Support optional 'path' config type

Initial work needed for this to work (config definition and config library)

21 files changed, 285 insertions, 17 deletions

diff --git a/config-lib/abi-spec.json b/config-lib/abi-spec.json
index caae2ebf1cf..07e61c63237 100644
--- a/config-lib/abi-spec.json
+++ b/config-lib/abi-spec.json
@@ -273,6 +273,7 @@
       "public static java.util.Map asNodeMap(java.util.Map, com.yahoo.config.LeafNode)",
       "public static java.util.Map asFileNodeMap(java.util.Map)",
       "public static java.util.Map asPathNodeMap(java.util.Map)",
+      "public static java.util.Map asOptionalPathNodeMap(java.util.Map)",
       "public static java.util.Map asUrlNodeMap(java.util.Map)",
       "public static java.util.Map asModelNodeMap(java.util.Map)"
     ],
@@ -289,6 +290,7 @@
       "public java.util.List asList()",
       "public static com.yahoo.config.LeafNodeVector createFileNodeVector(java.util.Collection)",
       "public static com.yahoo.config.LeafNodeVector createPathNodeVector(java.util.Collection)",
+      "public static com.yahoo.config.LeafNodeVector createOptionalPathNodeVector(java.util.Collection)",
       "public static com.yahoo.config.LeafNodeVector createUrlNodeVector(java.util.Collection)",
       "public static com.yahoo.config.LeafNodeVector createModelNodeVector(java.util.Collection)"
     ],
@@ -420,6 +422,27 @@
       "protected final java.util.ArrayList vector"
     ]
   },
+  "com.yahoo.config.OptionalPathNode" : {
+    "superClass" : "com.yahoo.config.LeafNode",
+    "interfaces" : [ ],
+    "attributes" : [
+      "public"
+    ],
+    "methods" : [
+      "public void <init>()",
+      "public void <init>(com.yahoo.config.FileReference)",
+      "public void <init>(java.util.Optional)",
+      "public java.util.Optional value()",
+      "public java.lang.String getValue()",
+      "public java.lang.String toString()",
+      "protected boolean doSetValue(java.lang.String)",
+      "public java.util.Optional getFileReference()",
+      "public static java.util.List toFileReferences(java.util.List)",
+      "public static java.util.Map toFileReferenceMap(java.util.Map)",
+      "public bridge synthetic java.lang.Object value()"
+    ],
+    "fields" : [ ]
+  },
   "com.yahoo.config.PathNode" : {
     "superClass" : "com.yahoo.config.LeafNode",
     "interfaces" : [ ],
diff --git a/config-lib/src/main/java/com/yahoo/config/LeafNodeMaps.java b/config-lib/src/main/java/com/yahoo/config/LeafNodeMaps.java
index 82663fa8bfd..214d8c52caa 100644
--- a/config-lib/src/main/java/com/yahoo/config/LeafNodeMaps.java
+++ b/config-lib/src/main/java/com/yahoo/config/LeafNodeMaps.java
@@ -4,6 +4,7 @@ package com.yahoo.config;
 import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.Map;
+import java.util.Optional;
 import java.util.stream.Collectors;
 
 /**
@@ -60,6 +61,14 @@ public class LeafNodeMaps {
         return Collections.unmodifiableMap(pathNodeMap);
     }
 
+    public static Map<String, OptionalPathNode> asOptionalPathNodeMap(Map<String, Optional<FileReference>> fileReferenceMap) {
+        Map<String, OptionalPathNode> pathNodeMap = new LinkedHashMap<>();
+        for (Map.Entry<String, Optional<FileReference>> e : fileReferenceMap.entrySet()) {
+            pathNodeMap.put(e.getKey(), new OptionalPathNode(e.getValue()));
+        }
+        return Collections.unmodifiableMap(pathNodeMap);
+    }
+
     public static Map<String, UrlNode> asUrlNodeMap(Map<String, UrlReference> urlReferenceMap) {
         return Collections.unmodifiableMap(
                     urlReferenceMap.entrySet().stream().collect(
diff --git a/config-lib/src/main/java/com/yahoo/config/LeafNodeVector.java b/config-lib/src/main/java/com/yahoo/config/LeafNodeVector.java
index a4fea95088d..cfb8cd4eebd 100644
--- a/config-lib/src/main/java/com/yahoo/config/LeafNodeVector.java
+++ b/config-lib/src/main/java/com/yahoo/config/LeafNodeVector.java
@@ -8,6 +8,7 @@ import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
+import java.util.Optional;
 
 /**
  * A vector of leaf nodes.
@@ -71,6 +72,13 @@ public class LeafNodeVector<REAL, NODE extends LeafNode<REAL>> extends NodeVecto
         return new LeafNodeVector<>(paths, new PathNode());
     }
 
