Make CynicClientError::Empty reachable for missing-data responses

[thedavidmeister]

Closes #178.

The defect

CynicClient::query matched on errors first and fell back to
data.ok_or(CynicClientError::Empty). cynic's GraphQlResponse deserializer
rejects any body where both data and errors are absent ("Either data or
errors must be present in a GraphQL response"), so errors: None already
implied data: Some(_) and the Empty arm was unreachable — a missing-data
body surfaced as CynicClientError::Request (reqwest Decode) instead.
Confirmed against the cynic 3.11.0 that Cargo.lock pins: src/result.rs, the
data.is_none() && errors.is_none() guard in the hand-written Deserialize
impl.

The one body shape that passes that deserializer while carrying neither an
error nor data is an empty errors array. The old match sent it down the
Some(errors) arm to GraphqlError([]) — an error whose Display renders
"Graphql errors: " with nothing after it.

The fix

Match on the (data, errors) pair and guard the errors arm on a non-empty
list:

• non-empty errors -> GraphqlError (unchanged)
• data present -> Ok(data), so errors: [] beside real data is no longer
  reported as an error carrying no messages
• no data -> Empty, now reachable and testable

Behaviour is unchanged for every shape a spec-conforming server sends (the
GraphQL spec forbids an empty errors list). Only the empty-errors shape
moves, and it moves off an error that said nothing.

test_get_metabytes_by_hash_null_data_is_request_decode_error still passes
unchanged — {"data": null} with no errors key is still rejected at the
deserializer. Its doc comment lost the now-false claim that Empty is
unreachable from query.

QA

• Discriminating tests: test_get_metabytes_by_hash_null_data_empty_errors_is_empty, test_get_metabytes_by_hash_empty_errors_returns_data — each fails on base (checked out origin/main's crates/metaboard/src/cynic_client.rs over this branch's tests and ran cargo test -p rain-metaboard-subgraph --lib: 12 passed, both new tests failed, the first on unexpected result: Err(RequestErrorByHash { .., source: GraphqlError([]) }))
• Mutations applied: cynic_client.rs (None, _) => Err(CynicClientError::Empty) → Err(CynicClientError::GraphqlError(vec![])) → killed by test_get_metabytes_by_hash_null_data_empty_errors_is_empty (this is the equivalent mutant issue CynicClientError::Empty is unreachable from CynicClient::query: missing/null data surfaces as a Request decode error #178 names; it now dies); cynic_client.rs (_, Some(errors)) if !errors.is_empty() → guard dropped, (_, Some(errors)) → killed by both new tests
• Oracle: the GraphQL spec (errors must be a non-empty list when present, and must be absent when no errors were raised, so errors: [] carries no error) plus cynic 3.11.0's GraphQlResponse deserializer read directly from the vendored source, which decides which bodies reach query's match at all — neither derived from this repo's implementation
• Category check: issue asks for one of two dispositions for the dead Empty arm — remove/document it as defensive, or surface missing-data deliberately. Took the second: the arm is now live for the empty-errors body, so nothing is dead and nothing needs a defensive comment. No panic path was introduced, which the first disposition would have needed since GraphQlResponse::data is Option<T> and the None case must produce some value.

cargo test -p rain-metaboard-subgraph --lib: 14 passed, 0 failed. cargo fmt --check and cargo clippy -p rain-metaboard-subgraph --all-targets -- -D warnings clean.

🤖 Generated with Claude Code

---

rainix-rs / static / rs-static fails on this branch and on main at the same
step — the pre-commit rustfmt hook exiting "Failed to find targets" — a
repo-wide rainix hook bug, not this change. cargo clippy --all-targets --all-features -- -D warnings -D clippy::all ran clean in that same job before
the hook failed. Every other check is green.

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📥 Commits

Reviewing files that changed from the base of the PR and between 45ca96c and 437fa03.

📒 Files selected for processing (2)

• crates/metaboard/src/cynic_client.rs
• crates/metaboard/src/metaboard_client.rs

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