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

f65b0f6 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 bitcoin@7878f97 from bitcoin#25494 and was reported by pstratem in bitcoin#26903 with an alternate fix.

ACKs for top commit:
  achow101:
    ACK f65b0f6
  ryanofsky:
    Code review ACK f65b0f6. Just moved log "Syncing" log line since last commit to avoid having to call now() twice.
  furszy:
    ACK f65b0f6
  TheCharlatan:
    ACK f65b0f6

Tree-SHA512: afa8f05786318d36346d167ff53ea0b3bc8abdb0ad04465d199dc3eb91e9f837369e24fcb7e24b5757b02d698ec504e61da6ac365eaf006c874fc07a424a7e20

Author: achow101

src/index/base.cpp

@@ -160,6 +160,18 @@ void BaseIndex::ThreadSync()
                 pindex = pindex_next;
             }
 
+            CBlock block;
+            if (!ReadBlockFromDisk(block, pindex, consensus_params)) {
+                FatalError("%s: Failed to read block %s from disk",
+                           __func__, pindex->GetBlockHash().ToString());
+                return;
+            }
+            if (!WriteBlock(block, pindex)) {
+                FatalError("%s: Failed to write block %s to index database",
+                           __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",
@@ -168,23 +180,11 @@ void BaseIndex::ThreadSync()
             }
 
             if (last_locator_write_time + SYNC_LOCATOR_WRITE_INTERVAL < current_time) {
-                SetBestBlockIndex(pindex->pprev);
+                SetBestBlockIndex(pindex);
                 last_locator_write_time = current_time;
                 // No need to handle errors in Commit. See rationale above.
                 Commit();
             }
-
-            CBlock block;
-            if (!ReadBlockFromDisk(block, pindex, consensus_params)) {
-                FatalError("%s: Failed to read block %s from disk",
-                           __func__, pindex->GetBlockHash().ToString());
-                return;
-            }
-            if (!WriteBlock(block, pindex)) {
-                FatalError("%s: Failed to write block %s to index database",
-                           __func__, pindex->GetBlockHash().ToString());
-                return;
-            }
         }
     }