Hi Antoine,
Thanks for bringing this up.
It seems spacechains[0] are impacted by this. Simply explained, the idea is to allow for fee-bidding Blind Merged Mining[1] by creating one transaction for each block, to which anyone can attach a block hash. The preferred mechanism utilizes sighash_anyprevout and is not affected, but there is also a practical variant that could be used without requiring the anyprevout soft fork, which unfortunately does seem to be impacted. Here's a brief description:
TX0:
input 0
output 1a*
output 1b
TX1:
input 1a*
output 2a**
output 2b
TX2:
input 2a**
output 3a***
output 3b
Etc.
Every TX has two outputs, one of which ("a") is used as the input for the next TX (these are pre-signed and act as a covenant), resulting in a continuous chain of transactions. The other output ("b") can be spent by anyone, and is meant to CPFP the parent TX and, importantly, be RBF replaceable by anyone. This allows whoever pays the highest CPFP fee to "win the RBF auction" and attach their TX to the output, containing the winning spacechain block hash.
With inherited signalling, this works because each pre-signed TX is RBF enabled, so each CPFP transaction inherits RBF as well. But if inherited signalling does not function, the first person who makes a CPFP transaction can simply disable RBF and win the auction, thus breaking the intended fee-bidding mechanism.
As it stands, this bug gets in the way of being able to deploy spacechains.
-- Ruben Somsen