api_extensions: introduce lxc_has_api_extension()

doc/api-extensions.md

+# API extensions
+
+The changes below were introduced to the LXC API after the 3.0 API was finalized.
+
+They are all backward compatible and can be detected by client tools by
+called the `lxc_has_api_extension` function.
+
+## lxc\_log
+
+This introduces a way to initialize a logging instance from the API for a given
+container.
+
+## lxc\_config\_item\_is\_supported
+
+This introduces the `lxc_config_item_is_supported` function. It allows users to
+check whether their LXC instance supports a given configuration key.
+
+## console\_log
+
+This adds support to container's console log. The console log is implemented as
+an efficient ringbuffer.
+
+## reboot2
+
+This adds `reboot2()` as a new API extension. This function properly waits
+until a reboot succeeded. It takes a timeout argument. When set to `> 0`
+`reboot2()` will block until the timeout is reached, if timeout is set to zero
+`reboot2()` will not block, if set to -1 `reboot2()` will block indefinitly.
+
+## mount\_injection
+
+This adds support for injecting and removing mounts into/from a running
+containers. Two new API functions `mount()` and `umount()` are added. They
+mirror the current mount and umount API of the kernel.

src/lxc/Makefile.am

@@ -2,7 +2,8 @@ pkginclude_HEADERS = attach_options.h \
 		     lxccontainer.h \
 		     version.h
 
-noinst_HEADERS = attach.h \
+noinst_HEADERS = api_extensions.h \
+		 attach.h \
 		 caps.h \
 		 cgroups/cgroup.h \
 		 cgroups/cgroup_utils.h \
@@ -85,6 +86,7 @@ endif
 
 lib_LTLIBRARIES = liblxc.la
 liblxc_la_SOURCES = af_unix.c af_unix.h \
+		    api_extensions.h \
 		    attach.c attach.h \
 		    caps.c caps.h \
 		    cgroups/cgfsng.c \

src/lxc/api_extensions.h

+/* liblxcapi
+ *
+ * Copyright © 2018 Christian Brauner <christian.brauner@ubuntu.com>.
+ * Copyright © 2018 Canonical Ltd.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2, as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef __LXC_API_EXTENSIONS_H
+#define __LXC_API_EXTENSIONS_H
+
+#include "config.h"
+#include <stdio.h>
+#include <stdlib.h>
+
+/*
+ * api_extensions is the list of all API extensions in the order they were
+ * added.
+
+ The following kind of changes come with a new extensions:
+
+ - New public functions
+ - New configuration key
+ - New valid values for a configuration key
+*/
+static char *api_extensions[] = {
+	"lxc_log",
+	"lxc_config_item_is_supported",
+	"console_log",
+	"reboot2",
+	"mount_injection",
+	"cgroup_relative",
+};
+
+static size_t nr_api_extensions = sizeof(api_extensions) / sizeof(*api_extensions);
+
+#endif /* __LXC_API_EXTENSIONS_H */

src/lxc/lxccontainer.c

@@ -42,6 +42,7 @@
 #include <unistd.h>
 
 #include "af_unix.h"
+#include "api_extensions.h"
 #include "attach.h"
 #include "cgroup.h"
 #include "commands.h"
@@ -5671,3 +5672,16 @@ bool lxc_config_item_is_supported(const char *key)
 {
 	return !!lxc_get_config(key);
 }
+
+bool lxc_has_api_extension(const char *extension)
+{
+	/* The NULL API extension is always present. :) */
+	if (!extension)
+		return true;
+
+	for (size_t i = 0; i < nr_api_extensions; i++)
+		if (strcmp(api_extensions[i], extension) == 0)
+			return true;
+
+	return false;
+}

src/lxc/lxccontainer.h

@@ -1123,6 +1123,13 @@ void lxc_log_close(void);
  */
 bool lxc_config_item_is_supported(const char *key);
 
+/*!
+ * \brief Check if an API extension is supported by this LXC instance.
+ *
+ * \param extension API extension to check for.
+ */
+bool lxc_has_api_extension(const char *extension);
+
 #ifdef  __cplusplus
 }
 #endif