath('data'), path_sep, 'site')
+end
+
+-- Try the package dir of the nvim-treesitter plugin first, followed by the
+-- "site" dir from "runtimepath". "site" dir will be created if it doesn't
+-- exist. Using only the package dir won't work when the plugin is installed
+-- with Nix, since the "/nix/store" is read-only.
+function M.get_parser_install_dir()
+  local package_path = M.get_package_path()
+  local package_path_parser_dir = join_path(package_path, "parser")
+
+  -- If package_path is read/write, use that
+  if luv.fs_access(package_path_parser_dir, 'RW') then
+    return package_path_parser_dir
+  end
+
+  local site_dir = M.get_site_dir()
+  local path_sep = M.get_path_sep()
+  local parser_dir = join_path(site_dir, path_sep, 'parser')
+
+  -- Try creating and using parser_dir if it doesn't exist
+  if not luv.fs_stat(parser_dir) then
+    local ok, error = pcall(vim.fn.mkdir, parser_dir, "p", "0755")
+    if not ok then
+      return nil, join_space('Couldn\'t create parser dir', parser_dir, ':', error)
+    end
+
+    return parser_dir
+  end
+
+  -- parser_dir exists, use it if it's read/write
+  if luv.fs_access(parser_dir, 'RW') then
+    return parser_dir
+  end
+
+  -- package_path isn't read/write, parser_dir exists but isn't read/write
+  -- either, give up
+  return nil, join_space('Invalid cache rights,', package_path, 'or', parser_dir, 'should be read/write')
 end
 
 -- Gets a property at path