"whitelisting" mechanism (list of files - stored in DB) - which
protecting us from downloading new files after upgrade/downgrade was
broken. And seems it became over-complicated with time.
I replacing it by 1 persistent flag inside downloader:
"prohibit_new_downloads.lock"
Erigon will turn downloader into this mode after
downloading/verification of first snapshots.
```
//Corner cases:
// - Erigon generated file X with hash H1. User upgraded Erigon. New version has preverified file X with hash H2. Must ignore H2 (don't send to Downloader)
// - Erigon "download once": means restart/upgrade/downgrade must not download files (and will be fast)
// - After "download once" - Erigon will produce and seed new files
```
------
`downloader --seedbox` is never "prohibit new downloads"
Reason:
- produce and seed snapshots earlier on chain tip. reduce depnedency on
"good peers with history" at p2p-network.
Some networks have no much archive peers, also ConsensusLayer clients
are not-good(not-incentivised) at serving history.
- avoiding having too much files:
more files(shards) - means "more metadata", "more lookups for
non-indexed queries", "more dictionaries", "more bittorrent
connections", ...
less files - means small files will be removed after merge (no peers for
this files).
ToDo:
[x] Recent 500K - merge up to 100K
[x] Older than 500K - merge up to 500K
[x] Start seeding 100k files
[x] Stop seeding 100k files after merge (right before delete)
In next PR:
[] Old version of Erigon must be able download recent hashes. To achieve
it - at first start erigon will download preverified hashes .toml from
s3 - if it's newer that what we have (build-in) - use it.
