conf: initial support for configuration file

* Look for configuration file in (in this order):
  - XDG_CONFIG_HOME/footrc
  - ~/.config/footrc
* Currently supports setting the font

config.c

@@ -0,0 +1,178 @@
+#include "config.h"
+
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+#include <stdbool.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <pwd.h>
+#include <assert.h>
+#include <errno.h>
+
+#define LOG_MODULE "config"
+#define LOG_ENABLE_DBG 0
+#include "log.h"
+
+static char *
+get_config_path_user_config(void)
+{
+    struct passwd *passwd = getpwuid(getuid());
+    if (passwd == NULL) {
+        LOG_ERRNO("failed to lookup user");
+        return NULL;
+    }
+
+    const char *home_dir = passwd->pw_dir;
+    LOG_DBG("user's home directory: %s", home_dir);
+
+    int len = snprintf(NULL, 0, "%s/.config/footrc", home_dir);
+    char *path = malloc(len + 1);
+    snprintf(path, len + 1, "%s/.config/footrc", home_dir);
+    return path;
+}
+
+static char *
+get_config_path_xdg(void)
+{
+    const char *xdg_config_home = getenv("XDG_CONFIG_HOME");
+    if (xdg_config_home == NULL)
+        return NULL;
+
+    int len = snprintf(NULL, 0, "%s/footrc", xdg_config_home);
+    char *path = malloc(len + 1);
+    snprintf(path, len + 1, "%s/footrc", xdg_config_home);
+    return path;
+}
+
+static char *
+get_config_path(void)
+{
+    struct stat st;
+
+    char *config = get_config_path_xdg();
+    if (config != NULL && stat(config, &st) == 0 && S_ISREG(st.st_mode))
+        return config;
+    free(config);
+
+    /* 'Default' XDG_CONFIG_HOME */
+    config = get_config_path_user_config();
+    if (config != NULL && stat(config, &st) == 0 && S_ISREG(st.st_mode))
+        return config;
+    free(config);
+
+    return NULL;
+}
+
+static bool
+parse_section_main(char *line, struct config *conf, const char *path, unsigned lineno)
+{
+    const char *key = strtok(line, "=");
+    const char *value = strtok(NULL, "\n");
+
+    LOG_DBG("%s:%u: key = %s, value=%s", path, lineno, key, value);
+
+    if (strcmp(key, "font") == 0) {
+        free(conf->font);
+        conf->font = strdup(value);
+    }
+
+    else {
+        LOG_ERR("%s:%u: invalid key: %s", path, lineno, key);
+        return false;
+    }
+
+    return true;
+}
+
+static bool
+parse_config_file(FILE *f, struct config *conf, const char *path)
+{
+    enum section {
+        SECTION_MAIN,
+    } section = SECTION_MAIN;
+
+    /* Function pointer, called for each key/value line */
+    typedef bool (*parser_fun_t)(
+        char *line, struct config *conf, const char *path, unsigned lineno);
+
+    /* Maps sections to line parser functions */
+    static const parser_fun_t section_parser_map[] = {
+        [SECTION_MAIN] = &parse_section_main,
+    };
+
+    unsigned lineno = 0;
+
+    while (true) {
+        errno = 0;
+        lineno++;
+
+        char *line = NULL;
+        size_t count = 0;
+        ssize_t ret = getline(&line, &count, f);
+
+        if (ret < 0) {
+            free(line);
+            if (errno != 0) {
+                LOG_ERRNO("failed to read from configuration");
+                return false;
+            }
+            break;
+        }
+
+        /* No sections yet */
+        if (line[0] == '[' && line[strlen(line) - 1] == ']') {
+            assert(false);
+            return false;
+        }
+
+        parser_fun_t section_parser = section_parser_map[section];
+        assert(section_parser != NULL);
+
+        if (!section_parser(line, conf, path, lineno)) {
+            free(line);
+            return false;
+        }
+
+        free(line);
+    }
+
+    return true;
+}
+
+struct config
+config_load(void)
+{
+    struct config conf = {
+        .font = strdup("monospace"),
+    };
+
+    char *path = get_config_path();
+    LOG_INFO("loading configuration from %s", path);
+
+    if (path == NULL) {
+        /* Default conf */
+        LOG_WARN("no configuration found, using defaults");
+        goto out;
+    }
+
+    FILE *f = fopen(path, "r");
+    if (f == NULL) {
+        LOG_ERR("%s: failed to open", path);
+        goto out;
+    }
+
+    parse_config_file(f, &conf, path);
+    fclose(f);
+
+out:
+    free(path);
+    return conf;
+}
+
+void
+config_free(struct config conf)
+{
+    free(conf.font);
+}