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hw/xen: Implement XenStore transactions

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Given that the whole thing supported copy on write from the beginning,
transactions end up being fairly simple. On starting a transaction, just
take a ref of the existing root; swap it back in on a successful commit.

The main tree has a transaction ID too, and we keep a record of the last
transaction ID given out. if the main tree is ever modified when it isn't
the latest, it gets a new transaction ID.

A commit can only succeed if the main tree hasn't moved on since it was
forked. Strictly speaking, the XenStore protocol allows a transaction to
succeed as long as nothing *it* read or wrote has changed in the interim,
but no implementations do that; *any* change is sufficient to abort a
transaction.

This does not yet fire watches on the changed nodes on a commit. That bit
is more fun and will come in a follow-on commit.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
This commit is contained in:
David Woodhouse 2023-01-22 22:05:37 +00:00
parent 6e1330090d
commit 7248b87cb0
2 changed files with 262 additions and 6 deletions
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118
tests/unit/test-xs-node.c
View file

@ -42,6 +42,7 @@ static void xs_impl_delete(XenstoreImplState *s)
g_assert(s->nr_nodes == 1);
g_hash_table_unref(s->watches);
g_hash_table_unref(s->transactions);
xs_node_unref(s->root);
g_free(s);
@ -271,12 +272,129 @@ static void test_xs_node_simple(void)
}
static void do_test_xs_node_tx(bool fail, bool commit)
{
XenstoreImplState *s = setup();
GString *watches = g_string_new(NULL);
GByteArray *data = g_byte_array_new();
unsigned int tx_id = XBT_NULL;
int err;
g_assert(s);
/* Set a watch */
err = xs_impl_watch(s, DOMID_GUEST, "some", "watch",
watch_cb, watches);
g_assert(!err);
g_assert(watches->len == strlen("somewatch"));
g_assert(!strcmp(watches->str, "somewatch"));
g_string_truncate(watches, 0);
/* Write something */
err = write_str(s, DOMID_GUEST, XBT_NULL, "some/relative/path",
"something");
g_assert(s->nr_nodes == 7);
g_assert(!err);
g_assert(!strcmp(watches->str,
"some/relative/pathwatch"));
g_string_truncate(watches, 0);
/* Create a transaction */
err = xs_impl_transaction_start(s, DOMID_GUEST, &tx_id);
g_assert(!err);
if (fail) {
/* Write something else in the root */
err = write_str(s, DOMID_GUEST, XBT_NULL, "some/relative/path",
"another thing");
g_assert(!err);
g_assert(s->nr_nodes == 7);
g_assert(!strcmp(watches->str,
"some/relative/pathwatch"));
g_string_truncate(watches, 0);
}
g_assert(!watches->len);
/* Perform a write in the transaction */
err = write_str(s, DOMID_GUEST, tx_id, "some/relative/path",
"something else");
g_assert(!err);
g_assert(s->nr_nodes == 7);
g_assert(!watches->len);
err = xs_impl_read(s, DOMID_GUEST, XBT_NULL, "some/relative/path", data);
g_assert(!err);
if (fail) {
g_assert(data->len == strlen("another thing"));
g_assert(!memcmp(data->data, "another thing", data->len));
} else {
g_assert(data->len == strlen("something"));
g_assert(!memcmp(data->data, "something", data->len));
}
g_byte_array_set_size(data, 0);
err = xs_impl_read(s, DOMID_GUEST, tx_id, "some/relative/path", data);
g_assert(!err);
g_assert(data->len == strlen("something else"));
g_assert(!memcmp(data->data, "something else", data->len));
g_byte_array_set_size(data, 0);
/* Attempt to commit the transaction */
err = xs_impl_transaction_end(s, DOMID_GUEST, tx_id, commit);
if (commit && fail) {
g_assert(err == EAGAIN);
} else {
g_assert(!err);
}
g_assert(!watches->len);
g_assert(s->nr_nodes == 7);
err = xs_impl_unwatch(s, DOMID_GUEST, "some", "watch",
watch_cb, watches);
g_assert(!err);
err = xs_impl_read(s, DOMID_GUEST, XBT_NULL, "some/relative/path", data);
g_assert(!err);
if (fail) {
g_assert(data->len == strlen("another thing"));
g_assert(!memcmp(data->data, "another thing", data->len));
} else if (commit) {
g_assert(data->len == strlen("something else"));
g_assert(!memcmp(data->data, "something else", data->len));
} else {
g_assert(data->len == strlen("something"));
g_assert(!memcmp(data->data, "something", data->len));
}
g_byte_array_unref(data);
g_string_free(watches, true);
xs_impl_delete(s);
}
static void test_xs_node_tx_fail(void)
{
do_test_xs_node_tx(true, true);
}
static void test_xs_node_tx_abort(void)
{
do_test_xs_node_tx(false, false);
do_test_xs_node_tx(true, false);
}
static void test_xs_node_tx_succeed(void)
{
do_test_xs_node_tx(false, true);
}
int main(int argc, char **argv)
{
g_test_init(&argc, &argv, NULL);
module_call_init(MODULE_INIT_QOM);
g_test_add_func("/xs_node/simple", test_xs_node_simple);
g_test_add_func("/xs_node/tx_abort", test_xs_node_tx_abort);
g_test_add_func("/xs_node/tx_fail", test_xs_node_tx_fail);
g_test_add_func("/xs_node/tx_succeed", test_xs_node_tx_succeed);
return g_test_run();
}