tree 3a944232bf0664f6856bd7ed43618ce61b838596
parent 87af6c083dca39250c29c9ad3a0a6627891eccb7
author Tianjie <xunchang@google.com> 1598312075 -0700
committer Tianjie <xunchang@google.com> 1598913148 -0700

Implement the topology sort in CreateCowMergeOperation

Generate a sequence of COW_COPY operations. The extents in these
operations won't be written as raw bytes in snapshots. Instead,
they will read from the source partitions.

So it's important to make sure no read after write happens on the
source partitions, similiar to the inplace update. If the topology
sort isn't possible due to cycles, we will omit some blocks to
break the cycles. And these blocks will be carried as raw bytes in
cow instead.

Bug: 162274240
Test: generate a payload, unittests pass
Change-Id: If4866704459b919d4bc09be48286b3e321b70497