+    public static LeafNodeVector<Optional<Path>, OptionalPathNode> createOptionalPathNodeVector(Collection<Optional<FileReference>> values) {
+        List<Optional<Path>> paths = new ArrayList<>();
+        for (Optional<FileReference> fileReference : values)
+            paths.add(fileReference.map(reference -> Paths.get(reference.value())));
+        return new LeafNodeVector<>(paths, new OptionalPathNode());
+    }
+
     public static LeafNodeVector<File, UrlNode> createUrlNodeVector(Collection<UrlReference> values) {
         List<File> files = new ArrayList<>();
         for (UrlReference urlReference : values)
diff --git a/config-lib/src/main/java/com/yahoo/config/OptionalPathNode.java b/config-lib/src/main/java/com/yahoo/config/OptionalPathNode.java
new file mode 100644
index 00000000000..8a6414d798f
--- /dev/null
+++ b/config-lib/src/main/java/com/yahoo/config/OptionalPathNode.java
@@ -0,0 +1,84 @@
+// Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+package com.yahoo.config;
+
+import java.nio.file.Path;
+import java.util.ArrayList;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+
+/**
+ * Represents a 'path' in a {@link ConfigInstance}, usually a filename, can be optional
+ *
+ * @author hmusum
+ */
+public class OptionalPathNode extends LeafNode<Optional<Path>> {
+
+    private final Optional<FileReference> fileReference;
+
+    public OptionalPathNode() {
+        fileReference = Optional.empty();
+    }
+
+    public OptionalPathNode(FileReference fileReference) {
+        super(true);
+        this.value = Optional.of(Path.of(fileReference.value()));
+        this.fileReference = Optional.of(fileReference);
+    }
+
+    public OptionalPathNode(Optional<FileReference> fileReference) {
+        super(true);
+        this.value = fileReference.map(reference -> Path.of(reference.value()));
+        this.fileReference = fileReference;
+    }
+
+    public Optional<Path> value() {
+        return value;
+    }
+
+    @Override
+    public String getValue() {
+        return value.toString();
+    }
+
+    @Override
+    public String toString() {
+        return (value.isEmpty()) ? "(empty)" : '"' + value.get().toString() + '"';
+    }
+
+    @Override
+    protected boolean doSetValue(String stringVal) {
+        throw new UnsupportedOperationException("doSetValue should not be necessary anymore!");
+    }
+
+    @Override
+    void serialize(String name, Serializer serializer) {
+        value.ifPresent(path -> serializer.serialize(name, path.toString()));
+    }
+
+    @Override
+    void serialize(Serializer serializer) {
+        value.ifPresent(path -> serializer.serialize(path.toString()));
+    }
+
+    public Optional<FileReference> getFileReference() {
+        return fileReference;
+    }
+
+    public static List<Optional<FileReference>> toFileReferences(List<OptionalPathNode> pathNodes) {
+        List<Optional<FileReference>> fileReferences = new ArrayList<>();
+        for (OptionalPathNode pathNode : pathNodes)
+            fileReferences.add(pathNode.getFileReference());
+        return fileReferences;
+    }
+
+    public static Map<String, Optional<FileReference>> toFileReferenceMap(Map<String, OptionalPathNode> map) {
+        Map<String, Optional<FileReference>> ret = new LinkedHashMap<>();
+        for (Map.Entry<String, OptionalPathNode> e : map.entrySet()) {
+            ret.put(e.getKey(), e.getValue().getFileReference());
+        }
+        return ret;
+    }
+
+}
diff --git a/config/src/main/java/com/yahoo/vespa/config/ConfigPayloadApplier.java b/config/src/main/java/com/yahoo/vespa/config/ConfigPayloadApplier.java
index e86b13b2c98..0dbc40a246c 100644
--- a/config/src/main/java/com/yahoo/vespa/config/ConfigPayloadApplier.java
+++ b/config/src/main/java/com/yahoo/vespa/config/ConfigPayloadApplier.java
@@ -21,6 +21,7 @@ import java.util.Deque;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
+import java.util.Optional;
 import java.util.Set;
 import java.util.logging.Logger;
 
@@ -213,7 +214,10 @@ public class ConfigPayloadApplier<T extends ConfigInstance.Builder> {
         Inspector value = (Inspector)rawValue;
         if (isPathField(builder, methodName))
             return resolvePath(value.asString());
-        else if (isUrlField(builder, methodName))
+        if (isOptionalPathField(builder, methodName)) {
+            String v = value.asString();
+            return resolvePath(v.isEmpty() ? Optional.empty() : Optional.of(v));
+        } else if (isUrlField(builder, methodName))
             return value.asString().isEmpty() ? "" : resolveUrl(value.asString());
         else if (isModelField(builder, methodName))
             return value.asString().isEmpty() ? "" : resolveModel(value.asString());
@@ -234,6 +238,10 @@ public class ConfigPayloadApplier<T extends ConfigInstance.Builder> {
         return new FileReference(path.toString());
     }
 
+    private Optional<FileReference> resolvePath(Optional<String> value) {
+        return value.isEmpty() ? Optional.empty() : Optional.of(resolvePath(value.get()));
+    }
+
     private UrlReference resolveUrl(String url) {
         if ( ! isClientside()) return new UrlReference(url);
         File file = urlDownloader.waitFor(new UrlReference(url), 60 * 60);
@@ -319,6 +327,16 @@ public class ConfigPayloadApplier<T extends ConfigInstance.Builder> {
         return isFieldType(pathFieldSet, builder, methodName, FileReference.class);
     }
 
