"tar tfv /dev/cd0" speedup patch

[Juergen Lock]

On Wed, Feb 17, 2010 at 10:38:30PM -0800, Tim Kientzle wrote:
> Juergen Lock wrote:
> > 
> >  ...  since bsdtar/libarchive know iso9660 I just did the command in the
> > Subject.  It worked, but it was sloow... :(  Apparently it read all of
> > the disc without seeking.  The following patch fixes this, is something
> > like this desired?  If yes I could look how to do the same for Linux,
> 
> Juergen,
> 
> This is great!  If you can figure out how to get this
> right, I would really appreciate it.  If you have a
> tape drive handy, definitely test with that.  My first
> attempts here actually broke reading from tape drives,
> which is why the current code is so conservative.
> 
Hmm I can't test on a tape atm but if I look at the kernel,
	http://fxr.watson.org/fxr/ident?i=DIOCGMEDIASIZE
DIOCGMEDIASIZE is only handled for geom, xen block devices, old CD drives
and pc98 floppies, and I'm pretty sure tapes don't use geom. :)

> Minor style comments:
> >  else if (S_ISCHR(st.st_mode) &&
> >     !ioctl(fd, DIOCGMEDIASIZE, &mediasize) && mediasize) {
> 
> Please be explicit:  S_ISCHR() && ioctl() == 0  && mediasize > 0
> 
> > 	archive_read_extract_set_skip_file(a, st.st_dev, st.st_ino);
> 
> extract_skip_file isn't needed here; we don't read the
> contents of device nodes.
> 
> Let me know as soon as you have something you're confident of.

 Ok here is a new version of the patch with these things fixed and the
Linux case added:  (Linux case not tested yet, and yes I did this on
stable/8.)

Index: src/lib/libarchive/archive_read_open_filename.c
===================================================================
RCS file: /home/scvs/src/lib/libarchive/archive_read_open_filename.c,v
retrieving revision 1.25.2.1
diff -u -p -r1.25.2.1 archive_read_open_filename.c
--- src/lib/libarchive/archive_read_open_filename.c	3 Aug 2009 08:13:06 -0000	1.25.2.1
+++ src/lib/libarchive/archive_read_open_filename.c	18 Feb 2010 18:14:16 -0000
@@ -44,6 +44,10 @@ __FBSDID("$FreeBSD: src/lib/libarchive/a
 #ifdef HAVE_UNISTD_H
 #include <unistd.h>
 #endif
+#ifdef __FreeBSD__
+#include <sys/ioctl.h>
+#include <sys/disk.h>
+#endif
 
 #include "archive.h"
 
@@ -83,6 +87,9 @@ archive_read_open_filename(struct archiv
 	struct read_file_data *mine;
 	void *b;
 	int fd;
+#ifdef __FreeBSD__
+	off_t mediasize = 0;
+#endif
 
 	archive_clear_error(a);
 	if (filename == NULL || filename[0] == '\0') {
@@ -143,6 +150,27 @@ archive_read_open_filename(struct archiv
 		 */
 		mine->can_skip = 1;
 	}
+#ifdef __FreeBSD__
+	/*
+	 * on FreeBSD if a device supports the DIOCGMEDIASIZE ioctl
+	 * it is a disk-like device and should be seekable.
+	 */
+	else if (S_ISCHR(st.st_mode) &&
+	    ioctl(fd, DIOCGMEDIASIZE, &mediasize) == 0 && mediasize > 0) {
+		mine->can_skip = 1;
+	}
+#endif
+#ifdef __linux__
+	/*
+	 * on Linux just check whether its a block device and that
+	 * lseek works.  (Tapes are character devices there.)
+	 */
+	else if (S_ISBLK(st.st_mode) &&
+	    lseek(fd, 0, SEEK_CUR) == 0 && lseek(fd, 0, SEEK_SET) == 0 &&
+	    lseek(fd, 0, SEEK_END) > 0 && lseek(fd, 0, SEEK_SET) == 0) {
+		mine->can_skip = 1;
+        }
+#endif
 	return (archive_read_open2(a, mine,
 		NULL, file_read, file_skip, file_close));
 }