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MXS-2609: Fix session command mixup on master failure

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If a master failed during an ongoing session command history replay, it
would be treated as if a normal session command failed which would result
in the already executed session command being re-executed on all servers
at the wrong logical position.

To fix this, the history replay must be distinguished from normal session
command execution. When a connection replaying the history fails, the
query routing simply needs to be attempted again.
This commit is contained in:
Markus Mäkelä 2019-08-07 14:04:49 +03:00
parent 547236b7a4
commit 1748e6599d
No known key found for this signature in database
GPG Key ID: 72D48FCE664F7B19
1 changed files with 28 additions and 3 deletions
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31
server/modules/routing/readwritesplit/rwsplitsession.cc
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@ -879,17 +879,40 @@ bool RWSplitSession::retry_master_query(SRWBackend& backend)
{
bool can_continue = false;
if (backend->has_session_commands())
if (backend->is_replaying_history())
{
// Try to route the session command again. If the master is not available, the response will be
// returned from one of the slaves.
// Master failed while it was replaying the session command history
mxb_assert(m_config.master_reconnection);
mxb_assert(!m_query_queue.empty());
retry_query(m_query_queue.front().release());
m_query_queue.pop_front();
can_continue = true;
}
else if (backend->has_session_commands())
{
// We were routing a session command to all servers but the master server from which the response
// was expected failed: try to route the session command again. If the master is not available,
// the response will be returned from one of the slaves if the configuration allows it.
mxb_assert(backend->next_session_command()->get_position() == m_recv_sescmd + 1);
mxb_assert(m_qc.current_route_info().target() == TARGET_ALL);
mxb_assert(!m_current_query.get());
mxb_assert(!m_sescmd_list.empty());
mxb_assert(m_sescmd_count >= 2);
MXS_INFO("Retrying session command due to master failure: %s",
backend->next_session_command()->to_string().c_str());
// MXS-2609: Maxscale crash in RWSplitSession::retry_master_query()
// To prevent a crash from happening, we make sure the session command list is not empty before
// we touch it. This should be converted into a debug assertion once the true root cause of the
// problem is found.
if (m_sescmd_count < 2 || m_sescmd_list.empty())
{
MXS_WARNING("Session command list was empty when it should not be");
return false;
}
// Before routing it, pop the failed session command off the list and decrement the number of
// executed session commands. This "overwrites" the existing command and prevents history duplication.
m_sescmd_list.pop_back();
@ -900,6 +923,8 @@ bool RWSplitSession::retry_master_query(SRWBackend& backend)
}
else if (m_current_query.get())
{
// A query was in progress, try to route it again
mxb_assert(m_prev_target == backend);
retry_query(m_current_query.release());
can_continue = true;
}