config-parser: Honor XDG_CONFIG_DIRS

This set of changes adds support for searching for a given config file
in the directories listed in $XDG_CONFIG_DIRS if it wasn't found in
$XDG_CONFIG_HOME or ~/.config.  This allows packages to install custom
config files in /etc/xdg/weston, for example, thus allowing them to
avoid dealing with home directories.

To avoid a TOCTOU race the config file is actually open()ed during the
search.  Its file descriptor is returned and stored in the compositor
for later use when performing subsequent config file parses.

Signed-off-by: Ossama Othman <ossama.othman@intel.com>

shared/config-parser.c

@@ -20,11 +20,17 @@
  * OF THIS SOFTWARE.
  */
 
+#define _GNU_SOURCE   /* for stchrnul() */
 #include <string.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <assert.h>
 #include <ctype.h>
+#include <limits.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
 
 #include "config-parser.h"
 
@@ -86,7 +92,7 @@ handle_key(const struct config_key *key, const char *value)
 }
 
 int
-parse_config_file(const char *path,
+parse_config_file(int fd,
 		  const struct config_section *sections, int num_sections,
 		  void *data)
 {
@@ -95,12 +101,17 @@ parse_config_file(const char *path,
 	const struct config_section *current = NULL;
 	int i;
 
-	fp = fopen(path, "r");
+	if (fd == -1)
+		return -1;
+
+	fp = fdopen(dup(fd), "r");
 	if (fp == NULL) {
-		fprintf(stderr, "couldn't open %s\n", path);
+            perror("couldn't open config file");
 		return -1;
 	}
 
+	rewind(fp);
+
 	while (fgets(line, sizeof line, fp)) {
 		if (line[0] == '#' || line[0] == '\n') {
 			continue;
@@ -151,37 +162,62 @@ parse_config_file(const char *path,
 	return 0;
 }
 
-char *
-config_file_path(const char *name)
+int
+open_config_file(const char *name)
 {
-	const char dotconf[] = "/.config/";
-	const char *config_dir;
-	const char *home_dir;
-	char *path;
-	size_t size;
+	const char *config_dir  = getenv("XDG_CONFIG_HOME");
+	const char *home_dir	= getenv("HOME");
+	const char *config_dirs = getenv("XDG_CONFIG_DIRS");
+	char path[PATH_MAX];
+	const char *p, *next;
+	int fd;
 
-	config_dir = getenv("XDG_CONFIG_HOME");
-	if (!config_dir) {
-		home_dir = getenv("HOME");
-		if (!home_dir) {
-			fprintf(stderr, "HOME is not set, using cwd.\n");
-			return strdup(name);
-		}
+	/* Precedence is given to config files in the home directory,
+	 * and then to directories listed in XDG_CONFIG_DIRS and
+	 * finally to the current working directory. */
 
-		size = strlen(home_dir) + sizeof dotconf + strlen(name);
-		path = malloc(size);
-		if (!path)
-			return NULL;
-
-		snprintf(path, size, "%s%s%s", home_dir, dotconf, name);
-		return path;
+	/* $XDG_CONFIG_HOME */
+	if (config_dir) {
+		snprintf(path, sizeof path, "%s/%s", config_dir, name);
+		fd = open(path, O_RDONLY | O_CLOEXEC);
+		if (fd >= 0)
+			return fd;
 	}
 
-	size = strlen(config_dir) + 1 + strlen(name) + 1;
-	path = malloc(size);
-	if (!path)
-		return NULL;
+	/* $HOME/.config */
+	if (home_dir) {
+		snprintf(path, sizeof path, "%s/.config/%s", home_dir, name);
+		fd = open(path, O_RDONLY | O_CLOEXEC);
+		if (fd >= 0)
+			return fd;
+	}
 
-	snprintf(path, size, "%s/%s", config_dir, name);
-	return path;
+	/* For each $XDG_CONFIG_DIRS: weston/<config_file> */
+	if (!config_dirs)
+		config_dirs = "/etc/xdg";  /* See XDG base dir spec. */
+
+	for (p = config_dirs; *p != '\0'; p = next) {
+		next = strchrnul(p, ':');
+		snprintf(path, sizeof path,
+			 "%.*s/weston/%s", (int)(next - p), p, name);
+		fd = open(path, O_RDONLY | O_CLOEXEC);
+		if (fd >= 0)
+			return fd;
+
+		if (*next == ':')
+			next++;
+	}
+
+	/* Current working directory. */
+	snprintf(path, sizeof path, "./%s", name);
+	fd = open(path, O_RDONLY | O_CLOEXEC);
+
+	if (fd >= 0)
+		fprintf(stderr,
+			"using config in current working directory: %s\n",
+			path);
+	else
+		fprintf(stderr, "config file \"%s\" not found.\n", name);
+
+	return fd;
 }

shared/config-parser.h

@@ -48,12 +48,12 @@ struct config_section {
 };
 
 int
-parse_config_file(const char *path,
+parse_config_file(int config_fd,
 		  const struct config_section *sections, int num_sections,
 		  void *data);
 
-char *
-config_file_path(const char *name);
+int
+open_config_file(const char *name);
 
 enum weston_option_type {
 	WESTON_OPTION_INTEGER,