rpc/jsonrpc, db/snapshotsync: seven review follow-ups from #23322

[lupin012]

Follow-up to #23322, which merged as approved with thirteen inline review notes deferred.
This closes the seven that are self-contained; the other six are grouped at the end.

Note	Site	Change
r3885706832	erigon_receipts.go	erigon_getLogs no longer answers [] for an unknown or non-canonical block hash
r3885706060	eth_receipts.go	resolveLogsRange returns the PrunedError it computes
r3885705986	eth_api.go, eth_system.go	the receipts gate and eth_capabilities follow history for a sentinel retention
r3885706103	receipts_generator.go	a transaction-free block is cached and answered before the exec semaphore
r3885706918	eth_api.go	the Byzantium check is read once, from receipts.PostStateCalculated
r3885706948	block_reader_test.go	the wall-clock assertion is replaced
r3885706992	block_reader.go, eth_api.go	three comments trimmed to the invariant they carry

Three of them are worth a line:

By-hash logs. resolveLogsRange's hash branch is extracted into resolveLogsBlockHash
and erigon_getLogs goes through it, so all three by-hash endpoints answer alike. The same
function no longer discards the refusal it derives; error class and message are unchanged.

Sentinel retention. Distance.Enabled() is false for three values and the switch
special-cased two, so KeepPostMergeBlocksPruneMode produced no gate while
historyRetireCutoffs retires the RCache with history for it. Both switches now ask one
predicate, which also drops the ordering dependency. It is an unexported helper rather than a
second exported method on prune.Mode, since the existing narrow ReceiptsFollowHistory()
still drives RCache download policy — see below.

Exec semaphore. It is now taken where execution starts, immediately before PrepareEnv,
so neither a transaction-free block nor the persisted-receipts fast path queues behind real
re-executions. The per-block mutex still covers the whole function.

Tests

Five new, one changed. TestErigonGetLogsByBlockHashRequiresACanonicalBlock;
a resolveLogsRange leg on TestLogsByBlockHashNamesThePruneBoundary; a
cache_sentinel_retention row on TestReceiptsGateFollowsRetention plus
TestCapabilitiesFollowHistoryForASentinelRetention; TestGetReceiptsCachesAnEmptyBlock and
TestGetReceiptsAnswersAnEmptyBlockWithoutAnExecSlot (semaphore full and context already
cancelled, so any path reaching the semaphore reports the cancellation).
make lint and make test-short are clean.

One deviation

The flaky-assert note suggested < 2*timeout or a channel signal; this asserts
client.calls.Load() <= 2 instead — each caller reaches the backend at most once, and every
fetch it runs is bounded by that caller's own deadline. Deterministic, but it pins the timing
property less tightly; require.Less(t, time.Since(started), 2*timeout) can go back alongside
it if preferred.

Not in this PR

Each is a behaviour change wanting its own test:

• erigon_getLatestLogs is the third by-hash log path and still hand-rolls HeaderByHash.
• Mode.ReceiptsFollowHistory() (narrow) still drives RCache download policy in
  db/snapshotsync, so for the sentinel retention download and retirement disagree.
• Generator.GetReceipt (singular) takes no exec slot at all, though it executes.

[AskAlexSharov]

Review pass for over-engineering only — correctness, security and perf are out of scope here. One line per finding: where, what to cut, what replaces it.

rpc/jsonrpc/check_prune_gates_test.go L1619-1638 — delete. TestCapabilitiesFollowHistoryForASentinelRetention is TestCapabilitiesFollowHistoryWhereTheCacheIsNotServed (L234) with one field changed — same setup helper, same three asserts. Add a row to pruneGatingConfigs (prune_gating_test.go L257) instead; TestCapabilitiesAgreeWithGates already pins caps-against-gate for every row there. The guard at prune_gating_test.go L290 has to widen to the new predicate for the row to pass, which is one line. The cache_sentinel_retention row in TestReceiptsGateFollowsRetention still earns its place — it pins the absolute boundary, which AgreeWithGates does not.

rpc/jsonrpc/eth_api.go L792-797 — shrink. receiptsRetiredWithHistory's amount != KeepAllReceiptsPruneMode half is dead: both call sites are the second arm of a switch whose first arm is case amount == prune.KeepAllReceiptsPruneMode. It also re-derives amount, which both callers already hold in scope. case !amount.Enabled(): inline at both sites, no helper, no doc comment.

receipts_generator_empty_block_test.go + receipts_generator_exec_sem_test.go — delete. Two new files for two tests of one code move. Fold into one test appended to post_state_calculated_test.go — same receipts package, newTestGenerator lives next door, and GetCachedReceipts is reachable from the exec-sem test, so the cache assertion costs two lines rather than a file. Drops two licence headers and two import blocks; also what CLAUDE.md asks for (append to an existing _test.go).

rpc/jsonrpc/check_prune_gates_test.go L575-583 — yagni. The resolveLogsRange leg opens its own transaction to test the function the two legs above it already route through. Nothing reaches the resolver except through one of them.

rpc/jsonrpc/eth_api.go L531-538 — shrink. holdsPreMergeBlockData's doc is still a seven-line paragraph after the trim. Keep the first sentence and "only a readable transaction of an early block settles it"; the TTL and fan-out rationale is commit-message material.

erigon_receipts_test.go L505-508, receipts_generator_empty_block_test.go L26-28 — delete. Doc comments restating names that already say it (...RequiresACanonicalBlock, ...CachesAnEmptyBlock). Keep only the second sentence of the erigon_getLogs one — the reorged-sibling case is the part the name does not carry.

net: about -95 of the 255 added lines.