GCHeap::NextObj fix

[Maoni0]

this is to fix #70231.

for regions we could run into this situation -

object is the last object before heap_segment_allocated (hs)
T0 calls NextObj, gets next obj which starts at heap_segment_allocated (hs)
T1 changes ephemeral_heap_segment to hs
T0 does these comparisons

        (nextobj >= heap_segment_allocated(hs) && hs != hp->ephemeral_heap_segment) ||
        (nextobj >= hp->alloc_allocated))

both still false because alloc_allocated hasn't been changed just yet (and the old alloc_allocated is larger than nextobj)

T0 validates next obj, concludes its m_alignpad is not 0, asserts

T1 forms an allocation context starting at heap_segment_allocated, clears memory so by the time the dump is taken, m_alignpad is already cleared (actually we clear it in a_fit_segment_end)

I'm fixing this by saving the ephemeral_heap_segment and alloc_allocated and bail if nextobj is not on the saved eph seg or if those 2 saved values are no long in sync.

[ghost]

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Issue Details

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for regions we could run into this situation -

object is the last object before heap_segment_allocated (hs)
T0 calls NextObj, gets next obj which starts at heap_segment_allocated (hs)
T1 changes ephemeral_heap_segment to hs
T0 does these comparisons

        (nextobj >= heap_segment_allocated(hs) && hs != hp->ephemeral_heap_segment) ||
        (nextobj >= hp->alloc_allocated))

both still false because alloc_allocated hasn't been changed just yet (and the old alloc_allocated is larger than nextobj)

T0 validates next obj, concludes its m_alignpad is not 0, asserts

T1 forms an allocation context starting at heap_segment_allocated, clears memory so by the time the dump is taken, m_alignpad is already cleared (actually we clear it in a_fit_segment_end)

I'm fixing this by saving the ephemeral_heap_segment and alloc_allocated and bail if nextobj is not on the saved eph seg or if those 2 saved values are no long in sync.

Author:	Maoni0
Assignees:	-
Labels:	area-GC-coreclr
Milestone:	-