Add ctor for ruleset reference id

pmd-core/src/main/java/net/sourceforge/pmd/RuleSetFactory.java

@@ -129,15 +129,14 @@ final class RuleSetFactory {
      * @return A new Rule.
      */
     private Rule createRule(RuleSetReferenceId ruleSetReferenceId, boolean withDeprecatedRuleReferences) {
-        if (ruleSetReferenceId.isAllRules()) {
+        RuleSetReferenceId parentRuleset = ruleSetReferenceId.getParentRulesetIfThisIsARule();
+        if (parentRuleset == null) {
             throw new IllegalArgumentException(
                 "Cannot parse a single Rule from an all Rule RuleSet reference: <" + ruleSetReferenceId + ">.");
         }
-        RuleSet ruleSet;
-        // java8: computeIfAbsent
-        if (parsedRulesets.containsKey(ruleSetReferenceId)) {
-            ruleSet = parsedRulesets.get(ruleSetReferenceId);
-        } else {
+        // can't use computeIfAbsent as creating a ruleset may add more entries to the map.
+        RuleSet ruleSet = parsedRulesets.get(parentRuleset);
+        if (ruleSet == null) {
             ruleSet = createRuleSet(ruleSetReferenceId, withDeprecatedRuleReferences);
             parsedRulesets.put(ruleSetReferenceId, ruleSet);
         }
@@ -500,7 +499,7 @@ final class RuleSetFactory {
         RuleSetFactory ruleSetFactory = toLoader().filterAbovePriority(RulePriority.LOW).warnDeprecated(false).toFactory();
 
         boolean isSameRuleSet = false;
-        RuleSetReferenceId otherRuleSetReferenceId = RuleSetReferenceId.parse(ref).get(0);
+        RuleSetReferenceId otherRuleSetReferenceId = RuleSetReferenceId.parse(ref).get(0); // fixme what is this get(0) doing here
         if (!otherRuleSetReferenceId.isExternal()
             && containsRule(ruleSetReferenceId, otherRuleSetReferenceId.getRuleName())) {
             otherRuleSetReferenceId = new RuleSetReferenceId(ref, ruleSetReferenceId);

pmd-core/src/main/java/net/sourceforge/pmd/RuleSetReferenceId.java

@@ -14,6 +14,7 @@ import java.util.ArrayList;
 import java.util.List;
 
 import org.apache.commons.lang3.StringUtils;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 import net.sourceforge.pmd.annotation.InternalApi;
 import net.sourceforge.pmd.util.ResourceLoader;
@@ -105,6 +106,14 @@ public class RuleSetReferenceId {
         this(id, null);
     }
 
+    private RuleSetReferenceId(final String ruleSetFileName, boolean allRules, boolean external, String ruleName, RuleSetReferenceId externalRuleSetReferenceId) {
+        this.ruleSetFileName = ruleSetFileName;
+        this.allRules = allRules;
+        this.external = external;
+        this.ruleName = ruleName;
+        this.externalRuleSetReferenceId = externalRuleSetReferenceId;
+    }
+
     /**
      * Construct a RuleSetReferenceId for the given single ID string. If an
      * external RuleSetReferenceId is given, the ID must refer to a non-external
@@ -221,6 +230,20 @@ public class RuleSetReferenceId {
         this.externalRuleSetReferenceId = externalRuleSetReferenceId;
     }
 
+    @Nullable RuleSetReferenceId getParentRulesetIfThisIsARule() {
+        if (ruleName == null) {
+            return null;
+        }
+        return new RuleSetReferenceId(
+            ruleSetFileName,
+            true,
+            external,
+            null,
+
+            null
+        );
+    }
+
     /**
      * Tries to load the given ruleset.
      *