Firstly, let me recommend my other favorite place, the WoodenBoat Magazine forum at
www.woodenboat.com, then look for the forum box down a bit on the right and click away. Lots of focused opinion on just this sort of thing.
A doug fir mast is probably not all that super special so you might consider a tin tube if the aesthetics match your style.
Box construction and other glue lines fail for many many reasons but there is no reason just off the bat to assume that the places that have not failed yet must be trouble waiting to happen.
I personally would start by digging out all rot, following along no matter how big a hole, both to let the interior of the mast dry out correctly and to get a look in there. In places where a glue line has failed, I'd very cautiously - maybe using a painters chisel - spread the failed area and perhaps even clean off some of the glue. You want to see if it's really the glue that failed or, as would be shown by raw wood on one side of the crack, the ajacent wood.
If with a little fiddling you get to the point where the joint is not just falling apart in your hands or the crack is running with the grain away from the glue line, drill a little hole on the glue line to act as a split stopper and later to be filled like a stopwater.
Get this far, make detailed notes, and consult your adoring public before going much further as there may be quite a few different solutions depending on the full extent of the problems.
G'luck
Ian