From 0873e99ebc7cecf072bf2384a0ff6c499c4288a4 Mon Sep 17 00:00:00 2001
From: Guillaume Nodet <gnodet@gmail.com>
Date: Wed, 27 May 2026 09:41:23 +0200
Subject: [PATCH] Fix deadlock in AbstractRequestCache.requests() due to
 unstable hashCode

The `requests()` method uses a HashMap to coordinate batch results between
the batch supplier and the individual CachingSupplier instances. When a
request object's hashCode() changes between HashMap.put() and
HashMap.containsKey() (e.g., because ResolverRequest includes a
RequestTrace with mutable ModelBuilderRequest data), the key is stored
in one bucket but looked up in another, causing containsKey() to return
false. The individualSupplier then waits forever for a result that is
already in the map but unreachable.

Switch to IdentityHashMap which uses System.identityHashCode() and
reference equality (==), both of which are stable regardless of
mutable object state. This is safe because the code always uses the
same object reference for put and get.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 .../org/apache/maven/impl/cache/AbstractRequestCache.java     | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/impl/maven-impl/src/main/java/org/apache/maven/impl/cache/AbstractRequestCache.java b/impl/maven-impl/src/main/java/org/apache/maven/impl/cache/AbstractRequestCache.java
index 0b7fac393bf8..0dc5a7d0f044 100644
--- a/impl/maven-impl/src/main/java/org/apache/maven/impl/cache/AbstractRequestCache.java
+++ b/impl/maven-impl/src/main/java/org/apache/maven/impl/cache/AbstractRequestCache.java
@@ -19,7 +19,7 @@
 package org.apache.maven.impl.cache;
 
 import java.util.ArrayList;
-import java.util.HashMap;
+import java.util.IdentityHashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.function.Function;
@@ -85,7 +85,7 @@ public <REQ extends Request<?>, REP extends Result<REQ>> REP request(REQ req, Fu
     @SuppressWarnings("unchecked")
     public <REQ extends Request<?>, REP extends Result<REQ>> List<REP> requests(
             List<REQ> reqs, Function<List<REQ>, List<REP>> supplier) {
-        final Map<REQ, Object> nonCachedResults = new HashMap<>();
+        final Map<REQ, Object> nonCachedResults = new IdentityHashMap<>();
         List<RequestResult<REQ, REP>> allResults = new ArrayList<>(reqs.size());
 
         Function<REQ, REP> individualSupplier = req -> {