Run fork function after cron for null block safety

[ZenGround0]

Actors state migrations will take place within a fork function called from handleStateForks. In general migrations need the epoch the transition is run at to work correctly. We are assuming the epoch passed to any migration function will be the same epoch passed to handleStateForks, i.

i is the fork epoch. In the common case of no null blocks it is the "previous epoch" in the sense that epoch i's cron has already run as part of processing the parent tipset and state.

If there are null blocks then for i > parentEpoch epoch i's cron will not have run before the fork function because of the current ordering of fork function before cron. We can't distinguish these two cases from within the migration function so the epoch we are passed in won't be well defined with respect to our state.

The fix is to run cron before the fork function.

[magik6k]

(TODO: Make sure this doesn't break consensus on the already performed forks (should be fairly easy with just lotus state --tipset @[epoch] compute-state))

[ZenGround0]

@magik6k this does not break consensus at breeze, smoke and ignition epochs

Before change

$ ./lotus version
Daemon:  0.8.1+git.efc1b24f.dirty+api0.16.0
Local: lotus version 0.8.1+git.efc1b24f.dirty
$ ./lotus state --tipset=@41281 compute-state
computed state cid:  bafy2bzaceajkwiwjw5hdwgp73u7f4nl6ayihxaxashcmtaqgxamww6xvufhpi
$ ./lotus state --tipset=@51001 compute-state
computed state cid:  bafy2bzaceaolfm5uaald2skcsfzamspm357hsaqyaq7mmlwlpl4oup5svaucu
$ ./lotus state --tipset=@94001 compute-state
computed state cid:  bafy2bzacecyce7bcu6yazuwwa5wn6vo5ksaztv73idmiw4w5rrpzi6shk4hu4

After change

$ ./lotus version
Daemon:  0.8.0+git.8091bb31.dirty+api0.16.0
Local: lotus version 0.8.0+git.8091bb31.dirty
$ ./lotus state --tipset=@41281 compute-state
computed state cid:  bafy2bzaceajkwiwjw5hdwgp73u7f4nl6ayihxaxashcmtaqgxamww6xvufhpi
$ ./lotus state --tipset=@51001 compute-state
computed state cid:  bafy2bzaceaolfm5uaald2skcsfzamspm357hsaqyaq7mmlwlpl4oup5svaucu
$ ./lotus state --tipset=@94001 compute-state
computed state cid:  bafy2bzacecyce7bcu6yazuwwa5wn6vo5ksaztv73idmiw4w5rrpzi6shk4hu4

Double checked that these match the next header to confirm compute-state is including cron/fork functions. I also checked 41820, 51000 and 94000 in case I was off by one and found matches

[Stebalien]

This makes sense.

[Stebalien]

nit cid.Undef.

[ZenGround0]

In case its not clear I don't have merge permissions so someone else should push the button.