gnodet commented on issue #12472:
URL: https://github.com/apache/maven/issues/12472#issuecomment-4955394126
## Root Cause Analysis
I investigated this hang and traced it to a **cross-thread deadlock** in
`AbstractRequestCache.requests()` — the batch request caching mechanism used
during dependency resolution.
### The deadlock mechanism (in rc-5)
1. **Thread A** calls `requests([artifactA, sharedDep])` — creates
`CachingSupplier` instances and stores them in the cache (`SoftIdentityMap`)
2. **Thread B** calls `requests([artifactB, sharedDep])` — gets back the
**same** `CachingSupplier` for `sharedDep` because `SoftIdentityMap` used
`equals()` (despite "Identity" in the name)
3. Thread B enters `CachingSupplier.apply()` for `sharedDep`, hits the
`individualSupplier` lambda which does `synchronized(nonCachedResults) { wait()
}` — waiting for Thread A's `IdentityHashMap`
4. Thread A's batch supplier completes and tries to populate results, but
Thread B holds the `CachingSupplier`'s monitor (via `synchronized(this)` in
`apply()`)
5. **Permanent deadlock**: Thread A can't deliver the result, Thread B can't
receive it
### Fix already on master
This was already fixed in commit c6de104bff
([#12446](https://github.com/apache/maven/pull/12446)) which replaced the
`individualSupplier` + `IdentityHashMap` wait pattern with:
- `CachingSupplier.complete()` — directly sets the value and calls
`notifyAll()`, no external map needed
- `ThreadLocal` re-entrancy guard — prevents same-thread deadlocks when
batch resolution triggers nested calls (e.g., parent POM resolution)
- `singleSupplier` fallback — re-entrant calls resolve individually instead
of waiting
### Regression test
I've added a specific cross-thread deadlock regression test in
[#12478](https://github.com/apache/maven/pull/12478) that reproduces the exact
scenario: two threads calling `requests()` with overlapping keys through a
shared equals-based cache, using a `CyclicBarrier` to force concurrent
execution. This test would deadlock on rc-5 code but passes on master.
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