session: process environment.d and allow empty variables

1. All '*.env' files in an 'environment.d' directory alongside each
   potential 'environment' file will be parsed and added to the
   environment.

2. For the purposes of configuration merging, an environment definition
   exists at one level if either the 'environment' file is defined or
   its corresponding 'environment.d' contains any valid '*.env' file.

3. Variable declarations of the form "VARIABLE=", with no following
   value, will be written to the environment as empty strings.

src/config/session.c

@@ -1,11 +1,13 @@
 // SPDX-License-Identifier: GPL-2.0-only
 #define _POSIX_C_SOURCE 200809L
 #include <assert.h>
+#include <dirent.h>
 #include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
+#include <sys/types.h>
 #include <wlr/backend/drm.h>
 #include <wlr/backend/multi.h>
 #include <wlr/util/log.h>
@@ -49,9 +51,10 @@ process_line(char *line)
 	buf_add(&value, string_strip(++p));
 	buf_expand_shell_variables(&value);
 	buf_expand_tilde(&value);
-	if (string_null_or_empty(key) || !value.len) {
+	if (string_null_or_empty(key)) {
 		goto error;
 	}
+
 	setenv(key, value.buf, 1);
 error:
 	free(value.buf);
@@ -80,6 +83,71 @@ read_environment_file(const char *filename)
 	return true;
 }
 
+static char *
+strdup_env_path_validate(const char *prefix, struct dirent *dirent)
+{
+	assert(prefix);
+
+	/* Valid environment files always end in '.env' */
+	if (!str_endswith(dirent->d_name, ".env")) {
+		return NULL;
+	}
+
+	char *full_path = strdup_printf("%s/%s", prefix, dirent->d_name);
+	if (!full_path) {
+		return NULL;
+	}
+
+	/* Valid environment files must be regular files */
+	struct stat statbuf;
+	if (stat(full_path, &statbuf) == 0 && S_ISREG(statbuf.st_mode)) {
+		return full_path;
+	}
+
+	free(full_path);
+	return NULL;
+}
+
+static bool
+read_environment_dir(const char *path_prefix)
+{
+	bool success = false;
+	char *path = strdup_printf("%s.d", path_prefix);
+
+	errno = 0;
+	DIR *envdir = opendir(path);
+
+	if (!envdir) {
+		if (errno != ENOENT) {
+			const char *err_msg = strerror(errno);
+			wlr_log(WLR_INFO,
+				"failed to read environment directory: %s",
+				err_msg ? err_msg : "reason unknown");
+		}
+
+		goto env_dir_cleanup;
+	}
+
+	struct dirent *dirent;
+	while ((dirent = readdir(envdir)) != NULL) {
+		char *env_file_path = strdup_env_path_validate(path, dirent);
+		if (!env_file_path) {
+			continue;
+		}
+
+		if (read_environment_file(env_file_path)) {
+			success = true;
+		}
+
+		free(env_file_path);
+	}
+
+env_dir_cleanup:
+	closedir(envdir);
+	free(path);
+	return success;
+}
+
 static void
 backend_check_drm(struct wlr_backend *backend, void *is_drm)
 {
@@ -176,7 +244,13 @@ session_environment_init(void)
 
 	for (struct wl_list *elm = iter(&paths); elm != &paths; elm = iter(elm)) {
 		struct path *path = wl_container_of(elm, path, link);
+
+		/* Process an environment file itself */
 		bool success = read_environment_file(path->string);
+
+		/* Process a correponding environment.d directory */
+		success |= read_environment_dir(path->string);
+
 		if (success && !should_merge_config) {
 			break;
 		}