wire-datafusion-function

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Wire Comet support for the $ARGUMENTS Spark expression by reusing an existing DataFusion or datafusion-spark function. No native Rust is written here — if upstream coverage is missing, stop and run implement-comet-expression.

Wiring patterns

Pick the lightest one that satisfies the Spark contract.

Pattern	When	What to change
A — passthrough	DF built-in (e.g. acos), or datafusion-spark UDF already registered in register_datafusion_spark_function	one line in the right map of QueryPlanSerde.scala: classOf[Foo] -> CometScalarFunction("foo")
B — register + passthrough	datafusion-spark UDF, not yet registered, semantics already match Spark	Pattern A line plus session_ctx.register_udf(ScalarUDF::new_from_impl(SparkFoo::default())); in native/core/src/execution/jni_api.rs::register_datafusion_spark_function
C — custom serde	Inputs need preprocessing (cast, nullIfNegative, +0.0 flip), or you need to set return type / failOnError, restrict input types via getSupportLevel, enforce foldable-only args, or attach getCompatibleNotes/getIncompatibleReasons/getUnsupportedReasons	new CometXxx object in the topic file (math.scala, strings.scala, …); see CometCeil, CometAtan2, CometLog, CometSha2, CometAbs

Function names must match: the string passed to CometScalarFunction("xyz") equals SparkXyz::name() upstream.

Workflow

1. Study the Spark contract

Find the case class $ARGUMENTS (PascalCase). Prefer the user's local Spark clone (agent instructions / project memory); fall back to git clone --depth 1 https://github.com/apache/spark.git /tmp/spark-master. If the clone is sandbox-blocked and no local clone is found, stop and ask — Spark is the ground truth.

Note inputTypes, dataType, eval / nullSafeEval, require guards, ANSI mode branches, and foldable-only arguments. These define what Comet must reproduce.

2. Find the upstream function (at the pinned version)

The pinned versions in native/Cargo.toml are the ground truth — a function on upstream main may not exist in the version Comet depends on. Resolve versions first, then search sources matching those exact versions. Pick one source path; do not mix.

REPO_ROOT=$(git rev-parse --show-toplevel)
# Portable regex — BSD awk on macOS does not support \b.
DF_SPARK_VER=$(awk -F'"' '/^datafusion-spark[ =]/ {print $2; exit}' "$REPO_ROOT/native/Cargo.toml")
DF_FUNCS_VER=$(awk -F'"' '/^datafusion[ =]/        {print $2; exit}' "$REPO_ROOT/native/Cargo.toml")
[ -z "$DF_SPARK_VER" ] || [ -z "$DF_FUNCS_VER" ] && { echo "ERROR: version extraction failed"; exit 1; }
EXPR='$ARGUMENTS'

Option A — cargo registry (preferred):

DF_SPARK=$(ls -d ~/.cargo/registry/src/*/datafusion-spark-${DF_SPARK_VER}/ 2>/dev/null | head -1)
DF_FUNCS=$(ls -d ~/.cargo/registry/src/*/datafusion-functions-${DF_FUNCS_VER}/ 2>/dev/null | head -1)
# If empty, run `cargo fetch` from native/.
grep -rin "fn name" "$DF_SPARK/src/function/" 2>/dev/null | grep -i "$EXPR"
grep -rin "fn name" "$DF_FUNCS/src/"          2>/dev/null | grep -i "$EXPR"

Option B — local DataFusion clone (only if agent instructions / memory point at one). Grep at the pinned tag — DataFusion uses lightweight tags (<version>, no v prefix), so use tag -l not rev-parse --verify ^{tag}:

DF_CLONE=<path from memory>
[ -z "$(git -C "$DF_CLONE" tag -l "$DF_SPARK_VER")" ] && echo "WARNING: tag missing — git fetch --tags or use Option A"
git -C "$DF_CLONE" grep -in "fn name" "$DF_SPARK_VER" -- 'datafusion/spark/src/function/' | grep -i "$EXPR"
git -C "$DF_CLONE" grep -in "fn name" "$DF_FUNCS_VER" -- 'datafusion/functions/src/'      | grep -i "$EXPR"

Never grep the clone's working tree directly — it may be on main.

