When building binary packages, portage pipes tar data through 'bzip2 -f' (without '-c' switch). Although standard bzip2 doesn't seem to have problems with it (pbzip2 has), according to the manual, will of outputting compressed data to STDOUT should be marked by use of '-c' switch. AFAICS similar bzip2 invocations in portage code use the '-c' switch.
Created attachment 197678 [details, diff] Patch to add '-c' switch to bzip2 invocation
I should add that I have used both '-c' and '-f' switches because portage does so in few places. I don't know why or what for…
Thanks, this is in svn r13934.
This is fixed in 2.2_rc37.
Not released in 2.1.*.
This is fixed in 2.1.7.