CLI: support defining custom commands via C++ & Python API

Add support for add-ons to define commands using the new argument
`-c` or `--command`.

Commands behave as follows:

- Passing in a command enables background mode without the need to pass
  in `--background`.
- All arguments following the command are passed to the command
  (without the need to use the `--` argument).
- Add-ons can define their own commands via
  `bpy.utils.register_cli_command` (see examples in API docs).
- Passing in `--command help` lists all available commands.

Ref !119115

source/creator/creator.cc

@@ -37,6 +37,7 @@
 /* Mostly initialization functions. */
 #include "BKE_appdir.hh"
 #include "BKE_blender.hh"
+#include "BKE_blender_cli_command.hh"
 #include "BKE_brush.hh"
 #include "BKE_cachefile.hh"
 #include "BKE_callbacks.hh"
@@ -566,8 +567,23 @@ int main(int argc,
 
 #ifndef WITH_PYTHON_MODULE
   if (G.background) {
+    int exit_code;
+    if (app_state.command.argv) {
+      const char *id = app_state.command.argv[0];
+      if (STREQ(id, "help")) {
+        BKE_blender_cli_command_print_help();
+        exit_code = EXIT_SUCCESS;
+      }
+      else {
+        exit_code = BKE_blender_cli_command_exec(
+            C, id, app_state.command.argc - 1, app_state.command.argv + 1);
+      }
+    }
+    else {
+      exit_code = G.is_break ? EXIT_FAILURE : EXIT_SUCCESS;
+    }
     /* Using window-manager API in background-mode is a bit odd, but works fine. */
-    WM_exit(C, G.is_break ? EXIT_FAILURE : EXIT_SUCCESS);
+    WM_exit(C, exit_code);
   }
   else {
     /* Shows the splash as needed. */

source/creator/creator_args.cc

@@ -698,6 +698,8 @@ static void print_help(bArgs *ba, bool all)
   PRINT("\n");
   BLI_args_print_arg_doc(ba, "-noaudio");
   BLI_args_print_arg_doc(ba, "-setaudio");
+  PRINT("\n");
+  BLI_args_print_arg_doc(ba, "--command");
 
   PRINT("\n");
 
@@ -924,6 +926,32 @@ static int arg_handle_background_mode_set(int /*argc*/, const char ** /*argv*/,
   return 0;
 }
 
+static const char arg_handle_command_set_doc[] =
+    "<command>\n"
+    "\tRun a command which consumes all remaining arguments.\n"
+    "\tUse '-c help' to list all other commands.\n"
+    "\tPass '--help' after the command to see its help text.\n"
+    "\n"
+    "\tThis implies '--background' mode.";
+static int arg_handle_command_set(int argc, const char **argv, void * /*data*/)
+{
+  if (argc < 2) {
+    fprintf(stderr, "%s requires at least one argument\n", argv[0]);
+    exit(EXIT_FAILURE);
+    BLI_assert_unreachable();
+  }
+
+  /* See `--background` implementation. */
+  G.background = true;
+  BKE_sound_force_device("None");
+
+  app_state.command.argc = argc - 1;
+  app_state.command.argv = argv + 1;
+
+  /* Consume remaining arguments. */
+  return argc - 1;
+}
+
 static const char arg_handle_log_level_set_doc[] =
     "<level>\n"
     "\tSet the logging verbosity level (higher for more details) defaults to 1,\n"
@@ -2374,6 +2402,8 @@ void main_args_setup(bContext *C, bArgs *ba, bool all)
       ba, nullptr, "--disable-abort-handler", CB(arg_handle_abort_handler_disable), nullptr);
 
   BLI_args_add(ba, "-b", "--background", CB(arg_handle_background_mode_set), nullptr);
+  /* Command implies background mode. */
+  BLI_args_add(ba, "-c", "--command", CB(arg_handle_command_set), nullptr);
 
   BLI_args_add(ba, "-a", nullptr, CB(arg_handle_playback_mode), nullptr);
 

source/creator/creator_intern.h

@@ -52,6 +52,12 @@ struct ApplicationState {
   struct {
     unsigned char python;
   } exit_code_on_error;
+
+  /** Storage for commands (see `--command` argument). */
+  struct {
+    int argc;
+    const char **argv;
+  } command;
 };
 extern struct ApplicationState app_state; /* creator.c */