+    /**
+     * Checks if this field is of type 'path', in which
+     * case some special handling might be needed. Caches the result.
+     */
+    private final Set<String> optionalPathFieldSet = new HashSet<>();
+    private boolean isOptionalPathField(Object builder, String methodName) {
+        // Paths are stored as Optional<FileReference> in Builder.
+        return isFieldType(optionalPathFieldSet, builder, methodName, Optional.class);
+    }
+
     private final Set<String> urlFieldSet = new HashSet<>();
     private boolean isUrlField(Object builder, String methodName) {
         // Urls are stored as UrlReference in Builder.
diff --git a/config/src/test/java/com/yahoo/config/subscription/CfgConfigPayloadBuilderTest.java b/config/src/test/java/com/yahoo/config/subscription/CfgConfigPayloadBuilderTest.java
index a6273ad5ccb..0e50be83e7a 100644
--- a/config/src/test/java/com/yahoo/config/subscription/CfgConfigPayloadBuilderTest.java
+++ b/config/src/test/java/com/yahoo/config/subscription/CfgConfigPayloadBuilderTest.java
@@ -34,6 +34,7 @@ public class CfgConfigPayloadBuilderTest {
                 "        'parent:'",
                 "    ],",
                 "    'pathVal': 'src/test/resources/configs/def-files/function-test.def',",
+                "    'optionalPathVal': 'src/test/resources/configs/def-files/function-test.def',",
                 "    'string_val': 'foo',",
                 "    'myStructMap': {",
                 "        'one': {",
diff --git a/config/src/test/java/com/yahoo/config/subscription/ConfigInstancePayloadTest.java b/config/src/test/java/com/yahoo/config/subscription/ConfigInstancePayloadTest.java
index c656bfe1a60..f09462eb634 100644
--- a/config/src/test/java/com/yahoo/config/subscription/ConfigInstancePayloadTest.java
+++ b/config/src/test/java/com/yahoo/config/subscription/ConfigInstancePayloadTest.java
@@ -13,6 +13,7 @@ import org.junit.Test;
 import java.io.File;
 import java.util.Arrays;
 import java.util.List;
+import java.util.Optional;
 
 import static com.yahoo.foo.FunctionTestConfig.*;
 import static org.junit.Assert.assertNotNull;
@@ -46,6 +47,7 @@ public class ConfigInstancePayloadTest {
                 refwithdef(":parent:").
                 fileVal("etc").
                 pathVal(FileReference.mockFileReferenceForUnitTesting(new File("src/test/resources/configs/def-files/function-test.def"))).
+                optionalPathVal(Optional.of(FileReference.mockFileReferenceForUnitTesting(new File("src/test/resources/configs/def-files/function-test.def")))).
                 boolarr(false).
                 longarr(9223372036854775807L).
                 longarr(-9223372036854775808L).
diff --git a/config/src/test/java/com/yahoo/config/subscription/FunctionTest.java b/config/src/test/java/com/yahoo/config/subscription/FunctionTest.java
index 8656c0e945f..7a3b0e437f2 100644
--- a/config/src/test/java/com/yahoo/config/subscription/FunctionTest.java
+++ b/config/src/test/java/com/yahoo/config/subscription/FunctionTest.java
@@ -30,7 +30,7 @@ public class FunctionTest {
     public static final String PATH = "src/test/resources/configs/function-test/";
 
     private FunctionTestConfig config;
-    private ConfigSourceSet sourceSet = new ConfigSourceSet("function-test");
+    private final ConfigSourceSet sourceSet = new ConfigSourceSet("function-test");
 
     public void configure(FunctionTestConfig config, ConfigSourceSet sourceSet) {
         this.config = config;
@@ -222,6 +222,8 @@ public class FunctionTest {
         assertEquals(":parent", config.refarr(1));
         assertEquals("parent:", config.refarr(2));
         assertEquals("bin", config.fileArr(0).value());
+        assertEquals("function-test.def", config.pathVal().toFile().getName());
+        assertEquals("function-test.def", config.optionalPathVal().get().toFile().getName());  // TODO
         assertEquals("pom.xml", config.pathArr(0).toString());
         assertEquals("pom.xml", config.pathMap("one").toString());
 
diff --git a/config/src/test/java/com/yahoo/vespa/config/ConfigDefinitionBuilderTest.java b/config/src/test/java/com/yahoo/vespa/config/ConfigDefinitionBuilderTest.java
index a9f09951d7e..fa1ec815047 100644
--- a/config/src/test/java/com/yahoo/vespa/config/ConfigDefinitionBuilderTest.java
+++ b/config/src/test/java/com/yahoo/vespa/config/ConfigDefinitionBuilderTest.java
@@ -77,10 +77,14 @@ public class ConfigDefinitionBuilderTest {
         assertThat(def.getFileDefs().size(), is(1));
         assertNotNull(def.getFileDefs().get("fileVal"));
 
