assumeutxo: Add documentation on dumptxoutset serialization format

1 files changed, 7 insertions, 0 deletions

diff --git a/src/rpc/blockchain.cpp b/src/rpc/blockchain.cpp
index 6402819ce6..cecad55aee 100644
--- a/src/rpc/blockchain.cpp
+++ b/src/rpc/blockchain.cpp
@@ -2702,6 +2702,13 @@ UniValue CreateUTXOSnapshot(
     size_t written_coins_count{0};
     std::vector<std::pair<uint32_t, Coin>> coins;
 
+    // To reduce space the serialization format of the snapshot avoids
+    // duplication of tx hashes. The code takes advantage of the guarantee by
+    // leveldb that keys are lexicographically sorted.
+    // In the coins vector we collect all coins that belong to a certain tx hash
+    // (key.hash) and when we have them all (key.hash != last_hash) we write
+    // them to file using the below lambda function.
+    // See also https://github.com/bitcoin/bitcoin/issues/25675
     auto write_coins_to_file = [&](AutoFile& afile, const Txid& last_hash, const std::vector<std::pair<uint32_t, Coin>>& coins, size_t& written_coins_count) {
         afile << last_hash;
         WriteCompactSize(afile, coins.size());