lua LSP client: initial implementation (#11336)

Mainly configuration and RPC infrastructure can be considered "done". Specific requests and their callbacks will be improved later (and also served by plugins). There are also some TODO:s for the client itself, like incremental updates.

Co-authored by at-tjdevries and at-h-michael, with many review/suggestion contributions.

runtime/lua/vim/lsp/log.lua

@@ -0,0 +1,95 @@
+-- Logger for language client plugin.
+
+local log = {}
+
+-- Log level dictionary with reverse lookup as well.
+--
+-- Can be used to lookup the number from the name or the name from the number.
+-- Levels by name: 'trace', 'debug', 'info', 'warn', 'error'
+-- Level numbers begin with 'trace' at 0
+log.levels = {
+  TRACE = 0;
+  DEBUG = 1;
+  INFO  = 2;
+  WARN  = 3;
+  ERROR = 4;
+  -- FATAL = 4;
+}
+
+-- Default log level is warn.
+local current_log_level = log.levels.WARN
+local log_date_format = "%FT%H:%M:%SZ%z"
+
+do
+  local path_sep = vim.loop.os_uname().sysname == "Windows" and "\\" or "/"
+  local function path_join(...)
+    return table.concat(vim.tbl_flatten{...}, path_sep)
+  end
+  local logfilename = path_join(vim.fn.stdpath('data'), 'vim-lsp.log')
+
+  --- Return the log filename.
+  function log.get_filename()
+    return logfilename
+  end
+
+  vim.fn.mkdir(vim.fn.stdpath('data'), "p")
+  local logfile = assert(io.open(logfilename, "a+"))
+  for level, levelnr in pairs(log.levels) do
+    -- Also export the log level on the root object.
+    log[level] = levelnr
+    -- Set the lowercase name as the main use function.
+    -- If called without arguments, it will check whether the log level is
+    -- greater than or equal to this one. When called with arguments, it will
+    -- log at that level (if applicable, it is checked either way).
+    --
+    -- Recommended usage:
+    -- ```
+    -- local _ = log.warn() and log.warn("123")
+    -- ```
+    --
+    -- This way you can avoid string allocations if the log level isn't high enough.
+    log[level:lower()] = function(...)
+      local argc = select("#", ...)
+      if levelnr < current_log_level then return false end
+      if argc == 0 then return true end
+      local info = debug.getinfo(2, "Sl")
+      local fileinfo = string.format("%s:%s", info.short_src, info.currentline)
+      local parts = { table.concat({"[", level, "]", os.date(log_date_format), "]", fileinfo, "]"}, " ") }
+      for i = 1, argc do
+        local arg = select(i, ...)
+        if arg == nil then
+          table.insert(parts, "nil")
+        else
+          table.insert(parts, vim.inspect(arg, {newline=''}))
+        end
+      end
+      logfile:write(table.concat(parts, '\t'), "\n")
+      logfile:flush()
+    end
+  end
+  -- Add some space to make it easier to distinguish different neovim runs.
+  logfile:write("\n")
+end
+
+-- This is put here on purpose after the loop above so that it doesn't
+-- interfere with iterating the levels
+vim.tbl_add_reverse_lookup(log.levels)
+
+function log.set_level(level)
+  if type(level) == 'string' then
+    current_log_level = assert(log.levels[level:upper()], string.format("Invalid log level: %q", level))
+  else
+    assert(type(level) == 'number', "level must be a number or string")
+    assert(log.levels[level], string.format("Invalid log level: %d", level))
+    current_log_level = level
+  end
+end
+
+-- Return whether the level is sufficient for logging.
+-- @param level number log level
+function log.should_log(level)
+  return level >= current_log_level
+end
+
+return log
+-- vim:sw=2 ts=2 et