-        assertThat(def.getArrayDefs().size(), is(9));
+        // An array does not have to have any elements set
+        assertThat(def.getArrayDefs().size(), is(10));
         assertNotNull(def.getArrayDefs().get("boolarr"));
         assertThat(def.getArrayDefs().get("boolarr").getTypeSpec().getType(), is("bool"));
 
+        assertNotNull(def.getArrayDefs().get("boolarrEmpty"));
+        assertThat(def.getArrayDefs().get("boolarrEmpty").getTypeSpec().getType(), is("bool"));
+
         assertNotNull(def.getArrayDefs().get("enumarr"));
         assertThat(def.getArrayDefs().get("enumarr").getTypeSpec().getType(), is("enum"));
         assertThat(def.getArrayDefs().get("enumarr").getTypeSpec().getEnumVals().toString(), is("[ARRAY, VALUES]"));
diff --git a/config/src/test/resources/configs/def-files/function-test.def b/config/src/test/resources/configs/def-files/function-test.def
index 4c4cb6bf08b..b97713b18f3 100644
--- a/config/src/test/resources/configs/def-files/function-test.def
+++ b/config/src/test/resources/configs/def-files/function-test.def
@@ -42,8 +42,10 @@ refval reference
 refwithdef reference default=":parent:"
 fileVal file
 pathVal path
+optionalPathVal path optional
 
 boolarr[] bool
+boolarrEmpty[] bool
 intarr[] int
 longarr[] long
 doublearr[] double
diff --git a/config/src/test/resources/configs/function-test/variableaccess.txt b/config/src/test/resources/configs/function-test/variableaccess.txt
index 997de21750d..8c2cadcdbbc 100644
--- a/config/src/test/resources/configs/function-test/variableaccess.txt
+++ b/config/src/test/resources/configs/function-test/variableaccess.txt
@@ -14,7 +14,8 @@ enumwithdef BAR2
 refval :parent:
 refwithdef ":parent:"
 fileVal "etc"
-pathVal "pom.xml"
+pathVal "function-test.def"
+optionalPathVal "function-test.def"
 boolarr[1]
 boolarr[0] false
 intarr[0]
diff --git a/configgen/src/main/java/com/yahoo/config/codegen/BuilderGenerator.java b/configgen/src/main/java/com/yahoo/config/codegen/BuilderGenerator.java
index 6cd344466e4..12469d7a3ef 100644
--- a/configgen/src/main/java/com/yahoo/config/codegen/BuilderGenerator.java
+++ b/configgen/src/main/java/com/yahoo/config/codegen/BuilderGenerator.java
@@ -4,10 +4,12 @@ package com.yahoo.config.codegen;
 import com.yahoo.config.codegen.LeafCNode.FileLeaf;
 import com.yahoo.config.codegen.LeafCNode.ModelLeaf;
 import com.yahoo.config.codegen.LeafCNode.PathLeaf;
+import com.yahoo.config.codegen.LeafCNode.OptionalPathLeaf;
 import com.yahoo.config.codegen.LeafCNode.UrlLeaf;
 
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Set;
 import java.util.stream.Collectors;
 
 import static com.yahoo.config.codegen.ConfigGenerator.boxedDataType;
@@ -89,7 +91,9 @@ public class BuilderGenerator {
     private static String getUninitializedScalars(InnerCNode node) {
         List<String> scalarsWithoutDefault = new ArrayList<>();
         for (CNode child : node.getChildren()) {
-            if (child instanceof LeafCNode && (!child.isArray && !child.isMap && ((LeafCNode) child).getDefaultValue() == null)) {
+            if (child instanceof LeafCNode
+                    && (!child.isArray && !child.isMap && ((LeafCNode) child).getDefaultValue() == null)
+                    && (! (child instanceof OptionalPathLeaf))) {
                 scalarsWithoutDefault.add("\"" + child.getName() + "\"");
             }
         }
@@ -109,7 +113,11 @@ public class BuilderGenerator {
         } else if (node instanceof InnerCNode) {
             return String.format("public %s %s = new %s();", builderType(node), node.getName(), builderType(node));
         } else if (node instanceof LeafCNode) {
-            return String.format("private %s %s = null;", boxedBuilderType((LeafCNode) node), node.getName());
+            String boxedBuilderType = boxedBuilderType((LeafCNode) node);
+            if (boxedBuilderType.startsWith("Optional<"))
+                return String.format("private %s %s = Optional.empty();", boxedBuilderType, node.getName());
+            else
+                return String.format("private %s %s = null;", boxedBuilderType, node.getName());
         } else {
             throw new IllegalStateException("Cannot produce builder field definition for node"); // Should not happen
         }
@@ -207,6 +215,11 @@ public class BuilderGenerator {
         private static String privateLeafNodeSetter(LeafCNode n) {
             if ("String".equals(builderType(n)) || "FileReference".equals(builderType(n))) {
                 return "";
+            } else if ("Optional<FileReference>".equals(builderType(n))) {
+                return "\n\n" + //
+                        "private Builder " + n.getName() + "(String " + INTERNAL_PREFIX + "value) {\n" + //
+                        "  return " + n.getName() + "(" + builderType(n) + ".of(" + INTERNAL_PREFIX + "value));\n" + //
+                        "}";
             } else {
                 return "\n\n" + //
                         "private Builder " + n.getName() + "(String " + INTERNAL_PREFIX + "value) {\n" + //
@@ -270,14 +283,24 @@ public class BuilderGenerator {
                     : "";
 
