core: ensure txindex will be triggered at least once (ethereum#27847)

Currently, we trigger the logic to (un)index transactions when the node receives a new
block. However, in some cases the node may not receive new blocks (eg, when the Geth node
is configured without peer discovery, or when it acts as an RPC node for historical-only
data).

In these situations, the Geth node user may not have previously configured txlookuplimit
(i.e. the default of around one year), but later realizes they need to index all
historical blocks. However, adding txlookuplimit=0 and restarting geth has no effect. This
change makes it check for required indexing work once, on startup, to fix the issue.

Co-authored-by: Martin Holst Swende <martin@swende.se>

core/blockchain.go

@@ -2421,6 +2421,12 @@ func (bc *BlockChain) skipBlock(err error, it *insertIterator) bool {
 func (bc *BlockChain) indexBlocks(tail *uint64, head uint64, done chan struct{}) {
 	defer func() { close(done) }()
 
+	// If head is 0, it means the chain is just initialized and no blocks are inserted,
+	// so don't need to indexing anything.
+	if head == 0 {
+		return
+	}
+
 	// The tail flag is not existent, it means the node is just initialized
 	// and all blocks(may from ancient store) are not indexed yet.
 	if tail == nil {
@@ -2480,6 +2486,14 @@ func (bc *BlockChain) maintainTxIndex() {
 	defer sub.Unsubscribe()
 	log.Info("Initialized transaction indexer", "limit", bc.TxLookupLimit())
 
+	// Launch the initial processing if chain is not empty. This step is
+	// useful in these scenarios that chain has no progress and indexer
+	// is never triggered.
+	if head := rawdb.ReadHeadBlock(bc.db); head != nil {
+		done = make(chan struct{})
+		go bc.indexBlocks(rawdb.ReadTxIndexTail(bc.db), head.NumberU64(), done)
+	}
+
 	for {
 		select {
 		case head := <-headCh: