feat: Choose writer by cluster communication (#2830)

Fixes #1974

src/cluster/WriterDecider.cpp

@@ -0,0 +1,98 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of clio: https://github.com/XRPLF/clio
+    Copyright (c) 2025, the clio developers.
+
+    Permission to use, copy, modify, and distribute this software for any
+    purpose with or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#include "cluster/WriterDecider.hpp"
+
+#include "cluster/Backend.hpp"
+#include "cluster/ClioNode.hpp"
+#include "etl/WriterState.hpp"
+#include "util/Assert.hpp"
+#include "util/Spawn.hpp"
+
+#include <boost/asio/thread_pool.hpp>
+
+#include <algorithm>
+#include <memory>
+#include <utility>
+#include <vector>
+
+namespace cluster {
+
+WriterDecider::WriterDecider(boost::asio::thread_pool& ctx, std::unique_ptr<etl::WriterStateInterface> writerState)
+    : ctx_(ctx), writerState_(std::move(writerState))
+{
+}
+
+void
+WriterDecider::onNewState(ClioNode::CUuid selfId, std::shared_ptr<Backend::ClusterData const> clusterData)
+{
+    if (not clusterData->has_value())
+        return;
+
+    util::spawn(
+        ctx_,
+        [writerState = writerState_->clone(),
+         selfId = std::move(selfId),
+         clusterData = clusterData->value()](auto&&) mutable {
+            auto const selfData =
+                std::ranges::find_if(clusterData, [&selfId](ClioNode const& node) { return node.uuid == selfId; });
+            ASSERT(selfData != clusterData.end(), "Self data should always be in the cluster data");
+
+            if (selfData->dbRole == ClioNode::DbRole::Fallback) {
+                return;
+            }
+
+            if (selfData->dbRole == ClioNode::DbRole::ReadOnly) {
+                writerState->giveUpWriting();
+                return;
+            }
+
+            // If any node in the cluster is in Fallback mode, the entire cluster must switch
+            // to the fallback writer decision mechanism for consistency
+            if (std::ranges::any_of(clusterData, [](ClioNode const& node) {
+                    return node.dbRole == ClioNode::DbRole::Fallback;
+                })) {
+                writerState->setWriterDecidingFallback();
+                return;
+            }
+
+            // We are not ReadOnly and there is no Fallback in the cluster
+            std::ranges::sort(clusterData, [](ClioNode const& lhs, ClioNode const& rhs) {
+                return *lhs.uuid < *rhs.uuid;
+            });
+
+            auto const it = std::ranges::find_if(clusterData, [](ClioNode const& node) {
+                return node.dbRole == ClioNode::DbRole::NotWriter or node.dbRole == ClioNode::DbRole::Writer;
+            });
+
+            if (it == clusterData.end()) {
+                // No writer nodes in the cluster yet
+                return;
+            }
+
+            if (*it->uuid == *selfId) {
+                writerState->startWriting();
+            } else {
+                writerState->giveUpWriting();
+            }
+        }
+    );
+}
+
+}  // namespace cluster