[gdb/dap] Fix race between dap startup and dap log file

In dap_gdb_start we do:
...
        append GDBFLAGS " -iex \"set debug dap-log-file $logfile\" -q -i=dap"
...

While the dap log file setting comes before the dap interpreter setting,
the order is the other way around:
- first, the dap interpreter is started
- second, the -iex commands are executed and the log file is initialized.

Consequently, there's a race between dap interpreter startup and dap log file
initialization.

This cannot be fixed by using -eiex instead.  Before the interpreter is
started, the "set debug dap-log-file" command is not yet registered.

Fix this by postponing the start of the DAP server until GDB has processed all
command files.

Tested on aarch64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

PR dap/31386
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31386

gdb/python/lib/gdb/dap/__init__.py

@@ -69,5 +69,23 @@ def run():
     os.close(wfd)
 
     # Note the inferior output is opened in text mode.
+    global server
     server = Server(open(saved_in, "rb"), open(saved_out, "wb"), open(rfd, "r"))
-    startup.start_dap(server.main_loop)
+
+
+# Whether the interactive session has started.
+session_started = False
+
+
+def pre_command_loop():
+    """DAP's pre_command_loop interpreter hook.  This is called by the GDB DAP
+    interpreter."""
+    global session_started
+    if not session_started:
+        # The pre_command_loop interpreter hook can be called several times.
+        # The first time it's called, it means we're starting an interactive
+        # session.
+        session_started = True
+        startup.thread_log("starting DAP server")
+        global server
+        startup.start_dap(server.main_loop)

gdb/python/py-dap.c

@@ -61,13 +61,18 @@ public:
     return m_ui_out.get ();
   }
 
+  void pre_command_loop () override;
+
 private:
 
   std::unique_ptr<ui_out> m_ui_out;
 };
 
-void
-dap_interp::init (bool top_level)
+
+/* Call function FN_NAME from module gdb.dap. */
+
+static void
+call_dap_fn (const char *fn_name)
 {
   gdbpy_enter enter_py;
 
@@ -75,18 +80,30 @@ dap_interp::init (bool top_level)
   if (dap_module == nullptr)
     gdbpy_handle_exception ();
 
-  gdbpy_ref<> func (PyObject_GetAttrString (dap_module.get (), "run"));
+  gdbpy_ref<> func (PyObject_GetAttrString (dap_module.get (), fn_name));
   if (func == nullptr)
     gdbpy_handle_exception ();
 
   gdbpy_ref<> result_obj (PyObject_CallObject (func.get (), nullptr));
   if (result_obj == nullptr)
     gdbpy_handle_exception ();
+}
+
+void
+dap_interp::init (bool top_level)
+{
+  call_dap_fn ("run");
 
   current_ui->input_fd = -1;
   current_ui->m_input_interactive_p = false;
 }
 
+void
+dap_interp::pre_command_loop ()
+{
+  call_dap_fn ("pre_command_loop");
+}
+
 void _initialize_py_interp ();
 void
 _initialize_py_interp ()