Esplora /block/:hash/txs/:start past the last tx returns 200 [] where electrs returns 404 #857

Version: observed on aa6de36c (0.7.99), mainnet. block_txs_impl is unchanged on origin/master 1aece752.

Related: #632, #814 (electrs drop-in).

What happens

$ H=000000000000000000017947388dd6b378913f40ace5f68e78407b4da4b9fcab     # height 969500, tx_count 2236
$ curl -s -w '  [HTTP %{http_code}]\n' --unix-socket /run/rbitcoin/esplora.sock http://x/block/$H/txs/5000
[]  [HTTP 200]
$ curl -s -w '  [HTTP %{http_code}]\n' https://mempool.space/api/block/$H/txs/5000
start index out of range  [HTTP 404]

The multiple-of-25 check already matches electrs (400 start_index must be a multiple of 25). Only the out-of-range case differs.

Expected

404 start index out of range when start >= tx_count, as electrs does. Clients that page until they get an error, rather than until they get an empty page, loop or misreport. Under the 0.8 "electrs HTTP drop-in" goal the status code is part of the contract.

Source

crates/rbitcoin-esplora/src/handlers.rs:280-311 (block_txs_impl) at 1aece752 and aa6de36: fks.into_iter().skip(start as usize).take(25) (:299) returns an empty page instead of checking start < fks.len().

Suggested fix

if start as usize >= fks.len() {
    return (StatusCode::NOT_FOUND, "start index out of range").into_response();
}

Workaround

Stop paging on an empty array.