[RFC PATCH 2/3] discover: Add support for device-mapper snapshots

[Samuel Mendoza-Jonas]

Device Mapper allows the creation of CoW snapshots backed by ramdisks.
This will allow Petitboot to perform potentially destructive actions
such as filesystem recovery without affecting the backing disk.
This commit adds several functions to create and destroy these snapshots
and to keep track of available ramdisks.

No functionality change in this patch.

Signed-off-by: Samuel Mendoza-Jonas <sam.mj at au1.ibm.com>
---
 discover/Makefile.am      |   4 ++
 discover/device-handler.c | 176 ++++++++++++++++++++++++++++++++++++++++++++++
 discover/device-handler.h |   6 ++
 3 files changed, 186 insertions(+)

diff --git a/discover/Makefile.am b/discover/Makefile.am
index 7808110..c6bc8af 100644
--- a/discover/Makefile.am
+++ b/discover/Makefile.am
@@ -58,6 +58,10 @@ discover_pb_discover_LDADD = \
 	$(core_lib) \
 	$(UDEV_LIBS)
 
+discover_pb_discover_LDFLAGS = \
+	$(AM_LDFLAGS) \
+	-ldevmapper
+
 discover_pb_discover_CPPFLAGS = \
 	$(AM_CPPFLAGS) \
 	-DLOCAL_STATE_DIR='"$(localstatedir)"' \
diff --git a/discover/device-handler.c b/discover/device-handler.c
index f411d9f..69cd699 100644
--- a/discover/device-handler.c
+++ b/discover/device-handler.c
@@ -10,6 +10,8 @@
 #include <sys/wait.h>
 #include <sys/mount.h>
 
+#include <libdevmapper.h>
+
 #include <talloc/talloc.h>
 #include <list/list.h>
 #include <log/log.h>
@@ -46,6 +48,12 @@ enum default_priority {
 	DEFAULT_PRIORITY_DISABLED	= 0xff,
 };
 
+struct ramdisk_device {
+	const char	*path;
+	bool		in_use;
+	char		*snapshot;
+};
+
 struct device_handler {
 	struct discover_server	*server;
 	int			dry_run;
@@ -57,6 +65,9 @@ struct device_handler {
 	struct discover_device	**devices;
 	unsigned int		n_devices;
 
+	struct ramdisk_device	**ramdisks;
+	unsigned int		n_ramdisks;
+
 	struct waitset		*waitset;
 	struct waiter		*timeout_waiter;
 	bool			autoboot_enabled;
@@ -747,6 +758,171 @@ void device_handler_add_device(struct device_handler *handler,
 		network_register_device(handler->network, device);
 }
 
