ESQL: Remaining serialization tests (elastic#143470)

This adds the remaining serialization tests. It makes a few production
code changes:
1. Registers `TO_DATE_RANGE` for serialization. It was serializable and
   not registered.
2. Normalizes the serialization of `SCORE` and explains how it used to
   work. It was not broken but held together strangely.
3. Replaces implementing `SurrogateExpression` with
   `OnlySurrogateExpression` in a few more cases so we don't expect
   serialization to work.
4. Adds some package private accessors so we can refer to children by
   name in serialization tests.

Author: nik9000

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/ExpressionWritables.java

@@ -23,6 +23,7 @@
 import org.elasticsearch.xpack.esql.expression.function.scalar.convert.ToCartesianPoint;
 import org.elasticsearch.xpack.esql.expression.function.scalar.convert.ToCartesianShape;
 import org.elasticsearch.xpack.esql.expression.function.scalar.convert.ToDateNanos;
+import org.elasticsearch.xpack.esql.expression.function.scalar.convert.ToDateRange;
 import org.elasticsearch.xpack.esql.expression.function.scalar.convert.ToDatetime;
 import org.elasticsearch.xpack.esql.expression.function.scalar.convert.ToDegrees;
 import org.elasticsearch.xpack.esql.expression.function.scalar.convert.ToDenseVector;
@@ -230,6 +231,7 @@ public static List<NamedWriteableRegistry.Entry> unaryScalars() {
         entries.add(ToCartesianPoint.ENTRY);
         entries.add(ToDatetime.ENTRY);
         entries.add(ToDateNanos.ENTRY);
+        entries.add(ToDateRange.ENTRY);
         entries.add(ToDegrees.ENTRY);
         entries.add(ToDenseVector.ENTRY);
         entries.add(ToDouble.ENTRY);

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/aggregate/PercentileOverTime.java

@@ -13,7 +13,7 @@
 import org.elasticsearch.xpack.esql.core.tree.NodeInfo;
 import org.elasticsearch.xpack.esql.core.tree.Source;
 import org.elasticsearch.xpack.esql.core.type.DataType;
-import org.elasticsearch.xpack.esql.expression.SurrogateExpression;
+import org.elasticsearch.xpack.esql.expression.OnlySurrogateExpression;
 import org.elasticsearch.xpack.esql.expression.function.Example;
 import org.elasticsearch.xpack.esql.expression.function.FunctionAppliesTo;
 import org.elasticsearch.xpack.esql.expression.function.FunctionAppliesToLifecycle;
@@ -27,7 +27,7 @@
 /**
  * Similar to {@link Percentile}, but it is used to calculate the percentile value over a time series of values from the given field.
  */
-public class PercentileOverTime extends TimeSeriesAggregateFunction implements SurrogateExpression, ToAggregator {
+public class PercentileOverTime extends TimeSeriesAggregateFunction implements OnlySurrogateExpression, ToAggregator {
     @FunctionInfo(
         returnType = "double",
         description = "Calculates the percentile over time of a field.",

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/aggregate/StddevOverTime.java

@@ -13,7 +13,7 @@
 import org.elasticsearch.xpack.esql.core.tree.NodeInfo;
 import org.elasticsearch.xpack.esql.core.tree.Source;
 import org.elasticsearch.xpack.esql.core.type.DataType;
-import org.elasticsearch.xpack.esql.expression.SurrogateExpression;
+import org.elasticsearch.xpack.esql.expression.OnlySurrogateExpression;
 import org.elasticsearch.xpack.esql.expression.function.Example;
 import org.elasticsearch.xpack.esql.expression.function.FunctionAppliesTo;
 import org.elasticsearch.xpack.esql.expression.function.FunctionAppliesToLifecycle;
@@ -30,7 +30,7 @@
 /**
  * Similar to {@link StdDev}, but it is used to calculate the standard deviation over a time series of values from the given field.
  */
-public class StddevOverTime extends TimeSeriesAggregateFunction implements SurrogateExpression, ToAggregator {
+public class StddevOverTime extends TimeSeriesAggregateFunction implements OnlySurrogateExpression, ToAggregator {
     @FunctionInfo(
         returnType = "double",
         description = "Calculates the population standard deviation over time of a numeric field.",

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/aggregate/VarianceOverTime.java

@@ -13,7 +13,7 @@
 import org.elasticsearch.xpack.esql.core.tree.NodeInfo;
 import org.elasticsearch.xpack.esql.core.tree.Source;
 import org.elasticsearch.xpack.esql.core.type.DataType;
-import org.elasticsearch.xpack.esql.expression.SurrogateExpression;
+import org.elasticsearch.xpack.esql.expression.OnlySurrogateExpression;
 import org.elasticsearch.xpack.esql.expression.function.Example;
 import org.elasticsearch.xpack.esql.expression.function.FunctionAppliesTo;
 import org.elasticsearch.xpack.esql.expression.function.FunctionAppliesToLifecycle;
@@ -30,7 +30,7 @@
 /**
  * Similar to {@link Variance}, but it is used to calculate the variance over a time series of values from the given field.
  */
-public class VarianceOverTime extends TimeSeriesAggregateFunction implements SurrogateExpression, ToAggregator {
+public class VarianceOverTime extends TimeSeriesAggregateFunction implements OnlySurrogateExpression, ToAggregator {
     @FunctionInfo(
         returnType = "double",
         description = "Calculates the population variance over time of a numeric field.",

