blockchain.block.header returns wrong response format when cp_height = 0

[eynhaender]

blockchain.block.header returns wrong response format when cp_height = 0

Summary

blockchain.block.header wraps its result in a JSON object instead of returning a plain hex string when cp_height is zero. This breaks Electrum wallet clients (confirmed with Sparrow) that expect the response format specified in the Electrum protocol.

Observed behavior

{"jsonrpc":"2.0","id":37,"result":{"hex":"0000092090a327..."}}

Expected behavior

{"jsonrpc":"2.0","id":37,"result":"0000092090a327..."}

Protocol spec

Per the Electrum protocol spec:

• cp_height = 0 → result is a plain hex string
• cp_height > 0 → result is a dict with header, branch, and root keys

Root cause

blockchain_block_headers() in src/protocols/electrum/protocol_electrum_headers.cpp is the shared implementation for both blockchain.block.header and blockchain.block.headers. It unconditionally builds a value_t{ object_t{} } for all code paths, including the single-header no-proof case that must return a plain string.

A second related bug exists in the same function: the pre-v1.6 branch sets result["hex"] for the proof dict (cp_height > 0, singular), but the spec key for that dict is "header", not "hex" — "hex" is only correct for the plural blockchain.block.headers concatenated response.

Impact

Sparrow retries the same request 5+ times per wallet load before dropping the connection. The wallet never finishes loading.

[evoskuil]

#745