+void device_handler_add_ramdisk(struct device_handler *handler,
+		const char *path)
+{
+	struct ramdisk_device *dev;
+	unsigned int i;
+
+	if (!path)
+		return;
+
+	for (i = 0; i < handler->n_ramdisks; i++)
+		if (!strcmp(handler->ramdisks[i]->path, path))
+			return;
+
+	dev = talloc_zero(handler, struct ramdisk_device);
+	dev->path = talloc_strdup(handler, path);
+
+	i = handler->n_ramdisks++;
+
+	handler->ramdisks = talloc_realloc(handler, handler->ramdisks,
+				struct ramdisk_device *, handler->n_ramdisks);
+	handler->ramdisks[i] = dev;
+}
+
+void device_handler_create_snapshot(struct device_handler *handler,
+		struct discover_device *device)
+{
+	unsigned int sectors, i;
+	struct dm_task *task;
+	char *ttype, *params;
+	uint32_t cookie = 0;
+	uint16_t flags = 0;
+	const char *tmp;
+	
+	tmp = discover_device_get_param(device, "ID_PART_ENTRY_SIZE");
+	if (!tmp) {
+		pb_log("Could not retrieve sector size for %s\n",
+		       device->device_path);
+		return;
+	}
+
+	sectors = strtoul(tmp, NULL, 0);
+	if (!sectors) {
+		pb_log("Error reading sector count for %s\n",
+		       device->device_path);
+		return;
+	}
+
+	pb_log("Creating a snapshot for %s, %u sectors, named %s\n",
+	       device->device_path, sectors, device->device->id);
+
+	/* Check if free ramdisk exists */
+	for (i = 0; i < handler->n_ramdisks; i++)
+		if (!handler->ramdisks[i]->in_use)
+			break;
+
+	/* Otherwise create a new one */
+	if (i == handler->n_ramdisks) {
+		char *name = talloc_asprintf(handler, "/dev/ram%d",
+				handler->n_ramdisks);
+		dev_t id = makedev(1, handler->n_ramdisks);
+		if (mknod(name, S_IFBLK, id)) {
+			if (errno == EEXIST) {
+				/* We haven't yet received updates for existing
+				 * ramdisks - add and use this one */
+				pb_debug("Using untracked ramdisk %s\n", name);
+			} else {
+				pb_log("Failed to create new ramdisk %s: %s\n",
+				       name, strerror(errno));
+				return;
+			}
+		}
+		device_handler_add_ramdisk(handler, name);
+		talloc_free(name);
+	}
+
+	ttype = talloc_asprintf(handler,  "snapshot");
+	params = talloc_asprintf(handler, "%s %s N 1",
+		 device->device_path, handler->ramdisks[i]->path);
+
+	task = dm_task_create(DM_DEVICE_CREATE);
+	if (!task) {
+		pb_log("Error creating new dm-task\n");
+		goto err1;
+	}
+
+	if (!dm_task_set_name(task, device->device->id))
+		goto err2;
+
+	/* Set the table for this dm-device */
+	if (!dm_task_add_target(task, 0, sectors, ttype, params))
+		goto err2;
+
+	if (!dm_task_set_add_node(task, DM_ADD_NODE_ON_CREATE))
+		goto err2;
+
+	/* Petitboot's libdm isn't compiled with --enable-udev_sync, so we set
+	 * empty cookie and flags */
+	if (!dm_task_set_cookie(task, &cookie, flags))
+		goto err2;
+
+	if (!dm_task_run(task)) {
+		pb_log("Error executing dm-task\n");
+		goto err2;
+	}
+
+	/* This is the magic incantation to get /dev/mapper/sdaX appearing */
+	dm_udev_wait(cookie);
+
+	device->ramdisk = handler->ramdisks[i];
+	handler->ramdisks[i]->in_use = true;
+	handler->ramdisks[i]->snapshot = talloc_asprintf(handler, "/dev/mapper/%s",
+						device->device->id);
+
+	pb_log("Snapshot created for device %s\n", device->device->id);
+
+err2:
+	dm_task_destroy(task);
+err1:
+	talloc_free(params);
+	talloc_free(ttype);
+}
+
+int discover_device_destroy_snapshot(struct discover_device *device)
+{
+	/* Assume we've already unmounted the snapshot */
+	struct dm_task *task;
+	uint32_t cookie = 0;
+	uint16_t flags = 0;
+	int rc = -1;
+
+	if (!device->ramdisk)
+		return 0;
+
+	task = dm_task_create(DM_DEVICE_REMOVE);
+	if (!task) {
+		pb_log("Could not create dm_task DM_DEVICE_REMOVE\n");
+		return -1;
+	}
+
+	if (!dm_task_set_name(task, device->device->id)) {
+		pb_log("No dm-snapshot named '%s'\n", device->device->id);
+		goto out;
+	}
+
+	/* Petitboot's libdm isn't compiled with --enable-udev_sync, so we set
+	 * empty cookie and flags */
+	if (!dm_task_set_cookie(task, &cookie, flags))
+		goto out;
+
+	if (!dm_task_run(task)) {
+		pb_log("Unable to remove snapshot '%s'\n", device->device->id);
+		goto out;
+	}
+
+	/* Wait for /dev/mapper/ entries to be removed */
+	dm_udev_wait(cookie);
+
+	device->ramdisk->in_use = false;
+	device->ramdisk = NULL;
+	rc = 0;
+out:
+	dm_task_destroy(task);
+	return rc;
+}
+
 /* Start discovery on a hotplugged device. The device will be in our devices
  * array, but has only just been initialised by the hotplug source.
  */
diff --git a/discover/device-handler.h b/discover/device-handler.h
index b592c46..2425b7b 100644
--- a/discover/device-handler.h
+++ b/discover/device-handler.h
@@ -26,6 +26,7 @@ struct discover_device {
 
 	char			*mount_path;
 	const char		*device_path;
+	struct ramdisk_device	*ramdisk;
 	bool			mounted;
 	bool			mounted_rw;
 	bool			unmount;
@@ -72,6 +73,11 @@ struct discover_device *discover_device_create(struct device_handler *handler,
 		const char *id);
 void device_handler_add_device(struct device_handler *handler,
 		struct discover_device *device);
+void device_handler_add_ramdisk(struct device_handler *handler,
+		const char *path);
+void device_handler_create_snapshot(struct device_handler *handler,
+		struct discover_device *device);
+int discover_device_destroy_snapshot(struct discover_device *device);
 int device_handler_discover(struct device_handler *handler,
 		struct discover_device *dev);
 int device_handler_dhcp(struct device_handler *handler,
-- 
2.1.0