config: support loading plugins

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

CHANGELOG.md

@@ -26,6 +26,7 @@
       ! fullscreen
       border_width = 0
   ```
+* picom now has a rudimentary plugin system. At the moment, the only thing you can do with it is loading custom backends.
 
 ## Notable changes
 

man/picom.1.asciidoc

@@ -76,6 +76,9 @@ OPTIONS
 *--write-pid-path* 'PATH'::
 	Write process ID to a file. it is recommended to use an absolute path.
 
+*--plugins* 'PATH'::
+	Specify plugins to load. Plugins will first be searched in current working directory (unless specified in the config file, in which case this step is skipped), then in `$XDG_CONFIG_HOME/picom/plugins`, then in `$XDG_CONFIG_DIRS/picom/plugins`. If all of the above fail, the plugin name is passed directly to the dynamic loader. Can be specified multiple times to load more than one plugins.
+
 *--shadow-color* 'STRING'::
 	Color of shadow, as a hex string ('#000000')
 

src/config.c

@@ -3,13 +3,11 @@
 // Copyright (c) 2013 Richard Grenville <pyxlcy@gmail.com>
 
 #include <ctype.h>
-#include <errno.h>
+#include <dlfcn.h>
 #include <fcntl.h>
 #include <limits.h>
-#include <math.h>
 #include <stdbool.h>
 #include <stddef.h>
-#include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -20,16 +18,11 @@
 
 #include <test.h>
 
-#include "c2.h"
 #include "common.h"
-#include "compiler.h"
 #include "kernel.h"
 #include "log.h"
-#include "region.h"
 #include "string_utils.h"
 #include "types.h"
-#include "utils.h"
-#include "win.h"
 
 #include "config.h"
 
@@ -632,6 +625,18 @@ void *parse_window_shader_prefix_with_cwd(const char *src, const char **end, voi
 	return parse_window_shader_prefix(src, end, cwd);
 }
 
+bool load_plugin(const char *name, const char *include_dir) {
+	scoped_charp path = locate_auxiliary_file("plugins", optarg, include_dir);
+	void *handle = NULL;
+	if (!path) {
+		handle = dlopen(name, RTLD_LAZY);
+	} else {
+		log_debug("Plugin %s resolved to %s", name, path);
+		handle = dlopen(path, RTLD_LAZY);
+	}
+	return handle != NULL;
+}
+
 bool parse_config(options_t *opt, const char *config_file) {
 	// clang-format off
 	*opt = (struct options){

src/config.h

@@ -354,6 +354,8 @@ typedef struct options {
 
 extern const char *const BACKEND_STRS[NUM_BKEND + 1];
 
+bool load_plugin(const char *name, const char *include_dir);
+
 bool must_use parse_long(const char *, long *);
 bool must_use parse_int(const char *, int *);
 struct conv **must_use parse_blur_kern_lst(const char *, int *count);

src/config_libconfig.c

@@ -628,6 +628,37 @@ bool parse_config_libconfig(options_t *opt, const char *config_file) {
 	// Get options from the configuration file. We don't do range checking
 	// right now. It will be done later
 
+	// Load user specified plugins at the very beginning, because list of backends
+	// depends on the plugins loaded.
+	auto plugins = config_lookup(&cfg, "plugins");
+	if (plugins != NULL) {
+		sval = config_setting_get_string(plugins);
+		if (sval) {
+			if (!load_plugin(sval, NULL)) {
+				log_fatal("Failed to load plugin \"%s\".", sval);
+				goto out;
+			}
+		} else if (config_setting_is_array(plugins) || config_setting_is_list(plugins)) {
+			for (int i = 0; i < config_setting_length(plugins); i++) {
+				sval = config_setting_get_string_elem(plugins, i);
+				if (!sval) {
+					log_fatal("Invalid value for \"plugins\" at line "
+					          "%d.",
+					          config_setting_source_line(plugins));
+					goto out;
+				}
+				if (!load_plugin(sval, NULL)) {
+					log_fatal("Failed to load plugin \"%s\".", sval);
+					goto out;
+				}
+			}
+		} else {
+			log_fatal("Invalid value for \"plugins\" at line %d.",
+			          config_setting_source_line(plugins));
+			goto out;
+		}
+	}
+
 	// --dbus
 	lcfg_lookup_bool(&cfg, "dbus", &opt->dbus);
 

src/options.c

@@ -332,6 +332,7 @@ static const struct picom_option picom_options[] = {
     [314] = {"show-all-xerrors", IGNORE(no_argument)},
     ['b'] = {"daemon"          , IGNORE(no_argument)      , "Daemonize process."},
     [256] = {"config"          , IGNORE(required_argument), "Path to the configuration file."},
+    [307] = {"plugins"         , IGNORE(required_argument), "Plugins to load. Can be specified multiple times, each time with a single plugin."},
 
     // Simple flags
     ['c'] = {"shadow"                   , ENABLE(shadow_enable)            , "Enabled client-side shadows on windows."},
@@ -691,6 +692,7 @@ bool get_early_config(int argc, char *const *argv, char **config_file, bool *all
                       bool *fork, int *exit_code) {
 	setup_longopts();
 
+	scoped_charp current_working_dir = getcwd(NULL, 0);
 	int o = 0, longopt_idx = -1;
 
 	// Pre-parse the command line arguments to check for --config and invalid
@@ -706,7 +708,6 @@ bool get_early_config(int argc, char *const *argv, char **config_file, bool *all
 		} else if (o == 'h') {
 			usage(argv[0], 0);
 			return true;
-
 		} else if (o == 'b') {
 			*fork = true;
 		} else if (o == 314) {
@@ -714,10 +715,17 @@ bool get_early_config(int argc, char *const *argv, char **config_file, bool *all
 		} else if (o == 318) {
 			printf("%s\n", PICOM_VERSION);
 			return true;
+		} else if (o == 307) {
+			// --plugin
+			if (!load_plugin(optarg, current_working_dir)) {
+				log_error("Failed to load plugin %s", optarg);
+				goto err;
+			}
 		} else if (o == '?' || o == ':') {
 			usage(argv[0], 1);
 			goto err;
 		}
+		// TODO(yshui) maybe log-level should be handled here.
 	}
 
 	// Check for abundant positional arguments