ESQL: Test utilities for functions

Adds utilities to generate test cases for `null` arguments and
combinations of arguments that aren't supported. The `null` arguments
cases are asserted to return `null` which is how *most* functions work.
The invalid argument type cominations are asserted to return an error.

This slightly modifies some functions to explicitly return `null` if
given an always-null argument. It's simpler to test this way and easier
to line up. So doing something like `GREATEST(1, 2, null)` will both
return `null` and have the type `null`.

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/operator/EvalOperator.java

@@ -51,5 +51,15 @@ public interface ExpressionEvaluator {
         Block eval(Page page);
     }
 
-    public static final ExpressionEvaluator CONSTANT_NULL = page -> Block.constantNullBlock(page.getPositionCount());
+    public static final ExpressionEvaluator CONSTANT_NULL = new ExpressionEvaluator() {
+        @Override
+        public Block eval(Page page) {
+            return Block.constantNullBlock(page.getPositionCount());
+        }
+
+        @Override
+        public String toString() {
+            return "ConstantNull";
+        }
+    };
 }

x-pack/plugin/esql/qa/testFixtures/src/main/resources/math.csv-spec

@@ -1025,7 +1025,7 @@ a:integer | b:integer | l:integer
 leastNull
 ROW l=LEAST(10, 5, null);
 
-l:integer
+l:null
 null
 ;
 
@@ -1052,7 +1052,7 @@ a:integer | b:integer | g:integer
 greatestNull
 ROW g=GREATEST(10, 5, null);
 
-g:integer
+g:null
 null
 ;
 

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/scalar/conditional/Greatest.java

@@ -60,12 +60,17 @@ protected TypeResolution resolveType() {
         }
 
         for (int position = 0; position < children().size(); position++) {
-            if (dataType == null || dataType == NULL) {
-                dataType = children().get(position).dataType();
+            Expression child = children().get(position);
+            if (child.dataType() == DataTypes.NULL) {
+                dataType = DataTypes.NULL;
+                break;
+            }
+            if (dataType == null) {
+                dataType = child.dataType();
                 continue;
             }
             TypeResolution resolution = TypeResolutions.isType(
-                children().get(position),
+                child,
                 t -> t == dataType,
                 sourceText(),
                 TypeResolutions.ParamOrdinal.fromIndex(position),

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/scalar/conditional/Least.java

@@ -60,12 +60,17 @@ protected TypeResolution resolveType() {
         }
 
         for (int position = 0; position < children().size(); position++) {
-            if (dataType == null || dataType == NULL) {
-                dataType = children().get(position).dataType();
+            Expression child = children().get(position);
+            if (child.dataType() == DataTypes.NULL) {
+                dataType = DataTypes.NULL;
+                break;
+            }
+            if (dataType == null) {
+                dataType = child.dataType();
                 continue;
             }
             TypeResolution resolution = TypeResolutions.isType(
-                children().get(position),
+                child,
                 t -> t == dataType,
                 sourceText(),
                 TypeResolutions.ParamOrdinal.fromIndex(position),

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/scalar/math/Abs.java

@@ -63,6 +63,9 @@ public Supplier<EvalOperator.ExpressionEvaluator> toEvaluator(
         if (dataType() == DataTypes.INTEGER) {
             return () -> new AbsIntEvaluator(field.get());
         }
+        if (dataType() == DataTypes.NULL) {
+            return () -> EvalOperator.CONSTANT_NULL;
+        }
         throw EsqlUnsupportedOperationException.unsupportedDataType(dataType());
     }
 

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/scalar/math/AbstractTrigonometricFunction.java

@@ -35,6 +35,9 @@ abstract class AbstractTrigonometricFunction extends UnaryScalarFunction impleme
     public final Supplier<EvalOperator.ExpressionEvaluator> toEvaluator(
         Function<Expression, Supplier<EvalOperator.ExpressionEvaluator>> toEvaluator
     ) {
+        if (field.dataType() == DataTypes.NULL) {
+            return () -> EvalOperator.CONSTANT_NULL;
+        }
         Supplier<EvalOperator.ExpressionEvaluator> fieldEval = Cast.cast(field().dataType(), DataTypes.DOUBLE, toEvaluator.apply(field()));
         return () -> doubleEvaluator(fieldEval.get());
     }
@@ -55,6 +58,9 @@ protected final TypeResolution resolveType() {
 
     @Override
     public final DataType dataType() {
+        if (field.dataType() == DataTypes.NULL) {
+            return DataTypes.NULL;
+        }
         return DataTypes.DOUBLE;
     }
 }

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/scalar/math/Atan2.java

@@ -57,6 +57,9 @@ static double process(double y, double x) {
 
     @Override
     public DataType dataType() {
+        if (y.dataType() == DataTypes.NULL || x.dataType() == DataTypes.NULL) {
+            return DataTypes.NULL;
+        }
         return DataTypes.DOUBLE;
     }
 
@@ -82,6 +85,9 @@ public boolean foldable() {
     public Supplier<EvalOperator.ExpressionEvaluator> toEvaluator(
         Function<Expression, Supplier<EvalOperator.ExpressionEvaluator>> toEvaluator
     ) {
+        if (y.dataType() == DataTypes.NULL || x.dataType() == DataTypes.NULL) {
+            return () -> EvalOperator.CONSTANT_NULL;
+        }
         Supplier<EvalOperator.ExpressionEvaluator> yEval = Cast.cast(y.dataType(), DataTypes.DOUBLE, toEvaluator.apply(y));
         Supplier<EvalOperator.ExpressionEvaluator> xEval = Cast.cast(x.dataType(), DataTypes.DOUBLE, toEvaluator.apply(x));
         return () -> new Atan2Evaluator(yEval.get(), xEval.get());

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/scalar/math/Round.java

@@ -157,6 +157,9 @@ public Supplier<EvalOperator.ExpressionEvaluator> toEvaluator(
         if (fieldType == DataTypes.UNSIGNED_LONG) {
             return toEvaluator(toEvaluator, Function.identity(), RoundUnsignedLongEvaluator::new);
         }
+        if (fieldType == DataTypes.NULL) {
+            return () -> EvalOperator.CONSTANT_NULL;
+        }
         throw EsqlUnsupportedOperationException.unsupportedDataType(fieldType);
     }