             String bType = builderType(n);
-            String stringSetter = "";
-            if ( ! "String".equals(bType) &&  ! "FileReference".equals(bType) && ! "ModelReference".equals(bType)) {
+            String privateSetter = "";
+            if ( ! Set.of("String", "FileReference", "ModelReference", "Optional<FileReference>").contains(bType)) {
                 String type = boxedDataType(n);
                 if ("UrlReference".equals(bType))
                     type = bType;
-                stringSetter = String.format("\nprivate Builder %s(String %svalue) {\n" +
-                        "  return %s(%s.valueOf(%svalue));\n" + //
-                        "}", name, INTERNAL_PREFIX, name, type, INTERNAL_PREFIX);
+                //
+                privateSetter = String.format("""
+
+                                                      private Builder %s(String %svalue) {
+                                                        return %s(%s.valueOf(%svalue));
+                                                      }""", name, INTERNAL_PREFIX, name, type, INTERNAL_PREFIX);
+            } else if ("Optional<FileReference>".equals(bType)) {
+                //
+                privateSetter = String.format("""
+
+                                                      private Builder %s(FileReference %svalue) {
+                                                        return %s(Optional.of(%svalue));
+                                                      }""", name, INTERNAL_PREFIX, name, INTERNAL_PREFIX);
             }
 
             String getNullGuard = bType.equals(boxedBuilderType(n)) ? String.format(
@@ -286,7 +309,7 @@ public class BuilderGenerator {
             return String.format("public Builder %s(%s %svalue) {%s\n" +
                     "  %s = %svalue;\n" + //
                     "%s", name, bType, INTERNAL_PREFIX, getNullGuard, name, INTERNAL_PREFIX, signalInitialized) +
-                    "  return this;" + "\n}\n" + stringSetter;
+                    "  return this;" + "\n}\n" + privateSetter;
         }
     }
 
@@ -307,6 +330,8 @@ public class BuilderGenerator {
             return name + "(" + nodeClass(child) + ".toFileReferenceMap(config." + name + "));";
         } else if (child instanceof PathLeaf) {
             return name + "(config." + name + ".getFileReference());";
+        } else if (child instanceof OptionalPathLeaf) {
+            return name + "(config." + name + ".getFileReference());";
         } else if (child instanceof UrlLeaf && isArray) {
             return name + "(" + nodeClass(child) + ".toUrlReferences(config." + name + "));";
         } else if (child instanceof UrlLeaf && isMap) {
@@ -408,6 +433,8 @@ public class BuilderGenerator {
             return "String";
         } else if (node instanceof PathLeaf) {
             return "FileReference";
+        } else if (node instanceof OptionalPathLeaf) {
+            return "Optional<FileReference>";
         } else if (node instanceof UrlLeaf) {
             return "UrlReference";
         } else if (node instanceof ModelLeaf) {
@@ -424,6 +451,8 @@ public class BuilderGenerator {
             return "String";
         } else if (node instanceof PathLeaf) {
             return "FileReference";
+        } else if (node instanceof OptionalPathLeaf) {
+            return "Optional<FileReference>";
         } else if (node instanceof UrlLeaf) {
             return "UrlReference";
         } else if (node instanceof ModelLeaf) {
diff --git a/configgen/src/main/java/com/yahoo/config/codegen/ConfigGenerator.java b/configgen/src/main/java/com/yahoo/config/codegen/ConfigGenerator.java
index cb10ffdc2be..903d8dc0865 100644
--- a/configgen/src/main/java/com/yahoo/config/codegen/ConfigGenerator.java
+++ b/configgen/src/main/java/com/yahoo/config/codegen/ConfigGenerator.java
@@ -7,6 +7,7 @@ import com.yahoo.config.codegen.LeafCNode.EnumLeaf;
 import com.yahoo.config.codegen.LeafCNode.FileLeaf;
 import com.yahoo.config.codegen.LeafCNode.IntegerLeaf;
 import com.yahoo.config.codegen.LeafCNode.LongLeaf;
+import com.yahoo.config.codegen.LeafCNode.OptionalPathLeaf;
 import com.yahoo.config.codegen.LeafCNode.PathLeaf;
 import com.yahoo.config.codegen.LeafCNode.ReferenceLeaf;
 import com.yahoo.config.codegen.LeafCNode.StringLeaf;
@@ -165,6 +166,8 @@ public class ConfigGenerator {
             return name + " = LeafNodeVector.createFileNodeVector(builder." + name + ");";
         } else if (child instanceof PathLeaf && isArray) {
             return name + " = LeafNodeVector.createPathNodeVector(builder." + name + ");";
+        } else if (child instanceof OptionalPathLeaf && isArray) {
+            return name + " = LeafNodeVector.createOptionalPathNodeVector(builder." + name + ");";
         } else if (child instanceof UrlLeaf && isArray) {
             return name + " = LeafNodeVector.createUrlNodeVector(builder." + name + ");";
         } else if (child instanceof ModelLeaf && isArray) {
@@ -175,6 +178,8 @@ public class ConfigGenerator {
             return name + " = LeafNodeMaps.asFileNodeMap(builder." + name + ");";
         } else if (child instanceof PathLeaf && isMap) {
             return name + " = LeafNodeMaps.asPathNodeMap(builder." + name + ");";
+        } else if (child instanceof OptionalPathLeaf && isMap) {
+            return name + " = LeafNodeMaps.asOptionalPathNodeMap(builder." + name + ");";
         } else if (child instanceof UrlLeaf && isMap) {
             return name + " = LeafNodeMaps.asUrlNodeMap(builder." + name + ");";
         } else if (child instanceof ModelLeaf && isMap) {
@@ -401,6 +406,8 @@ public class ConfigGenerator {
             return "FileNode";
         } else if (node instanceof PathLeaf) {
             return "PathNode";
+        } else if (node instanceof OptionalPathLeaf) {
+            return "OptionalPathNode";
         } else if (node instanceof UrlLeaf) {
             return "UrlNode";
         } else if (node instanceof ModelLeaf) {
@@ -431,6 +438,8 @@ public class ConfigGenerator {
             return "FileReference";
         } else if (node instanceof PathLeaf) {
             return "Path";
+        } else if (node instanceof OptionalPathLeaf) {
+            return "Optional<Path>";
         } else if (node instanceof UrlLeaf) {
             return "File";
         } else if (node instanceof ModelLeaf) {
@@ -456,6 +465,8 @@ public class ConfigGenerator {
             return "Integer";
         } else if (rawType.toLowerCase().equals(rawType)) {
             return ConfiggenUtil.capitalize(rawType);
+        } else if (rawType.startsWith("Optional<")) {
+            return "Optional";
         } else {
             return rawType;
         }
diff --git a/configgen/src/main/java/com/yahoo/config/codegen/DefLine.java b/configgen/src/main/java/com/yahoo/config/codegen/DefLine.java
index 385c7f1979e..d6bffe349a8 100644
--- a/configgen/src/main/java/com/yahoo/config/codegen/DefLine.java
+++ b/configgen/src/main/java/com/yahoo/config/codegen/DefLine.java
@@ -7,6 +7,7 @@ import java.util.regex.Pattern;
 public class DefLine {
 
