It is a bug for tar to enter an infinite loop writing an ever expanding file of trailing zeroes because the current file was truncated to a size smaller than it

It is a bug for tar to enter an infinite loop writing an ever expanding file of trailing zeroes because the current file was truncated to a size smaller than its stat returned. It's basically a classic TOCTOU race, tar needs to honor the EOF returned from read() as authoritative. This is actually a very common trap for junior *nix programmers; treating stat() and a subsequent mmap() or read() loop as if they were atomic with regards to stat.st_size. The size returned by stat() can be used as an estimate, but otherwise can't be applied to subsequent operations.