Help with finding checkpoint code

I am trying to find when WAL log files are rotated.  The message is:
 2002-02-11 21:18:13 DEBUG: recycled transaction log file 0000000000000005

and it is printed in MoveOfflineLogs(), and MoveOfflineLogs() is only
called by CreateCheckPoint(), but I can't see where CreateCheckPoint()
is called in normal operation.  I see it called by CHECKPOINT, and on
startup and shutdown, and from bootstrap, but where is it called during
normal backend operation.

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