MXS-1220: Add alter maxscale MaxAdmin command

A set of the core MaxScale parameters can now be altered at runtime. This
set consists of the authentication timeouts and the admin interface
authentication. Other parameters either can't be modified due to internal
limitations or aren't sensible to modify at runtime.

server/core/config.cc

@@ -3907,3 +3907,68 @@ json_t* config_maxscale_to_json(const char* host)
 
     return mxs_json_resource(host, MXS_JSON_API_MAXSCALE, obj);
 }
+
+/**
+ * Creates a global configuration at the location pointed by @c filename
+ *
+ * @param filename Filename where configuration is written
+ * @return True on success, false on error
+ */
+static bool create_global_config(const char *filename)
+{
+    int file = open(filename, O_EXCL | O_CREAT | O_WRONLY, S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH);
+
+    if (file == -1)
+    {
+        MXS_ERROR("Failed to open file '%s' when serializing global configuration: %d, %s",
+                  filename, errno, mxs_strerror(errno));
+        return false;
+    }
+
+    dprintf(file, "[maxscale]\n");
+    dprintf(file, "%s=%u\n", CN_AUTH_CONNECT_TIMEOUT, gateway.auth_conn_timeout);
+    dprintf(file, "%s=%u\n", CN_AUTH_READ_TIMEOUT, gateway.auth_read_timeout);
+    dprintf(file, "%s=%u\n", CN_AUTH_WRITE_TIMEOUT, gateway.auth_write_timeout);
+    dprintf(file, "%s=%s\n", CN_ADMIN_AUTH, gateway.admin_auth ? "true" : "false");
+
+    close(file);
+
+    return true;
+}
+
+bool config_global_serialize()
+{
+    static const char* GLOBAL_CONFIG_NAME = "global-options";
+    bool rval = false;
+    char filename[PATH_MAX];
+
+    snprintf(filename, sizeof(filename), "%s/%s.cnf.tmp", get_config_persistdir(),
+             GLOBAL_CONFIG_NAME);
+
+    if (unlink(filename) == -1 && errno != ENOENT)
+    {
+        MXS_ERROR("Failed to remove temporary global configuration at '%s': %d, %s",
+                  filename, errno, mxs_strerror(errno));
+    }
+    else if (create_global_config(filename))
+    {
+        char final_filename[PATH_MAX];
+        strcpy(final_filename, filename);
+
+        char *dot = strrchr(final_filename, '.');
+        ss_dassert(dot);
+        *dot = '\0';
+
+        if (rename(filename, final_filename) == 0)
+        {
+            rval = true;
+        }
+        else
+        {
+            MXS_ERROR("Failed to rename temporary server configuration at '%s': %d, %s",
+                      filename, errno, mxs_strerror(errno));
+        }
+    }
+
+    return rval;
+}