From eb24e95731b9f865b95b821c1745264fdc58119d Mon Sep 17 00:00:00 2001
From: Namgyu Kim <namgyu@apache.org>
Date: Sat, 16 Jan 2021 06:49:23 +0900
Subject: [PATCH] LUCENE-9661: Fix deadlock in TermsEnum.EMPTY

---
 lucene/CHANGES.txt                                |  3 +++
 .../java/org/apache/lucene/index/TermsEnum.java   | 15 +++++++++++++--
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/lucene/CHANGES.txt b/lucene/CHANGES.txt
index dfb4efa964e..7affde9e8c2 100644
--- a/lucene/CHANGES.txt
+++ b/lucene/CHANGES.txt
@@ -298,6 +298,9 @@ Bug Fixes
 * LUCENE-9642: When encoding triangles in ShapeField, make sure generated triangles are CCW by rotating 
   triangle points before checking triangle orientation. (Ignacio Vera)  
 
+* LUCENE-9661: Fix deadlock in TermsEnum.EMPTY that occurs when trying to initialize TermsEnum and BaseTermsEnum
+  at the same time (Namgyu Kim)
+
 Other
 ---------------------
 
diff --git a/lucene/core/src/java/org/apache/lucene/index/TermsEnum.java b/lucene/core/src/java/org/apache/lucene/index/TermsEnum.java
index 0f03c94a60b..de0234be908 100644
--- a/lucene/core/src/java/org/apache/lucene/index/TermsEnum.java
+++ b/lucene/core/src/java/org/apache/lucene/index/TermsEnum.java
@@ -180,7 +180,10 @@ public abstract class TermsEnum implements BytesRefIterator {
    * of unused Attributes does not matter.
    */
   public static final TermsEnum EMPTY =
-      new BaseTermsEnum() {
+      new TermsEnum() {
+
+        private AttributeSource atts = null;
+
         @Override
         public SeekStatus seekCeil(BytesRef term) {
           return SeekStatus.END;
@@ -226,7 +229,15 @@ public abstract class TermsEnum implements BytesRefIterator {
 
         @Override // make it synchronized here, to prevent double lazy init
         public synchronized AttributeSource attributes() {
-          return super.attributes();
+          if (atts == null) {
+            atts = new AttributeSource();
+          }
+          return atts;
+        }
+
+        @Override
+        public boolean seekExact(BytesRef text) throws IOException {
+          return seekCeil(text) == SeekStatus.FOUND;
         }
 
         @Override