2a. Decision gate — confirm the source crate

• Found in datafusion-spark → proceed (Spark-tuned by design). If listed in register_datafusion_spark_function → Pattern A; otherwise Pattern B.
• Found only in datafusion-functions → STOP and AskUserQuestion. Pure DataFusion follows standard SQL semantics and frequently diverges from Spark (NULL vs error, negatives, overflow, return type, NaN). Surface what you found in each crate and the divergences you've already identified, then offer:
  1. Wire from datafusion-functions with Pattern C bridging — list the divergences and the masking/casting that closes them.
  2. Stop and switch to implement-comet-expression to port to datafusion-spark upstream first.
• Found in neither → stop and run implement-comet-expression.

If semantics diverge in a way Pattern C can't bridge, escalate.

3. Apply the Scala wiring

Add the entry to the matching map in spark/src/main/scala/org/apache/comet/serde/QueryPlanSerde.scala: mathExpressions, stringExpressions, arrayExpressions, mapExpressions, structExpressions, predicateExpressions, conditionalExpressions, bitwiseExpressions, temporalExpressions, hashExpressions, conversionExpressions, miscExpressions.

Spark expression classes are in scope via import org.apache.spark.sql.catalyst.expressions._. For Pattern C, the topic file uses explicit imports — extend the existing import …expressions.{…} line and place the new object alongside its peers.

Helpers from QueryPlanSerde:

• scalarFunctionExprToProto(name, args*) — return type inferred, failOnError = false.
• scalarFunctionExprToProtoWithReturnType(name, returnType, failOnError, args*) — explicit type / fail-on-error.
• optExprWithInfo(optExpr, expr, children*) — wrap final result; propagates "why we couldn't convert" tags.
• withInfo(expr, "reason") — tag a fallback when returning None.

getSupportLevel returning Incompatible(Some("…")) gates behind spark.comet.expr.<name>.allowIncompatible=true. Unsupported(…) always falls back.

4. Register the UDF (Pattern B only)

In native/core/src/execution/jni_api.rs::register_datafusion_spark_function, add the use and the register_udf line, grouped with neighbors. Patterns A and C never touch native code.

5. Add SQL file tests

Create spark/src/test/resources/sql-tests/expressions/<category>/$ARGUMENTS.sql. Mandatory rules:

1. Test only the directly supported input types — read inputTypes from the Spark source. Do not add columns for implicit-cast widening (ImplicitCastInputTypes); the cast path has its own tests. E.g. inputTypes = Seq(IntegerType) (Factorial) → one int column, not byte/short/long/float/double/decimal. For TypeCollection(A, B), add one column per member.
2. All SELECTs read from a parquet table — no literal-only queries (the constant folder skips the read path and column-vector kernel).
3. Mix NULL into every column — exercises the validity bitmap.
4. Per type, cover the edge-case set:
   • Float / Double: NaN, +0.0, -0.0, ±Infinity via cast('NaN' as double) etc.
   • Integer / decimal: each type's max and min (127/-128, 2147483647/-2147483648, 9223372036854775807/-9223372036854775808, Decimal(38, 0) boundary) for overflow / wrap.
   • String / binary: empty, multi-byte UTF-8 ('é', '日本'), embedded NULs if relevant.
5. Cover the Spark-specific edges found in step 1: in-range boundaries, just-out-of-range values, ANSI error paths, foldable-only args.

Use query expect_fallback(...) for inputs where the serde returns None. Format: docs/source/contributor-guide/sql-file-tests.md.

6. Update docs

Hand-curated (make does not regenerate these):

• docs/source/user-guide/latest/expressions.md — add | <ExpressionClass> | \<sql_name>` |` to the matching category table, alphabetical.
• docs/source/user-guide/latest/expressions.md — update the function's Status from 🔜/💤 to ✅ (or ⚠️) in the matching category table.

Per-expression compatibility pages under compatibility/expressions/*.md are auto-generated from getCompatibleNotes / getIncompatibleReasons / getUnsupportedReasons — do not hand-edit.

7. Build, audit, finalize

make
cd native && cargo clippy --all-targets --workspace -- -D warnings

Then run audit-comet-expression on $ARGUMENTS to compare against Spark 3.4.3 / 3.5.8 / 4.0.1 and surface coverage gaps; iterate on tests.

The user generally runs tests themselves. If asked for a smoke test:

./mvnw test -Dsuites="org.apache.comet.CometSqlFileTestSuite $ARGUMENTS" -Dtest=none

PR: fill in .github/pull_request_template.md; under "What changes are included", note the skill was used and which upstream function (datafusion::… built-in or datafusion-spark::function::…) is being wired.