Add mechanism to retrieve missing blocks

[Gabriel-Trintinalia]

Signed-off-by: Gabriel Trintinalia gabriel.trintinalia@consensys.net

PR description

Add mechanism to retrieve missing blocks when a pending block is saved

Fixed Issue(s)

#3955

Documentation

• I thought about documentation and added the doc-change-required label to this PR if
  updates are required.

Changelog

• I thought about the changelog and included a changelog update if required.

[garyschulte]

Good test coverage 👍

[fab-10]

It is worth to know that the BlockPropagationManager runs concurrently with full sync, and full sync should be able to recover from situations where a parent block was missing, so it could be that the real cause of the issue is related to full sync, in the way the sync target is selected, rather that the BlockPropagationManager.
Anyway this could be a fallback mechanism to workaround full sync issues, and if it works well and since this block propagation is not needed after the merge, putting more effort on fixing full sync could not be worth.

[fab-10]

What happens in case more than one parent block is missing?

[matkt]

It is worth to know that the BlockPropagationManager runs concurrently with full sync, and full sync should be able to recover from situations where a parent block was missing, so it could be that the real cause of the issue is related to full sync, in the way the sync target is selected, rather that the BlockPropagationManager. Anyway this could be a fallback mechanism to workaround full sync issues, and if it works well and since this block propagation is not needed after the merge, putting more effort on fixing full sync could not be worth.

the fullsync only starts when we are too far from the head. so indeed it can be strange that the fullsync does not trigger after a certain time. but at the same time I also prefer this fix because we don't have to wait to be too far to fix the problem

[fab-10]

Double checked with @matkt, and full sync and BlockPropagationManager always run concurrently.

[Gabriel-Trintinalia]

What happens in case more than one parent block is missing?

So if block +3 arrives and we do not have +1 and +2:

1. we will request +3's parent, which is +2.
2. Whenever +2 arrives, it would be saved for future and then request +1.
3. When +1 arrives, it is imported, then its children (+2) and then +2's children (+3)