dash(stratum): serve real work — wire GBT work data into session template path + X11 submit scoring

[frstrtr]

Fixes #732 — the v0.2.2 DASH stratum accepts and registers miners but never serves work: DASHWorkSource stages 4c/4d were unimplemented skeletons, so StratumSession::send_notify_work() saw an empty template forever and re-armed its 1 s timer in a loop while the binary logged a misleading [LTC] wait line.

Stage 4c — template serving

Bridges the armed get_work() seam (#726: embedded coin-state when seeded, retained dashd GBT fallback otherwise — the duality is preserved untouched) into the template trio the coin-agnostic session consumes:

• Template cache (cached_work()): the DashWorkData snapshot is cached under template_mutex_, keyed on work_generation_ + a 30 s staleness TTL, with a 5 s negative cache so a down dashd is not hammered by every session's 1 s retry timer. A refresh that observes a moved tip bumps work_generation_ so sessions re-push work between timer firings.
• get_current_work_template(): GBT-shaped JSON with exactly the fields send_notify_work() reads (previousblockhash BE display hex, version, bits as 8-char BE hex, curtime, height, coinbasevalue), mirroring the LTC/BTC conventions. Empty object on an honest set-gap — never fabricated.
• get_stratum_merkle_branches(): sibling list over [coinbase placeholder, tx1..txN] in the LE-internal-bytes wire encoding (the existing dash::coinbase::merkle_branches_raw/_hex SSOT).
• build_connection_coinbase(): goes through the EXISTING verifier-shared SSOTs — compute_dash_payouts (worker_tx ‖ masternode/superblock payments ‖ donation tail, the share_check.hpp gentx order) → coinbase::build (BIP34 height + pool tag scriptSig, DIP3/DIP4 payload, ref_hash + nonce64 OP_RETURN tail) → split_coinb (coinb1/coinb2 around the 8-byte nonce64 extranonce slot = en1(4) ‖ en2(4)). No second payout or serialization implementation exists by construction, so the coinbase a miner hashes is byte-identical to the verifier split.
  • Genesis case (fresh pool, empty sharechain — the deployment reality): the single connecting miner's P2PKH script carries the full worker payout (weights {script:1}, total 1) + the GBT-mandated masternode outputs + donation tail. The PPLNS multi-output path rides the new set_pplns_weights_fn seam (ShareTracker walk bound by the run-loop) through the same compute_dash_payouts arm.
  • get_coinbase_parts() serves an all-to-donation pool fallback so the core's legacy hardcoded (LTC-shaped) default coinbase is unreachable for DASH.
• get_current_gbt_prevhash(): same cached snapshot as the template — one truthful source.
• set_difficulty scale fix: the ctor overrides set_difficulty_multiplier to 1.0 (X11 uses the standard diff-1 scale, p2pool-dash DUMB_SCRYPT_DIFF = 1), same as the SHA256d BTC work source. The default 65536 (scrypt convention) inflated the advertised difficulty 65536× (the wire=32.768 in the field report).

Stage 4d — submit scoring

• mining_submit(): reconstructs coinbase = coinb1 ‖ en1 ‖ en2 ‖ coinb2, sha256d txid, ascends the frozen branches, serializes the 80-byte header via the block_producer.hpp SSOT, runs the --selftest-pinned dash::crypto::hash_x11, and classifies tighten-first:
  • WonBlock (pow ≤ block target): full block assembled with the exact --mine-block serialization (header ‖ CompactSize(1+ntx) ‖ coinbase ‖ txs; DASH has no segwit) → submit_block_fn_, the dual-path won-block dispatcher bound in main_dash.cpp (dashd submitblock RPC arm; embedded P2P relay leg folds into that same closure when it lands). A won block reaching no sink logs at ERROR — never a silent drop.
  • ShareAccept (pow ≤ share target): routed into the new set_mint_share_fn seam (header/coinbase/branches/payout script/pow). While unbound it accepts for vardiff and logs loudly.
  • else low-difficulty reject (code 23); missing JobSnapshot rejects 21.
• compute_share_difficulty(): same reconstruction ending in diff1 / x11(header), so the coin-agnostic vardiff gate engages; 0.0 on malformed input (the documented sentinel).

Secondary fix — [LTC] mistag in the coin-agnostic core

stratum_server.cpp hardcoded [LTC] in the "Waiting for block template" warning. StratumConfig now carries a coin_symbol set at runtime (DASH/BTC/DGB set it in their work-source ctors, LTC in main_ltc.cpp), with a neutral [Stratum] fallback. No coin name is hardcoded in src/core.

Test gate (the miss that let v0.2.2 ship)

Extended the existing allowlisted test_dash_stratum_work_source target (already in the build.yml --target list and ctest registration — the #729 compile-into-existing-target pattern, no workflow-file change needed):

• stub work source returning a fixed DashWorkData → non-empty template with correct prevhash/bits/version/curtime encodings; set-gap fixture → honestly empty trio;
• merkle branches fold to the canonical compute_merkle_root over the fixture tx set;
• coinb1 ‖ extranonce ‖ coinb2 reassembles byte-identical to the compute_dash_payouts + coinbase::build SSOT output, with value splits pinned (masternode output = exact GBT amount, miner + donation = worker payout, total = coinbasevalue);
• compute_share_difficulty KAT: fixture header → pinned X11-derived value; malformed input → 0.0;
• mining_submit classification: deterministic (test-mined nonce) ShareAccept / low-diff reject / WonBlock, the WonBlock asserting the dispatched block bytes are the exact --mine-block serialization; plus the mint-seam routing test.

Verified

• test_dash_stratum_work_source — 19/19 KATs green locally (Release/gcc-13, conan profile ci/conan/linux-gcc13.profile); test_stratum_extensions (44), test_dash_stratum_extranonce_split (6), test_dash_stratum_submit_reassembly (7) all green.
• c2pool-dash --selftest green (X11 KATs + coinbase parity untouched).
• Live smoke (--run --stratum against a stubbed dashd GBT endpoint): subscribe → authorize → mining.notify arrives with a real job — BIP34 height push + c2pool tag + PPLNS/masternode/donation/OP_RETURN outputs, correct stratum prevhash chunking, set_difficulty wire value now 0.0005 (was 32.768). The exact dash: stratum serves no work — DASHWorkSource 4c/4d skeleton (v0.2.2 field report); [LTC] mistag in core #732 field failure mode (miner registered, no job, 60 s EOF loop) is gone. With no dashd reachable, the honest set-gap path logs [DASH] Waiting for block template ... — the coin-tag fix observed live.
• c2pool (LTC), btc_stratum, dgb_stratum targets rebuilt green (core StratumConfig change is additive; no positional aggregate inits exist).

Explicitly deferred (follow-ups, called out in-code)

• Sharechain mint wiring: main_dash.cpp does not yet stand up a DASH ShareTracker in the --run loop, so the node-side share-creation seam is not cleanly reachable; set_mint_share_fn is the one-line binding point and unbound share-target submissions accept-for-vardiff with a loud log.
• PPLNS multi-output weights: set_pplns_weights_fn is the ShareTracker-walk binding point; until bound the genesis single-miner split is served (correct for a fresh pool's empty sharechain).
• Embedded P2P won-block relay leg remains S8 (unchanged; the RPC arm is live).