Serialize VSD DispatchCache unlinking with cache write lock (#128868)

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.

Fixes #128859

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jkotas <6668460+jkotas@users.noreply.github.com>

Author: Copilot

src/coreclr/vm/virtualcallstub.cpp

@@ -417,6 +417,10 @@ void VirtualCallStubManager::ResetCache()
     // Go through each cache entry and if the cache element there is in
     // the cache entry heap of the manager being deleted, then we just
     // set the cache entry to empty.
+#ifdef CHAIN_LOOKUP
+    // Serialize cache chain unlinking against concurrent insert/promote writers.
+    CrstHolder lh(g_resolveCache->GetWriteLock());
+#endif
     DispatchCache::Iterator it(g_resolveCache);
     while (it.IsValid())
     {
@@ -681,6 +685,10 @@ VirtualCallStubManager::~VirtualCallStubManager()
     // Go through each cache entry and if the cache element there is in
     // the cache entry heap of the manager being deleted, then we just
     // set the cache entry to empty.
+#ifdef CHAIN_LOOKUP
+    // Serialize cache chain unlinking against concurrent insert/promote writers.
+    CrstHolder lh(g_resolveCache->GetWriteLock());
+#endif
     DispatchCache::Iterator it(g_resolveCache);
     while (it.IsValid())
     {

src/coreclr/vm/virtualcallstub.h

@@ -1306,6 +1306,14 @@ class DispatchCache
           cache[idx] = elem;
         }
 
+#ifdef CHAIN_LOOKUP
+    inline Crst *GetWriteLock()
+    {
+        LIMITED_METHOD_CONTRACT;
+        return &m_writeLock;
+    }
+#endif
+
     inline void ClearCacheEntry(size_t idx)
     {
         LIMITED_METHOD_CONTRACT;