Serialize VSD DispatchCache unlinking with cache write lock

[Copilot]

Collectible ALC teardown can unlink g_resolveCache bucket entries concurrently with Insert/PromoteChainEntry, violating the cache’s single-writer assumption and allowing self-referential chain cycles that hang VSD resolution. This change restores writer serialization for unlink paths.

• DispatchCache API surface

  • Added DispatchCache::GetWriteLock() (under CHAIN_LOOKUP) to expose the existing cache writer lock for safe external mutation paths.

• Collectible-unload unlink serialization

  • Wrapped DispatchCache::Iterator::UnlinkEntry() loops with CrstHolder in:
    • VirtualCallStubManager::ResetCache()
    • VirtualCallStubManager::~VirtualCallStubManager()
  • Locking is conditional on CHAIN_LOOKUP, matching where m_writeLock exists and where chain mutation races matter.

• Resulting invariant

  • All chain mutations (Insert, PromoteChainEntry, unlink purge) are serialized by the same lock.

#ifdef CHAIN_LOOKUP
CrstHolder lh(g_resolveCache->GetWriteLock());
#endif
DispatchCache::Iterator it(g_resolveCache);
while (it.IsValid())
{
    it.UnlinkEntry();
}

Fixes #128859

[dotnet-policy-service]

Tagging subscribers to this area: @agocke
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

Fixes a race in the VSD DispatchCache where collectible ALC teardown could unlink cache chain entries concurrently with Insert/PromoteChainEntry, breaking the single-writer assumption and producing self-referential chains that hang VSD resolution. The fix exposes the existing cache write lock and acquires it around the two unlink loops invoked during cache reset and manager teardown.

Changes:

• Expose DispatchCache::GetWriteLock() (under CHAIN_LOOKUP).
• Take m_writeLock around the Iterator::UnlinkEntry() loops in VirtualCallStubManager::ResetCache() and ~VirtualCallStubManager().

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/coreclr/vm/virtualcallstub.h	Adds DispatchCache::GetWriteLock() accessor (CHAIN_LOOKUP only) to allow external callers to serialize with cache writers.
src/coreclr/vm/virtualcallstub.cpp	Wraps the two DispatchCache::Iterator::UnlinkEntry() loops in ResetCache and the destructor with CrstHolder(g_resolveCache->GetWriteLock()) under CHAIN_LOOKUP.

[wtgodbe]

Thanks @jkotas!