log: print reason for why should_write was triggered in FlushStateToDisk

Co-authored-by: Jon Atack <jon@atack.com>

src/validation.cpp

@@ -2874,6 +2874,9 @@ bool Chainstate::FlushStateToDisk(
         bool should_write = (mode == FlushStateMode::ALWAYS) || fCacheLarge || fCacheCritical || fPeriodicWrite || fFlushForPrune;
         // Write blocks, block index and best chain related state to disk.
         if (should_write) {
+            LogDebug(BCLog::COINDB, "Writing chainstate to disk: flush mode=%s, prune=%d, large=%d, critical=%d, periodic=%d",
+                     FlushStateModeNames[size_t(mode)], fFlushForPrune, fCacheLarge, fCacheCritical, fPeriodicWrite);
+
             // Ensure we can write block index
             if (!CheckDiskSpace(m_blockman.m_opts.blocks_dir)) {
                 return FatalError(m_chainman.GetNotifications(), state, _("Disk space is too low!"));

src/validation.h

@@ -437,7 +437,8 @@ enum DisconnectResult
 class ConnectTrace;
 
 /** @see Chainstate::FlushStateToDisk */
-enum class FlushStateMode {
+inline constexpr std::array FlushStateModeNames{"NONE", "IF_NEEDED", "PERIODIC", "ALWAYS"};
+enum class FlushStateMode: uint8_t {
     NONE,
     IF_NEEDED,
     PERIODIC,