xenstore: add support for reading directory with many children

As the payload size for one xenstore wire command is limited to 4096
bytes it is impossible to read the children names of a node with a
large number of children (e.g. /local/domain in case of a host with
more than about 2000 domains). This effectively limits the maximum
number of domains a host can support.

In order to support such long directory outputs add a new wire command
XS_DIRECTORY_PART which will return only some entries in each call and
can be called in a loop to get all entries.

Input data are the path of the node and the byte offset into the child
list where returned data should start.

Output is the generation count of the node (which will change each time
the node is being modified) and a list of child names starting with
the specified index. The end of the list is indicated by an empty
child name. It is the responsibility of the caller to check for data
consistency by comparing the generation counts of all returned data
sets to be the same for one node.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>

tools/xenstore/xenstored_core.c

@@ -16,6 +16,7 @@
     along with this program; If not, see <http://www.gnu.org/licenses/>.
 */
 
+#include <inttypes.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <poll.h>
@@ -147,6 +148,7 @@ static char *sockmsg_string(enum xsd_sockmsg_type type)
 	case XS_RESUME: return "RESUME";
 	case XS_SET_TARGET: return "SET_TARGET";
 	case XS_RESET_WATCHES: return "RESET_WATCHES";
+	case XS_DIRECTORY_PART: return "DIRECTORY_PART";
 	default:
 		return "**UNKNOWN**";
 	}
@@ -812,6 +814,67 @@ static void send_directory(struct connection *conn, struct buffered_data *in)
 	send_reply(conn, XS_DIRECTORY, node->children, node->childlen);
 }
 
+static void send_directory_part(struct connection *conn,
+				struct buffered_data *in)
+{
+	unsigned int off, len, maxlen, genlen;
+	char *name, *child, *data;
+	struct node *node;
+	char gen[24];
+
+	if (xs_count_strings(in->buffer, in->used) != 2) {
+		send_error(conn, EINVAL);
+		return;
+	}
+
+	/* First arg is node name. */
+	name = canonicalize(conn, in->buffer);
+
+	/* Second arg is childlist offset. */
+	off = atoi(in->buffer + strlen(in->buffer) + 1);
+
+	node = get_node(conn, in, name, XS_PERM_READ);
+	if (!node) {
+		send_error(conn, errno);
+		return;
+	}
+
+	genlen = snprintf(gen, sizeof(gen), "%"PRIu64, node->generation) + 1;
+
+	/* Offset behind list: just return a list with an empty string. */
+	if (off >= node->childlen) {
+		gen[genlen] = 0;
+		send_reply(conn, XS_DIRECTORY_PART, gen, genlen + 1);
+		return;
+	}
+
+	len = 0;
+	maxlen = XENSTORE_PAYLOAD_MAX - genlen - 1;
+	child = node->children + off;
+
+	while (len + strlen(child) < maxlen) {
+		len += strlen(child) + 1;
+		child += strlen(child) + 1;
+		if (off + len == node->childlen)
+			break;
+	}
+
+	data = talloc_array(in, char, genlen + len + 1);
+	if (!data) {
+		send_error(conn, ENOMEM);
+		return;
+	}
+
+	memcpy(data, gen, genlen);
+	memcpy(data + genlen, node->children + off, len);
+	if (off + len == node->childlen) {
+		data[genlen + len] = 0;
+		len++;
+	}
+
+	send_reply(conn, XS_DIRECTORY_PART, data, genlen + len);
+}
+
 static void do_read(struct connection *conn, struct buffered_data *in)
 {
 	struct node *node;
@@ -1334,6 +1397,10 @@ static void process_message(struct connection *conn, struct buffered_data *in)
 		do_reset_watches(conn, in);
 		break;
 
+	case XS_DIRECTORY_PART:
+		send_directory_part(conn, in);
+		break;
+
 	default:
 		eprintf("Client unknown operation %i", in->hdr.msg.type);
 		send_error(conn, ENOSYS);

xen/include/public/io/xs_wire.h

@@ -50,6 +50,7 @@ enum xsd_sockmsg_type
     XS_SET_TARGET,
     XS_RESTRICT,
     XS_RESET_WATCHES,
+    XS_DIRECTORY_PART,
 
     XS_INVALID = 0xffff /* Guaranteed to remain an invalid type */
 };