nvim: Fix molten statusline icon

Fixed issue when pynvim did not exist in the environment but we tried to
invoke a molten-nvim command. Now we first fire up python to check for
pynvim existence. This takes a moment so we do it asynchronously and
cache the result for the rest of the program runtime.

Also we only show the molten icon if we are actually connected to a
running kernel.

nvim/.config/nvim/lua/plugins/ui.lua

@@ -4,16 +4,31 @@ return {
 		"nvim-lualine/lualine.nvim",
 		dependencies = { { "nvim-tree/nvim-web-devicons", config = true } },
 		config = function()
-			-- FIXME: Errors out on no pynvim installed
-			-- local function molten()
-			-- 	if
-			-- 		require("core.util").is_available("molten.status")
-			-- 		and require("molten.status").initialized() ~= ""
-			-- 	then
-			-- 		return "󱪄"
-			-- 	end
-			-- 	return ""
-			-- end
+			local has_pynvim = -1
+			-- if molten exists, is initialized and connected to a kernel
+			-- show it in the statusline
+			local function molten()
+				local function checked_pynvim(_, exitcode)
+					if exitcode == 0 then
+						has_pynvim = 1
+					else
+						has_pynvim = 0
+					end
+				end
+				if has_pynvim == 0 then
+					return ""
+				elseif has_pynvim == -1 then
+					vim.fn.jobstart({ "python", "-c", "from neovim import VERSION" }, { on_exit = checked_pynvim })
+				end
+				if
+					has_pynvim == 1
+					and require("core.util").is_available("molten.status")
+					and require("molten.status").kernels() ~= ""
+				then
+					return "󱪄"
+				end
+				return ""
+			end
 			require("lualine").setup({
 				options = {
 					icons_enabled = true,
@@ -27,7 +42,7 @@ return {
 					lualine_a = { "mode" },
 					lualine_b = { "branch", "diff", "diagnostics" },
 					lualine_c = { "filename" },
-					lualine_x = { "encoding", "fileformat", "filetype", "molten" },
+					lualine_x = { "encoding", "fileformat", "filetype", molten },
 					lualine_y = { "progress" },
 					lualine_z = { "location" },
 				},