     private final static Pattern defaultPattern =     Pattern.compile("^\\s*default\\s*=\\s*(\\S+)");
+    private final static Pattern optionalPattern =    Pattern.compile("^\\s*optional\\s*");
     private final static Pattern rangePattern =       Pattern.compile("^\\s*range\\s*=\\s*([\\(\\[].*?[\\)\\]])");
     private final static Pattern restartPattern =     Pattern.compile("^\\s*restart\\s*");
     private final static Pattern wordPattern =        Pattern.compile("\\S+");
@@ -21,6 +22,7 @@ public class DefLine {
     private final Type type = new Type();
 
     private DefaultValue defaultValue = null;
+    private boolean optional = false;
 
     private String range = null;
     private boolean restart = false;
@@ -74,6 +76,9 @@ public class DefLine {
     }
 
     public Type getType() {
+        if (optional && type.name.equals("path"))
+            type.name = "optionalPath";
+
         return type;
     }
 
@@ -89,6 +94,8 @@ public class DefLine {
         return enumArray;
     }
 
+    public boolean isOptional() { return optional; }
+
     /**
      * Special function that searches through s and returns the index
      * of the first occurrence of " that is not escaped.
@@ -114,6 +121,7 @@ public class DefLine {
     private int parseOptions(CharSequence string) {
         Matcher defaultNullMatcher = defaultNullPattern.matcher(string);
         Matcher defaultMatcher = defaultPattern.matcher(string);
+        Matcher optionalMatcher = optionalPattern.matcher(string);
         Matcher rangeMatcher = rangePattern.matcher(string);
         Matcher restartMatcher = restartPattern.matcher(string);
 
@@ -133,6 +141,12 @@ public class DefLine {
                 defaultValue = new DefaultValue(deflt, type);
             }
             return defaultMatcher.end();
+        } else if (optionalMatcher.find()) {
+            if ( ! type.name.equals("path"))
+                throw new IllegalArgumentException("optional can only be used for 'path'");
+            optional = true;
+            type.name = "optionalPath";
+            return optionalMatcher.end();
         } else if (rangeMatcher.find()) {
             range = rangeMatcher.group(1);
             return rangeMatcher.end();
diff --git a/configgen/src/main/java/com/yahoo/config/codegen/LeafCNode.java b/configgen/src/main/java/com/yahoo/config/codegen/LeafCNode.java
index afd6acfbabf..c2470b0c703 100644
--- a/configgen/src/main/java/com/yahoo/config/codegen/LeafCNode.java
+++ b/configgen/src/main/java/com/yahoo/config/codegen/LeafCNode.java
@@ -26,6 +26,7 @@ public abstract class LeafCNode extends CNode {
                 case "reference" -> new ReferenceLeaf(parent, name);
                 case "file" -> new FileLeaf(parent, name);
                 case "path" -> new PathLeaf(parent, name);
+                case "optionalPath" -> new OptionalPathLeaf(parent, name);
                 case "enum" -> new EnumLeaf(parent, name, type.enumArray);
                 case "url" -> new UrlLeaf(parent, name);
                 case "model" -> new ModelLeaf(parent, name);
@@ -217,6 +218,17 @@ public abstract class LeafCNode extends CNode {
         }
     }
 
