implement callback for headless instance

ref #43

doc/osv.txt

@@ -162,6 +162,26 @@ Checks whether |osv| is currently running.
 	True if osv is running.
 
 
+=========================================================================
+osv callbacks                                             *osv-callbacks*
+
+Certain default bevahviours can be overriden using callbacks.  All callbacks
+are stored in the `on` table. If no function is assigned, the default
+bevahiour is executed, otherwise the callback will be called instead. Only one
+callback can be assigned to an action.
+
+For example:
+>
+  require"osv".on["action"] = function(...)
+  end
+<
+The `start_server` event is called when osv starts the headless instance for
+the DAP server. The default behaviour is:
+>
+  require"osv".on["start_server"] = function(args, env)
+    return vim.fn.jobstart(args, {rpc = true, env = env})
+  end
+
 =========================================================================
 osv logging                                                 *osv-logging*
 

lua/osv/init.lua

@@ -89,6 +89,8 @@ function M.launch(opts)
   	vim.api.nvim_echo({{"Server is already running.", "ErrorMsg"}}, true, {})
     return
   end
+
+
   vim.validate {
     opts = {opts, 't', true}
   }
@@ -149,7 +151,13 @@ function M.launch(opts)
   	end
   end
 
-  nvim_server = vim.fn.jobstart(args, {rpc = true, env = env})
+  if M.on["start_server"] then
+    nvim_server = M.callback["start_server"](args, env)
+    assert(nvim_server)
+
+  else
+    nvim_server = vim.fn.jobstart(args, {rpc = true, env = env})
+  end
 
   local mode = vim.fn.rpcrequest(nvim_server, "nvim_get_mode")
   if mode.blocking then
@@ -1363,6 +1371,8 @@ function M.wait_attach()
   end))
 end
 
+M.on = {}
+
 function log(str)
   if log_filename then
     local f = io.open(log_filename, "a")

src/launch.lua.t

@@ -26,7 +26,10 @@ local nvim_server
 @copy_env
 @fill_env_with_custom
 @fill_args_with_custom
-nvim_server = vim.fn.jobstart(args, {rpc = true, env = env})
+@if_exists_use_custom_for_launching_server
+else
+  nvim_server = vim.fn.jobstart(args, {rpc = true, env = env})
+end
 
 @script_variables+=
 local hook_address
@@ -137,3 +140,13 @@ if M.is_running() then
 	vim.api.nvim_echo({{"Server is already running.", "ErrorMsg"}}, true, {})
   return
 end
+
+
+@implement+=
+M.on = {}
+
+@if_exists_use_custom_for_launching_server+=
+if M.on["start_server"] then
+  nvim_server = M.callback["start_server"](args, env)
+  assert(nvim_server)
+