easy way to determine if a stream or fd is seekable

[Alexander Best]

On Thu Nov 17 11, Dieter BSD wrote:
> > lseek() on a tape drive does not return an error, nor does it
> > actually do anything.
> 
> IIRC some tape drives can seek, while others cannot.
> Vague memories that it is supposed to be possible to put a
> filesystem on a DECtape and mount the filesystem.

i took a look at the bsdtar sources and before the lseek() a function called
test_for_append() is run. apart from other things it will do the following:

	if (!S_ISREG(s.st_mode) && !S_ISBLK(s.st_mode))
		lafe_errc(1, 0,
		    "Cannot append to %s: not a regular file.",
		    bsdtar->filename);

but again this is too restrictive. take a look at /dev/zero e.g.:

otaku% ./mode /dev/zero 
Block special file:        0
Character special file:    1
Directory:                 0
Pipe or FIFO special file: 0
Symbolic link:             0
Regular file:              0
Socket:                    0
Whiteout:                  0
Seekable:                  1

it is not regular file and no block special file, so bsdtar will bail out
for no reason!

i think there are two possibilities:

1) add code to lseek() that will return -1 for all tape drives.
2) deal with those tape drives that don't support seeking individually.

identifying a tape drive is easy. here's dd's code snippet:

	if (S_ISCHR(sb.st_mode) || S_ISBLK(sb.st_mode)) {
		if (ioctl(io->fd, FIODTYPE, &type) == -1)
			err(1, "%s", io->name);

		if (type & D_TAPE)
			io->flags |= ISTAPE;
	}

this could be added to lseek() and when "type & D_TAPE" == TRUE, then we could
return -1. this would be method 1). no idea how to fix this issue using method
2) though. probably driving into the driver code of the according tape drives.

cheers.
alex

ps: thanks to nox@ for pointing me to the initial thread, where this issue was
discussed [1]. however the purpose of that thread was mainly fixing
libarchive/bsdtar and not so much really fixing the issue at hand.

also i'll consider re-opening problem report [2], wince this was never really
fixed.

[1] http://markmail.org/message/n6tp5dbv7sbzzujc
[2] http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/60313

> 
> It might be that FreeBSD doesn't currently support seeking
> on a tape, but we shouldn't paint ourselves into a corner
> by assuming that it is fundimentally impossible.