Enable GC double reporting detection

src/coreclr/inc/clrconfigvalues.h

@@ -278,6 +278,7 @@ CONFIG_STRING_INFO(INTERNAL_SkipGCCoverage, W("SkipGcCoverage"), "Specify a list
 RETAIL_CONFIG_DWORD_INFO(UNSUPPORTED_StatsUpdatePeriod, W("StatsUpdatePeriod"), 60, "Specifies the interval, in seconds, at which to update the statistics")
 RETAIL_CONFIG_DWORD_INFO(UNSUPPORTED_GCRetainVM, W("GCRetainVM"), 0, "When set we put the segments that should be deleted on a standby list (instead of releasing them back to the OS) which will be considered to satisfy new segment requests (note that the same thing can be specified via API which is the supported way)")
 RETAIL_CONFIG_DWORD_INFO(EXTERNAL_gcAllowVeryLargeObjects, W("gcAllowVeryLargeObjects"), 1, "Allow allocation of 2GB+ objects on GC heap")
+RETAIL_CONFIG_DWORD_INFO(UNSUPPORTED_EnableGCHoleMonitoring, W("EnableGCHoleMonitoring"), 0, "Enable checks to proactively watch for possible GC holes")
 RETAIL_CONFIG_DWORD_INFO(EXTERNAL_GCStress, W("GCStress"), 0, "Trigger GCs at regular intervals")
 CONFIG_DWORD_INFO(INTERNAL_GcStressOnDirectCalls, W("GcStressOnDirectCalls"), 0, "Whether to trigger a GC on direct calls")
 RETAIL_CONFIG_DWORD_INFO(UNSUPPORTED_HeapVerify, W("HeapVerify"), 0, "When set verifies the integrity of the managed heap on entry and exit of each GC")

src/coreclr/vm/eeconfig.cpp

@@ -425,6 +425,8 @@ HRESULT EEConfig::sync()
     iGCConservative =  (CLRConfig::GetConfigValue(CLRConfig::UNSUPPORTED_gcConservative) != 0);
 #endif // FEATURE_CONSERVATIVE_GC
 
+    fEnableGCHoleMonitoring = (CLRConfig::GetConfigValue(CLRConfig::UNSUPPORTED_EnableGCHoleMonitoring) != 0);
+
 #ifdef HOST_64BIT
     iGCAllowVeryLargeObjects = (CLRConfig::GetConfigValue(CLRConfig::EXTERNAL_gcAllowVeryLargeObjects) != 0);
 #endif

src/coreclr/vm/eeconfig.h

@@ -389,6 +389,7 @@ class EEConfig
 #ifdef FEATURE_CONSERVATIVE_GC
     bool    GetGCConservative()             const {LIMITED_METHOD_CONTRACT; return iGCConservative;}
 #endif
+    bool    GetEnableGCHoleMonitoring()     const {LIMITED_METHOD_CONTRACT; return fEnableGCHoleMonitoring; }
 #ifdef HOST_64BIT
     bool    GetGCAllowVeryLargeObjects()    const {LIMITED_METHOD_CONTRACT; return iGCAllowVeryLargeObjects;}
 #endif
@@ -572,6 +573,8 @@ class EEConfig
     bool iGCAllowVeryLargeObjects;
 #endif // HOST_64BIT
 
+    bool fEnableGCHoleMonitoring;
+
     bool fGCBreakOnOOM;
 
 #ifdef _DEBUG

src/coreclr/vm/gcenv.ee.common.cpp

@@ -137,6 +137,11 @@ inline bool SafeToReportGenericParamContext(CrawlFrame* pCF)
     return true;
 }
 
+thread_local void **doubleReportTracking = NULL;
+thread_local int doubleReportTrackingSize = 0;
+thread_local int doubleReportTrackingIndex = 0;
+static const int DoubleReportTrackingInitialSize = 256;
+
 /*
  * GcEnumObject()
  *
@@ -149,6 +154,22 @@ void GcEnumObject(LPVOID pData, OBJECTREF *pObj, uint32_t flags)
     Object ** ppObj = (Object **)pObj;
     GCCONTEXT   * pCtx  = (GCCONTEXT *) pData;
 
+    if (g_pConfig->GetEnableGCHoleMonitoring() && ((flags & GC_CALL_PINNED) == 0) && pCtx->sc->promotion)
+    {
+        if (doubleReportTrackingIndex == doubleReportTrackingSize)
+        {
+            doubleReportTrackingSize = (doubleReportTrackingSize == 0) ? DoubleReportTrackingInitialSize : doubleReportTrackingSize * 2;
+            doubleReportTracking = (void**)realloc(doubleReportTracking, doubleReportTrackingSize * sizeof(void*));
+        }
+        for (int i = 0; i < doubleReportTrackingIndex; i++)
+        {
+            // Double reporting of the same slot detected
+            _ASSERTE_ALL_BUILDS(doubleReportTracking[i] != pObj);
+        }
+
+        doubleReportTracking[doubleReportTrackingIndex++] = pObj;
+    }
+
     // Since we may be asynchronously walking another thread's stack,
     // check (frequently) for stack-buffer-overrun corruptions after
     // any long operation

src/coreclr/vm/gcenv.ee.cpp

@@ -32,6 +32,8 @@ extern void FinalizeWeakReference(Object* obj);
 extern GCHeapHardLimitInfo g_gcHeapHardLimitInfo;
 extern bool g_gcHeapHardLimitInfoSpecified;
 
+extern thread_local int doubleReportTrackingIndex;
+
 #include <generatedumpflags.h>
 #include "gcrefmap.h"
 
@@ -201,6 +203,7 @@ static void ScanStackRoots(Thread * pThread, promote_func* fn, ScanContext* sc)
 #if defined(FEATURE_EH_FUNCLETS)
         flagsStackWalk |= GC_FUNCLET_REFERENCE_REPORTING;
 #endif // defined(FEATURE_EH_FUNCLETS)
+        doubleReportTrackingIndex = 0;
         pThread->StackWalkFrames( GcStackCrawlCallBack, &gcctx, flagsStackWalk);
     }