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Fix to precommit/commit order inversion bug #108

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merged 1 commit into from
Mar 6, 2020
Merged

Fix to precommit/commit order inversion bug #108

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36 changes: 32 additions & 4 deletions src/handle_append_entries.cxx
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Original file line number Diff line number Diff line change
Expand Up @@ -666,11 +666,39 @@ ptr<resp_msg> raft_server::handle_append_entries(req_msg& req)
// on next `append_entries()` call, due to racing
// between BG commit thread and appending logs.
// Hence, we always should take smaller one.
precommit_index_ = req.get_last_log_idx() + req.log_entries().size();
commit( std::min( req.get_commit_idx(),
req.get_last_log_idx() + req.log_entries().size() ) );
ulong target_precommit_index = req.get_last_log_idx() + req.log_entries().size();

resp->accept(req.get_last_log_idx() + req.log_entries().size() + 1);
// WARNING:
// Since `peer::set_free()` is called prior than response handler
// without acquiring `raft_server::lock_`, there can be an edge case
// that leader may send duplicate logs, and their last log index may not
// be greater than the last log index this server already has. We should
// always compare the target index with current precommit index, and take
// it only when it is greater than the previous one.
const size_t MAX_ATTEMPTS = 10;
size_t num_attempts = 0;
ulong prev_precommit_index = precommit_index_;
while ( prev_precommit_index < target_precommit_index &&
num_attempts < MAX_ATTEMPTS ) {
if ( precommit_index_.compare_exchange_strong( prev_precommit_index,
target_precommit_index ) ) {
break;
}
// Otherwise: retry until `precommit_index_` is equal to or greater than
// `target_precommit_index`.
num_attempts++;
}
if (num_attempts >= MAX_ATTEMPTS) {
// If updating `precommit_index_` failed, we SHOULD NOT update
// commit index as well.
p_er("updating precommit_index_ failed after %zu attempts, "
"last seen precommit_index_ %zu, target %zu",
num_attempts, prev_precommit_index, target_precommit_index);
} else {
commit( std::min( req.get_commit_idx(), target_precommit_index ) );
}

resp->accept(target_precommit_index + 1);

int32 time_ms = tt.get_us() / 1000;
if (time_ms >= ctx_->get_params()->heart_beat_interval_) {
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