Dac changes for regions (#50092)

There are two sets of changes here:

- Make DacHeapWalker work for empty regions - I had observed a crash with the empty regions injected with STRESS_REGIONS.
- Make GetGCHeapStaticData robust against globals saved_sweep_ephemeral_seg and saved_sweep_ephemeral_start being null.

The changes should not cause issues for a coreclr NOT running with regions.

src/coreclr/debug/daccess/dacdbiimpl.cpp

@@ -6540,17 +6540,20 @@ HRESULT DacHeapWalker::NextSegment()
 
     do
     {
-        mCurrSeg++;
-        while (mCurrSeg >= mHeaps[mCurrHeap].SegmentCount)
+        do
         {
-            mCurrSeg = 0;
-            mCurrHeap++;
-
-            if (mCurrHeap >= mHeapCount)
+            mCurrSeg++;
+            while (mCurrSeg >= mHeaps[mCurrHeap].SegmentCount)
             {
-                return S_FALSE;
+                mCurrSeg = 0;
+                mCurrHeap++;
+
+                if (mCurrHeap >= mHeapCount)
+                {
+                    return S_FALSE;
+                }
             }
-        }
+        } while (mHeaps[mCurrHeap].Segments[mCurrSeg].Start >= mHeaps[mCurrHeap].Segments[mCurrSeg].End);
 
         mCurrObj = mHeaps[mCurrHeap].Segments[mCurrSeg].Start;
 
@@ -6657,6 +6660,12 @@ HRESULT DacHeapWalker::Reset(CORDB_ADDRESS start, CORDB_ADDRESS end)
     mCurrHeap = 0;
     mCurrSeg = 0;
 
+    HRESULT hr = S_OK;
+
+    // it's possible the first segment is empty
+    if (mCurrObj >= mHeaps[0].Segments[0].End)
+        hr = MoveToNextObject();
+
     if (!mCache.ReadMT(mCurrObj, &mCurrMT))
         return E_FAIL;
 
@@ -6664,9 +6673,9 @@ HRESULT DacHeapWalker::Reset(CORDB_ADDRESS start, CORDB_ADDRESS end)
         return E_FAIL;
 
     if (mCurrObj < mStart || mCurrObj > mEnd)
-        MoveToNextObject();
+        hr = MoveToNextObject();
 
-    return S_OK;
+    return hr;
 }
 
 HRESULT DacHeapWalker::ListNearObjects(CORDB_ADDRESS obj, CORDB_ADDRESS *pPrev, CORDB_ADDRESS *pContaining, CORDB_ADDRESS *pNext)

src/coreclr/debug/daccess/request.cpp

@@ -2786,8 +2786,18 @@ ClrDataAccess::GetGCHeapStaticData(struct DacpGcHeapDetails *detailsData)
     detailsData->mark_array = (CLRDATA_ADDRESS)*g_gcDacGlobals->mark_array;
     detailsData->current_c_gc_state = (CLRDATA_ADDRESS)*g_gcDacGlobals->current_c_gc_state;
     detailsData->next_sweep_obj = (CLRDATA_ADDRESS)*g_gcDacGlobals->next_sweep_obj;
-    detailsData->saved_sweep_ephemeral_seg = (CLRDATA_ADDRESS)*g_gcDacGlobals->saved_sweep_ephemeral_seg;
-    detailsData->saved_sweep_ephemeral_start = (CLRDATA_ADDRESS)*g_gcDacGlobals->saved_sweep_ephemeral_start;
+    if (g_gcDacGlobals->saved_sweep_ephemeral_seg != nullptr)
+    {
+        detailsData->saved_sweep_ephemeral_seg = (CLRDATA_ADDRESS)*g_gcDacGlobals->saved_sweep_ephemeral_seg;
+        detailsData->saved_sweep_ephemeral_start = (CLRDATA_ADDRESS)*g_gcDacGlobals->saved_sweep_ephemeral_start;
+    }
+    else
+    {
+        // with regions, we don't have these variables anymore
+        // use special value -1 in saved_sweep_ephemeral_seg to signal the region case
+        detailsData->saved_sweep_ephemeral_seg = (CLRDATA_ADDRESS)-1;
+        detailsData->saved_sweep_ephemeral_start = 0;
+    }
     detailsData->background_saved_lowest_address = (CLRDATA_ADDRESS)*g_gcDacGlobals->background_saved_lowest_address;
     detailsData->background_saved_highest_address = (CLRDATA_ADDRESS)*g_gcDacGlobals->background_saved_highest_address;