Avoid static dependency from TermsEnum to one of its subclasses (#15319)

Fixes a potential hang with initialisation of TermsEnum and BaseTermsEnum, by simply removing the dependency and replicating the small amount of code.

Problem description: the static TermsEnum.EMPTY initialises to an implementation of BaseTermsEnum. TermsEnum is a superclass of BaseTermsEnum, so there is a clear dependency between these classes. If a subclass of BaseTermsEnum is initialising it may grab the lock on BaseTermsEnum, and prevent TermsEnum from initialising.

---------

Co-authored-by: Robert Muir <[email protected]>

Author: ChrisHegarty

lucene/CHANGES.txt

@@ -199,6 +199,8 @@ Bug Fixes
   removing co-planar points to do it once after all holes are removed instead of doing it for each
   removed hole.
 
+* GITHUB#15319: Fix potential hang in initialisation of TermsEnum and BaseTermsEnum (Chris Hegarty)
+
 Other
 ---------------------
 * GITHUB#15237: Fix SmartChinese to only deserialize dictionary data from classpath with a native array

lucene/core/src/java/org/apache/lucene/index/TermsEnum.java

@@ -196,9 +196,30 @@ public final PostingsEnum postings(PostingsEnum reuse) throws IOException {
    * Attributes to it. This should not be a problem, as the enum is always empty and the existence
    * of unused Attributes does not matter.
    */
-  @SuppressWarnings("ClassInitializationDeadlock") // FIXME: may cause hangs!
+  // Avoid refactoring that results in a dependency on a subclass, like BaseTermsEnum.
+  // See: https://github.com/apache/lucene/issues/15317
   public static final TermsEnum EMPTY =
-      new BaseTermsEnum() {
+      new TermsEnum() {
+
+        private AttributeSource atts;
+
+        @Override
+        public AttributeSource attributes() {
+          if (atts == null) {
+            atts = new AttributeSource();
+          }
+          return atts;
+        }
+
+        @Override
+        public boolean seekExact(BytesRef text) {
+          return seekCeil(text) == SeekStatus.FOUND;
+        }
+
+        @Override
+        public IOBooleanSupplier prepareSeekExact(BytesRef text) {
+          return () -> seekExact(text);
+        }
 
         @Override
         public SeekStatus seekCeil(BytesRef term) {