Re: pg_rewind WAL segments deletion pitfall

Regarding the the patch, pg_rewind starts reading segments from the
divergence point back to the nearest checkpoint, then moves foward
during rewinding. So, the fact that SimpleXLogPageRead have read a
segment suggests that the segment is required during the next startup.
So I don't think we need to move around the keepWalSeg flag.  All
files that are wanted while rewinding should be preserved
unconditionally.

+               /*
+                * Some entries (WAL segments) already have an action assigned
+                * (see SimpleXLogPageRead()).
+                */
+               if (entry->action == FILE_ACTION_NONE)
+                       continue;
                entry->action = decide_file_action(entry);

It might be more reasonable to call decide_file_action() when action
is UNDECIDED.