Merge bitcoin/bitcoin#32404: log: print reason when writing chainstate

53e9b71 log: print reason for why should_write was triggered in `FlushStateToDisk` (Lőrinc)

Pull request description:

  This PR addresses a leftover logging nit found while reviewing bitcoin/bitcoin#30611 (review).
  This was also needed to validate its behavior properly, because currently there's no way to visualize how often (and why) we're flushing/syncing.

  Starting with `-debug=coindb` will now add log lines such as
  ```
  2025-05-03T08:34:57Z [coindb] Writing chainstate to disk: flush mode=PERIODIC, prune=0, large=0, critical=0, periodic=1
  2025-05-03T09:26:52Z [coindb] Writing chainstate to disk: flush mode=PERIODIC, prune=0, large=0, critical=0, periodic=1
  2025-05-03T10:27:58Z [coindb] Writing chainstate to disk: flush mode=PERIODIC, prune=0, large=0, critical=0, periodic=1
  2025-05-03T11:39:20Z [coindb] Writing chainstate to disk: flush mode=PERIODIC, prune=0, large=0, critical=0, periodic=1
  2025-05-03T12:41:48Z [coindb] Writing chainstate to disk: flush mode=PERIODIC, prune=0, large=0, critical=0, periodic=1
  2025-05-03T13:40:08Z [coindb] Writing chainstate to disk: flush mode=PERIODIC, prune=0, large=0, critical=0, periodic=1
  2025-05-03T14:49:16Z [coindb] Writing chainstate to disk: flush mode=PERIODIC, prune=0, large=0, critical=0, periodic=1
  2025-05-03T15:14:37Z [coindb] Writing chainstate to disk: flush mode=ALWAYS, prune=0, large=0, critical=0, periodic=0
  2025-05-03T15:17:28Z [coindb] Writing chainstate to disk: flush mode=ALWAYS, prune=0, large=0, critical=0, periodic=0
  ```

ACKs for top commit:
  davidgumberg:
    ACK bitcoin/bitcoin@53e9b71
  jonatack:
    diff-only review ACK 53e9b71 per `git range-diff 2df824 cfc8056 53e9b71`
  achow101:
    ACK 53e9b71
  andrewtoth:
    utACK 53e9b71

Tree-SHA512: 09f3a38cf3ecaa32cf3aba350a9e9dff9345c5468a05070c8b20987f0fdb23a8b1dc59370829c64ea356d2fc0ce99a66cc7240de7fa8c19ef3133da06db6bf3d

Author: achow101

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,