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retain the last file handle even if it's an IO reference
It's always been possible to extract a reference to the IO SV from a glob, and use that for I/O, but such use would set PL_last_in_gv to a temporary GV which would almost immediately be released, clearing PL_last_in_gv.
This would result in reads from $. returning its most recently read
value (even from another handle) and modification being ignored, while
${^LAST_FH} would return undef, even though the file handle is still
open.
To fix this, store the underlying IO object as well as the GV.
Retaining the GV lets errors continue to report the GV name where
possible, while the IO object means $. and ${^LAST_FH} can still work
if the IO object still exists.
This does not try to fix the problem where localization can leave PL_last_in_gv (and now PL_last_in_io) pointing at a SV that has been destroyed, the original changes that introduced PL_last_in_gv didn't keep a reference count for the GV, since this would prevent the file being close automatically when the reference to to the glob went out of scope.
Fixes #1420
My gut feeling tells me this will either fix a lot of unexplored format/write problems too. OTOH it might also cause some new :)
Anyway this is beyond my area of expertise to give a +1 or -1 on the code as shown.
@tonycoz this seems reasonable, but stalled and in conflict now. Do you want to pick it up? Should I try to resolve conflicts on it?
I think I left it draft (it's been a while) because of that last problem - IIRC it was PL_last_in_gv wasn't being reference counted when pushed to the save stack, so we could be restoring a freed GV to PL_last_in_gv when the save stack was popped.
I suspect the fix is to increment the reference count when saving, and decrement when restoring, setting PL_last_in_gv to NULL if the reference count is zero.
That could result in a user visible change of behaviour though, and might need a custom save type.
Feel free to pick it up.