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/fulltext/Score.java

@@ -59,11 +59,7 @@ public Score(
             description = "Boolean expression that contains full text function(s) to be scored."
         ) Expression scorableQuery
     ) {
-        this(source, List.of(scorableQuery));
-    }
-
-    protected Score(Source source, List<Expression> children) {
-        super(source, children);
+        super(source, List.of(scorableQuery));
     }
 
     @Override
@@ -73,7 +69,7 @@ public DataType dataType() {
 
     @Override
     public Expression replaceChildren(List<Expression> newChildren) {
-        return new Score(source(), newChildren);
+        return new Score(source(), newChildren.getFirst());
     }
 
     @Override
@@ -95,12 +91,26 @@ public EvalOperator.ExpressionEvaluator.Factory toEvaluator(EvaluatorMapper.ToEv
     @Override
     public void writeTo(StreamOutput out) throws IOException {
         source().writeTo(out);
-        out.writeNamedWriteableCollection(this.children());
+        out.writeOptionalNamedWriteable(children().getFirst());
     }
 
     private static Expression readFrom(StreamInput in) throws IOException {
         Source source = Source.readFrom((PlanStreamInput) in);
+        /*
+         * This is not truly optional. But when the SCORE scalar was originally
+         * created we serialized it with this pair:
+         *
+         * in.readOptionalNamedWriteable(Expression.class);
+         * out.writeNamedWriteableCollection(this.children());
+         *
+         * This pair *is* compatible if we send a single element list. Single
+         * element lists are serialized as `0x01 <name> <body>`. That's also
+         * how optional named writeables are serialized.
+         */
         Expression query = in.readOptionalNamedWriteable(Expression.class);
+        if (query == null) {
+            throw new IllegalStateException("query isn't really optional");
+        }
         return new Score(source, query);
     }
 

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

@@ -14,7 +14,7 @@
 import org.elasticsearch.xpack.esql.core.tree.NodeInfo;
 import org.elasticsearch.xpack.esql.core.tree.Source;
 import org.elasticsearch.xpack.esql.core.type.DataType;
-import org.elasticsearch.xpack.esql.expression.SurrogateExpression;
+import org.elasticsearch.xpack.esql.expression.OnlySurrogateExpression;
 import org.elasticsearch.xpack.esql.expression.function.Example;
 import org.elasticsearch.xpack.esql.expression.function.FunctionInfo;
 import org.elasticsearch.xpack.esql.expression.function.OptionalArgument;
@@ -43,7 +43,7 @@
  * </ul>
  */
 
-public class ToIntegerSurrogate extends EsqlScalarFunction implements SurrogateExpression, OptionalArgument, ConvertFunction {
+public class ToIntegerSurrogate extends EsqlScalarFunction implements OnlySurrogateExpression, OptionalArgument, ConvertFunction {
 
     private final Expression field;
     private final Expression base;

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

@@ -16,7 +16,7 @@
 import org.elasticsearch.xpack.esql.core.tree.NodeInfo;
 import org.elasticsearch.xpack.esql.core.tree.Source;
 import org.elasticsearch.xpack.esql.core.type.DataType;
-import org.elasticsearch.xpack.esql.expression.SurrogateExpression;
+import org.elasticsearch.xpack.esql.expression.OnlySurrogateExpression;
 import org.elasticsearch.xpack.esql.expression.function.Example;
 import org.elasticsearch.xpack.esql.expression.function.FunctionInfo;
 import org.elasticsearch.xpack.esql.expression.function.MapParam;
@@ -62,7 +62,7 @@
  *     expose a single method to users.
  * </p>
  */
-public class ToIp extends EsqlScalarFunction implements SurrogateExpression, OptionalArgument, ConvertFunction {
+public class ToIp extends EsqlScalarFunction implements OnlySurrogateExpression, OptionalArgument, ConvertFunction {
     private static final String LEADING_ZEROS = "leading_zeros";
     public static final Map<String, DataType> ALLOWED_OPTIONS = Map.ofEntries(Map.entry(LEADING_ZEROS, KEYWORD));
 

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

@@ -14,7 +14,7 @@
 import org.elasticsearch.xpack.esql.core.tree.NodeInfo;
 import org.elasticsearch.xpack.esql.core.tree.Source;
 import org.elasticsearch.xpack.esql.core.type.DataType;
-import org.elasticsearch.xpack.esql.expression.SurrogateExpression;
+import org.elasticsearch.xpack.esql.expression.OnlySurrogateExpression;
 import org.elasticsearch.xpack.esql.expression.function.Example;
 import org.elasticsearch.xpack.esql.expression.function.FunctionInfo;
 import org.elasticsearch.xpack.esql.expression.function.OptionalArgument;
@@ -46,7 +46,7 @@
  * </ul>
  */
 
-public class ToLongSurrogate extends EsqlScalarFunction implements SurrogateExpression, OptionalArgument, ConvertFunction {
+public class ToLongSurrogate extends EsqlScalarFunction implements OnlySurrogateExpression, OptionalArgument, ConvertFunction {
 
     private final Expression field;
     private final Expression base;

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

@@ -210,6 +210,22 @@ public String getWriteableName() {
         return ENTRY.name;
     }
 
+    Expression value() {
+        return value;
+    }
+
+    Expression origin() {
+        return origin;
+    }
+
+    Expression scale() {
+        return scale;
+    }
+
+    Expression options() {
+        return options;
+    }
+
     @Override
     protected TypeResolution resolveType() {
         if (childrenResolved() == false) {