+    public static class OptionalPathLeaf extends NoClassLeafCNode {
+        OptionalPathLeaf(InnerCNode parent, String name) {
+            super(parent, name);
+        }
+
+        @Override
+        public String getType() {
+            return "optionalPath";
+        }
+    }
+
     public static class UrlLeaf extends NoClassLeafCNode {
         UrlLeaf(InnerCNode parent, String name) {
             super(parent, name);
diff --git a/configgen/src/test/java/com/yahoo/config/codegen/DefLineParsingTest.java b/configgen/src/test/java/com/yahoo/config/codegen/DefLineParsingTest.java
index 0e2f6cc4d05..bb6b8eb64b4 100644
--- a/configgen/src/test/java/com/yahoo/config/codegen/DefLineParsingTest.java
+++ b/configgen/src/test/java/com/yahoo/config/codegen/DefLineParsingTest.java
@@ -1,6 +1,7 @@
 // Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
 package com.yahoo.config.codegen;
 
+import java.util.Optional;
 import org.junit.jupiter.api.Test;
 
 import static org.junit.jupiter.api.Assertions.*;
@@ -246,4 +247,24 @@ public class DefLineParsingTest {
         assertTrue(r1.getRestart());
     }
 
+    @Test
+    void testParseOptionalPathWithDefault() {
+        DefLine l = new DefLine("pathWithDef path optional");
+
+        assertEquals("pathWithDef", l.getName());
+        assertNull(l.getDefault());
+        assertTrue(l.isOptional());
+        assertEquals("optionalPath", l.getType().getName());
+    }
+
+    @Test
+    void testParsPathWithDefault() {
+        DefLine l = new DefLine("pathWithDef path");
+
+        assertEquals("pathWithDef", l.getName());
+        assertNull(l.getDefault());
+        assertFalse(l.isOptional());
+        assertEquals("path", l.getType().getName());
+    }
+
 }
diff --git a/configgen/src/test/java/com/yahoo/config/codegen/DefParserTest.java b/configgen/src/test/java/com/yahoo/config/codegen/DefParserTest.java
index 45d1f21763c..e5227282c05 100644
--- a/configgen/src/test/java/com/yahoo/config/codegen/DefParserTest.java
+++ b/configgen/src/test/java/com/yahoo/config/codegen/DefParserTest.java
@@ -28,7 +28,7 @@ public class DefParserTest {
         CNode root = new DefParser("test", new FileReader(defFile)).getTree();
         assertNotNull(root);
         CNode[] children = root.getChildren();
-        assertEquals(37, children.length);
+        assertEquals(38, children.length);
 
         int numGrandChildren = 0;
         int numGreatGrandChildren = 0;
@@ -70,7 +70,7 @@ public class DefParserTest {
     void testMd5Sum() throws IOException {
         File defFile = new File(DEF_NAME);
         CNode root = new DefParser("test", new FileReader(defFile)).getTree();
-        assertEquals("0501f9e2c4ecc8c283e100e0b1178ca4", root.defMd5);
+        assertEquals("ee37973499305fde315da46256e64b2e", root.defMd5);
     }
 
     @Test
diff --git a/configgen/src/test/java/com/yahoo/config/codegen/JavaClassBuilderTest.java b/configgen/src/test/java/com/yahoo/config/codegen/JavaClassBuilderTest.java
index 428576e340f..c3145c03fff 100644
--- a/configgen/src/test/java/com/yahoo/config/codegen/JavaClassBuilderTest.java
+++ b/configgen/src/test/java/com/yahoo/config/codegen/JavaClassBuilderTest.java
@@ -120,7 +120,7 @@ public class JavaClassBuilderTest {
         }
         for (int i = 0; i < referenceClassLines.size(); i++) {
             if (configClassLines.length <= i)
-                fail("Missing lines i generated config class. First missing line:\n" + referenceClassLines.get(i));
+                fail("Missing lines in generated config class. First missing line:\n" + referenceClassLines.get(i));
             assertEquals(referenceClassLines.get(i), configClassLines[i], "Line " + i);
         }
     }
diff --git a/configgen/src/test/java/com/yahoo/config/codegen/NormalizedDefinitionTest.java b/configgen/src/test/java/com/yahoo/config/codegen/NormalizedDefinitionTest.java
index 57b3ed962eb..18608102ffa 100644
--- a/configgen/src/test/java/com/yahoo/config/codegen/NormalizedDefinitionTest.java
+++ b/configgen/src/test/java/com/yahoo/config/codegen/NormalizedDefinitionTest.java
@@ -70,7 +70,7 @@ public class NormalizedDefinitionTest {
         }
 
         assertNotNull(out);
-        assertEquals(75, out.size());
+        assertEquals(76, out.size());
 
         assertNotNull(fileReader);
         fileReader.close();
diff --git a/configgen/src/test/resources/allfeatures.reference b/configgen/src/test/resources/allfeatures.reference
index b7a79f663e7..79508b3a25f 100644
--- a/configgen/src/test/resources/allfeatures.reference
+++ b/configgen/src/test/resources/allfeatures.reference
@@ -35,7 +35,7 @@ import com.yahoo.config.*;
  */
 public final class AllfeaturesConfig extends ConfigInstance {
 
