index: Enable reindex-chainstate with active indexes

This is achieved by letting the index sync thread wait until
reindex-chainstate is finished.

This also disables the pruning check when reindexing the chainstate (which is
incompatible with prune mode) because there would be no chain at this point
in init.

src/index/base.cpp

@@ -23,6 +23,8 @@
 #include <string>
 #include <utility>
 
+using node::g_indexes_ready_to_sync;
+
 constexpr uint8_t DB_BEST_BLOCK{'B'};
 
 constexpr auto SYNC_LOG_INTERVAL{30s};
@@ -105,7 +107,9 @@ bool BaseIndex::Init()
     // datadir and an index enabled. If this is the case, indexation will happen solely
     // via `BlockConnected` signals until, possibly, the next restart.
     m_synced = m_best_block_index.load() == active_chain.Tip();
-    if (!m_synced) {
+
+    // Skip pruning check if indexes are not ready to sync (because reindex-chainstate has wiped the chain).
+    if (!m_synced && g_indexes_ready_to_sync) {
         bool prune_violation = false;
         if (!m_best_block_index) {
             // index is not built yet
@@ -161,6 +165,12 @@ static const CBlockIndex* NextSyncBlock(const CBlockIndex* pindex_prev, CChain&
 void BaseIndex::ThreadSync()
 {
     SetSyscallSandboxPolicy(SyscallSandboxPolicy::TX_INDEX);
+    // Wait for a possible reindex-chainstate to finish until continuing
+    // with the index sync
+    while (!g_indexes_ready_to_sync) {
+        if (!m_interrupt.sleep_for(std::chrono::milliseconds(500))) return;
+    }
+
     const CBlockIndex* pindex = m_best_block_index.load();
     if (!m_synced) {
         std::chrono::steady_clock::time_point last_log_time{0s};