Re: [bug] git checkout lies about number of ahead commits when switching from detached HEAD

Jeff King [off-list ref] writes:

For the case of 2 traversals, I suspect that clearing everything between
is not so different from clearing from the tips, since most everything
parsed was probably from the first traversal. But as we lib-ify more, we
may end up with more and more traversals in a single program, so it's
probably better to go the more efficient route from the beginning.

"Let the one that has more information to do the clearing" is not about
performance but about correctness and reusability potential.

When you insert a new traversal B in the existing codeflow before another
traversal C, B knows not just which commits it started from (hence knows
which commits were marked by it), but more importantly it also knows what
mark bits were potentially modified. If the existing codeflow had another
traversal A before you added B, and C took the marks A left on the objects
into account while doing its work, the only sensible clean-up is to clear
marks B touched (and no other marks) immediately after B, and we obviously
do not want C (and any later traversals in other codepaths that we may
later want to insert B) to have too much knowledge of which marks may have
been clobbered by B.

Of course, C could be coded defensively to clear marks other than what it
cares about (namely, the ones other than what A would have left and the
ones that would affect itself e.g. UNINTERESTING), and whoever inserts B
into existing codeflow needs to make sure that B does not stomp on marks
deliberately left by earlier traversals like A for later users like C
regardless of how the latter is coded.

So how about this?

  [1/3]: checkout: add basic tests for detached-orphan warning
  [2/3]: checkout: clear commit marks after detached-orphan check
  [3/3]: checkout: tweak detached-orphan warning format

Looks very sensible, except for the clear_marks(-1) that clears everything
I have a slight doubt about.

I think 3/3 was way overdue and it was my fault. Thanks for cleaning up my
mess.