And as I said in a mail I sent just before, the patch looks too complex. How about just comparing WAL file name aginst the last common checkpoint's tli and lsn? We can tell filemap.c about the last checkpoint and decide_file_action can compare the file name with it.
It is sufficient to preserve WAL files if tli matches and the segment number of the WAL file is equal to or later than the checkpoint location.
What if the last common checkpoint was on a previous timeline?
I.e., standby was promoted to primary, the timeline changed from 1 to 2, and after that the node crashed _before_ the CHECKPOINT after promote has finished.
The next node will advance the timeline from 2 to 3.
In this case, the last common checkpoint will be on timeline 1, and the check becomes more complex because we will have to consider both timelines, 1 and 2.
Also, we need to take into account the divergency LSN. Files after it are not required.