server.features should not be tied to peer discovery (returns -32603 when --electrum-public-hosts unset) #229

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1. EddieHouston commented on Jun 18, 2026

   

   EddieHouston

   on Jun 18, 2026

   Last edited by EddieHouston

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   For reference: mempool/electrs (the fork this repo's JSON-RPC 2.0 error objects were ported from, in mempool/electrs#103) has already implemented exactly this decoupling, so there's a proven reference rather than a novel design.

   In their src/electrum/server.rs, ServerFeatures is stored as a field on the connection (built once at startup from config, with hosts taken from electrum_public_hosts), and server.features just returns it unconditionally:

   fn server_features(&self) -> Result<Value> {
       Ok(json!(self.server_features.as_ref()))
   }

   It is always present in the dispatch match and never returns an error — so there's no -32603 "discovery is disabled" for server.features.

   server.add_peer stays gated on the electrum-discovery feature / DiscoveryManager (as proposed here):

   • built without electrum-discovery → falls through to the &_ arm → -32601 method not found
   • built with discovery but electrum_public_hosts unset → -32603 "discovery is disabled"

   Cross-fork comparison:

   Method	romanz/electrs	mempool/electrs	Blockstream/electrs (current)
   server.features (no hosts)	success	success	-32603 "discovery is disabled"
   server.add_peer (no discovery)	n/a (no such method)	-32601 / -32603	-32603

   So the fix is essentially porting mempool's server_features field pattern — mirroring how mempool/electrs#103 was ported.
2. EddieHouston commented on Jun 18, 2026

   

   EddieHouston

   on Jun 18, 2026

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   Direct reference implementation: mempool/electrs has since landed this exact change in mempool/electrs#150 ("feat[electrum]: Support server.features on all feature combinations", closes mempool/electrs#149). It's a small diff (+22/-19) across src/config.rs and src/electrum/server.rs and matches the proposal above:

   1. config.rs — move electrum_public_hosts out of #[cfg(feature = "electrum-discovery")] so the field always exists (defaults to None when the feature is off).
   2. server.rs — build a server_features: Arc<ServerFeatures> once at RPC startup with hosts = electrum_public_hosts.unwrap_or_default() (empty map when unconfigured), and thread it into each Connection.
   3. server_features() becomes Ok(json!(self.server_features.as_ref())), and the dispatch arm is ungated so it works in all build configurations.
   4. Discovery still receives the same struct (server_features.as_ref().clone()); server.add_peer stays gated on the discovery feature.

   So this is a ready-to-port reference, same as how the JSON-RPC 2.0 error objects were ported from mempool/electrs#103.

   One nuance when porting: mempool/electrs#150 sets server_version from VERSION_STRING, whereas this repo currently constructs it inline as format!("electrs-esplora {}", ELECTRS_VERSION) — that existing string should be preserved rather than adopting mempool's value verbatim.
3. devdavidejesus commented on Sep 28, 2026

   

   devdavidejesus

   on Sep 28, 2026

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   Additional client-side impact, reproduced on regtest today: Electrum 4.8.2 cannot connect to a default build of this repo at all.

   Since Electrum 4.7.0 (spesmilo/electrum#10281, "interface: check genesis hash on connection"), the client requests server.features right after server.version and disconnects if it fails (electrum/interface.py, L1063–1067 in the 4.8.2 tag). Against electrs new-index at cb7c72f built without electrum-discovery, Electrum's log shows, in a retry loop:

   disconnecting due to GracefulDisconnect(RPCError(-32601, 'unknown method server.features'))
   

   and getinfo stays at "connected": false. server.version itself succeeds (["electrs-esplora 0.4.1","1.4"]).

   Confirmed by execution: the same Electrum 4.8.2 connects to the same commit once electrs is built with --features electrum-discovery and started with --electrum-public-hosts.

   So beyond protocol correctness, porting mempool/electrs#150 would restore compatibility with current Electrum releases for anyone running a default build.

   Is anyone already working on the port? If not, I'd be happy to open a PR following the plan above (keeping the existing electrs-esplora {ELECTRS_VERSION} string), and verify it end-to-end with Electrum 4.8.2 on regtest.