Merge bitcoin/bitcoin#29671: index: avoid "failed to commit" errors on initialization

f65b0f6401 index: Move last_locator_write_time and logging to end of threadsync loop (Fabian Jahr)

Pull request description:

  In the index sync thread, when initializing an index for the first time, stop callng BaseIndex::Commit when m_best_block_index is null, to avoid a spurious "failed to commit" error from that function. This error started happening in commit 7878f97bf1 from https://github.com/bitcoin/bitcoin/pull/25494 and was reported by pstratem in https://github.com/bitcoin/bitcoin/pull/26903 with an alternate fix.

ACKs for top commit:
  achow101:
    ACK f65b0f6401
  ryanofsky:
    Code review ACK f65b0f6401. Just moved log "Syncing" log line since last commit to avoid having to call now() twice.
  furszy:
    ACK f65b0f6401
  TheCharlatan:
    ACK f65b0f6401

Tree-SHA512: afa8f05786318d36346d167ff53ea0b3bc8abdb0ad04465d199dc3eb91e9f837369e24fcb7e24b5757b02d698ec504e61da6ac365eaf006c874fc07a424a7e20

src/index/base.cpp

@@ -174,20 +174,6 @@ void BaseIndex::Sync()
             pindex = pindex_next;
 
 
-            auto current_time{std::chrono::steady_clock::now()};
-            if (last_log_time + SYNC_LOG_INTERVAL < current_time) {
-                LogPrintf("Syncing %s with block chain from height %d\n",
-                          GetName(), pindex->nHeight);
-                last_log_time = current_time;
-            }
-
-            if (last_locator_write_time + SYNC_LOCATOR_WRITE_INTERVAL < current_time) {
-                SetBestBlockIndex(pindex->pprev);
-                last_locator_write_time = current_time;
-                // No need to handle errors in Commit. See rationale above.
-                Commit();
-            }
-
             CBlock block;
             interfaces::BlockInfo block_info = kernel::MakeBlockInfo(pindex);
             if (!m_chainstate->m_blockman.ReadBlockFromDisk(block, *pindex)) {
@@ -202,6 +188,20 @@ void BaseIndex::Sync()
                            __func__, pindex->GetBlockHash().ToString());
                 return;
             }
+
+            auto current_time{std::chrono::steady_clock::now()};
+            if (last_log_time + SYNC_LOG_INTERVAL < current_time) {
+                LogPrintf("Syncing %s with block chain from height %d\n",
+                          GetName(), pindex->nHeight);
+                last_log_time = current_time;
+            }
+
+            if (last_locator_write_time + SYNC_LOCATOR_WRITE_INTERVAL < current_time) {
+                SetBestBlockIndex(pindex);
+                last_locator_write_time = current_time;
+                // No need to handle errors in Commit. See rationale above.
+                Commit();
+            }
         }
     }