bin/108990: incorrect error handling in tar(1) for files that change size

[Tim Kientzle]

Ade Lovett wrote:
> Synopsis: incorrect error handling in tar(1) for files that change size
> 
> http://www.freebsd.org/cgi/query-pr.cgi?pr=108990

This is caused by a recent fix to libarchive, which no longer
silently consumes data written past the end of the entry.

I think the patch presented in the PR is acceptable,
but I like this better.  Any concerns?

Index: write.c
===================================================================
--- write.c     (revision 120)
+++ write.c     (working copy)
@@ -851,11 +851,19 @@
         bytes_read = read(fd, buff, sizeof(buff));
         while (bytes_read > 0) {
                 bytes_written = archive_write_data(a, buff, bytes_read);
-               if (bytes_written <= 0) {
+               if (bytes_written < 0) {
                         /* Write failed; this is bad */
                         bsdtar_warnc(bsdtar, 0, "%s", 
archive_error_string(a));
                         return (-1);
                 }
+               if (bytes_written < bytes_read) {
+                       /* Write was truncated; warn but continue. */
+                       bsdtar_warnc(bsdtar, 0,
+                           "Truncated write; file may have grown while 
being archived.");
+                       /* Make bsdtar return a final error. */
+                       bsdtar->return_value = 1;
+                       return (0);
+               }
                 bytes_read = read(fd, buff, sizeof(buff));
         }
         return 0;