-  public final static String CONFIG_DEF_MD5 = "0501f9e2c4ecc8c283e100e0b1178ca4";
+  public final static String CONFIG_DEF_MD5 = "ee37973499305fde315da46256e64b2e";
   public final static String CONFIG_DEF_NAME = "allfeatures";
   public final static String CONFIG_DEF_NAMESPACE = "configgen";
   public final static String[] CONFIG_DEF_SCHEMA = {
@@ -56,6 +56,7 @@ public final class AllfeaturesConfig extends ConfigInstance {
     "refwithdef reference default=\":parent:\"",
     "fileVal file",
     "pathVal path",
+    "optionalPathVal path optional",
     "urlVal url",
     "modelVal model",
     "boolarr[] bool",
@@ -130,6 +131,7 @@ public final class AllfeaturesConfig extends ConfigInstance {
     private String refwithdef = null;
     private String fileVal = null;
     private FileReference pathVal = null;
+    private Optional<FileReference> optionalPathVal = Optional.empty();
     private UrlReference urlVal = null;
     private ModelReference modelVal = null;
     public List<Boolean> boolarr = new ArrayList<>();
@@ -171,6 +173,7 @@ public final class AllfeaturesConfig extends ConfigInstance {
       refwithdef(config.refwithdef());
       fileVal(config.fileVal().value());
       pathVal(config.pathVal.getFileReference());
+      optionalPathVal(config.optionalPathVal.getFileReference());
       urlVal(config.urlVal.getUrlReference());
       modelVal(config.modelVal.getModelReference());
       boolarr(config.boolarr());
@@ -231,6 +234,8 @@ public final class AllfeaturesConfig extends ConfigInstance {
         fileVal(__superior.fileVal);
       if (__superior.pathVal != null)
         pathVal(__superior.pathVal);
+      if (__superior.optionalPathVal != null)
+        optionalPathVal(__superior.optionalPathVal);
       if (__superior.urlVal != null)
         urlVal(__superior.urlVal);
       if (__superior.modelVal != null)
@@ -412,6 +417,17 @@ public final class AllfeaturesConfig extends ConfigInstance {
     }
 
 
+    public Builder optionalPathVal(Optional<FileReference> __value) {
+    if (__value == null) throw new IllegalArgumentException("Null value is not allowed.");
+      optionalPathVal = __value;
+      __uninitialized.remove("optionalPathVal");
+      return this;
+    }
+
+    private Builder optionalPathVal(FileReference __value) {
+      return optionalPathVal(Optional.of(__value));
+    }
+
     public Builder urlVal(UrlReference __value) {
     if (__value == null) throw new IllegalArgumentException("Null value is not allowed.");
       urlVal = __value;
@@ -759,6 +775,7 @@ public final class AllfeaturesConfig extends ConfigInstance {
   private final ReferenceNode refwithdef;
   private final FileNode fileVal;
   private final PathNode pathVal;
+  private final OptionalPathNode optionalPathVal;
   private final UrlNode urlVal;
   private final ModelNode modelVal;
   private final LeafNodeVector<Boolean, BooleanNode> boolarr;
@@ -822,6 +839,8 @@ public final class AllfeaturesConfig extends ConfigInstance {
         new FileNode() : new FileNode(builder.fileVal);
     pathVal = (builder.pathVal == null) ?
         new PathNode() : new PathNode(builder.pathVal);
+    optionalPathVal = (builder.optionalPathVal == null) ?
+        new OptionalPathNode() : new OptionalPathNode(builder.optionalPathVal);
     urlVal = (builder.urlVal == null) ?
         new UrlNode() : new UrlNode(builder.urlVal);
     modelVal = (builder.modelVal == null) ?
@@ -960,6 +979,13 @@ public final class AllfeaturesConfig extends ConfigInstance {
   }
 
   /**
+   * @return allfeatures.optionalPathVal
+   */
+  public Optional<Path> optionalPathVal() {
+    return optionalPathVal.value();
+  }
+
+  /**
    * @return allfeatures.urlVal
    */
   public File urlVal() {
diff --git a/configgen/src/test/resources/configgen.allfeatures.def b/configgen/src/test/resources/configgen.allfeatures.def
index 1f93e29b73b..eee39dc18f3 100644
--- a/configgen/src/test/resources/configgen.allfeatures.def
+++ b/configgen/src/test/resources/configgen.allfeatures.def
@@ -39,6 +39,7 @@ refVal reference
 refwithdef reference default=":parent:"
 fileVal file
 pathVal path
+optionalPathVal path optional
 urlVal url
 modelVal model