Factor XML discard predicates into OverlayXml.qll

csharp/ql/lib/semmle/code/csharp/internal/Overlay.qll

@@ -2,6 +2,8 @@
  * Defines entity discard predicates for C# overlay analysis.
  */
 
+private import OverlayXml
+
 /**
  * Holds always for the overlay variant and never for the base variant.
  * This local predicate is used to define local predicates that behave
@@ -110,36 +112,6 @@ private predicate discardLocation(@location_default loc) {
   exists(string path | discardableLocation(loc, path) | overlayChangedFiles(path))
 }
 
-/**
- * A class of Xml locatables that can be discarded from the base.
- */
-overlay[local]
-private class DiscardableXmlEntity extends DiscardableEntityBase instanceof @xmllocatable {
-  /** Gets the path to the file in which this element occurs. */
-  override string getFilePath() {
-    exists(@location_default loc | result = getLocationFilePath(loc) | xmllocations(this, loc))
-  }
-}
-
-overlay[local]
-private predicate overlayXmlExtracted(string file) {
-  exists(DiscardableXmlEntity dxe |
-    dxe.existsInOverlay() and
-    file = dxe.getFilePath() and
-    not files(dxe, _) and
-    not xmlNs(dxe, _, _, _)
-  )
-}
-
-overlay[discard_entity]
-private predicate discardXmlEntity(@xmllocatable xml) {
-  overlayChangedFiles(xml.(DiscardableXmlEntity).getFilePath())
-  or
-  // The XML extractor is not incremental and may extract more
-  // XML files than those included in overlayChangedFiles.
-  overlayXmlExtracted(xml.(DiscardableXmlEntity).getFilePath())
-}
-
 overlay[local]
 private class DiscardableAspEntity extends DiscardableEntityBase instanceof @asp_element {
   /** Gets the path to the file in which this element occurs. */

csharp/ql/lib/semmle/code/csharp/internal/OverlayXml.qll

@@ -0,0 +1,45 @@
+overlay[local]
+module;
+
+/**
+ * A local predicate that always holds for the overlay variant and never holds for the base variant.
+ * This is used to define local predicates that behave differently for the base and overlay variant.
+ */
+private predicate isOverlay() { databaseMetadata("isOverlay", "true") }
+
+private @file getXmlFile(@xmllocatable locatable) {
+  exists(@location_default location | xmllocations(locatable, location) |
+    locations_default(location, result, _, _, _, _)
+  )
+}
+
+private @file getXmlFileInBase(@xmllocatable locatable) {
+  not isOverlay() and
+  result = getXmlFile(locatable)
+}
+
+/**
+ * Holds if the given `file` was extracted as part of the overlay and was extracted by the HTML/XML
+ * extractor.
+ */
+private predicate overlayXmlExtracted(@file file) {
+  isOverlay() and
+  exists(@xmllocatable locatable |
+    not files(locatable, _) and not xmlNs(locatable, _, _, _) and file = getXmlFile(locatable)
+  )
+}
+
+/**
+ * Holds if the given XML `locatable` should be discarded, because it is part of the overlay base
+ * and is in a file that was also extracted as part of the overlay database.
+ */
+overlay[discard_entity]
+private predicate discardXmlLocatable(@xmllocatable locatable) {
+  exists(@file file | file = getXmlFileInBase(locatable) |
+    exists(string path | files(file, path) | overlayChangedFiles(path))
+    or
+    // The HTML/XML extractor is currently not incremental and may extract more files than those
+    // included in overlayChangedFiles.
+    overlayXmlExtracted(file)
+  )
+}