When encrypted_data_source::get() caches a trailing block in
_next, the next call takes it directly — bypassing
input_stream::read(), which checks _eof. It then calls
input_stream::read_exactly() on the already-drained stream.
Unlike read(), read_up_to(), and consume(), read_exactly()
does not check _eof when the buffer is empty, so it calls
_fd.get() on a source that already returned EOS.
In production this manifested as stuck encrypted SSTable
component downloads during tablet restore: the underlying
chunked_download_source hung forever on the post-EOS get(),
causing 4 tablets to never complete. The stuck files were
always block-aligned sizes (8k, 12k) where _next gets
populated and the source is fully consumed in the same call.
Fix by checking _input.eof() before calling read_exactly().
When the stream already reached EOF, buf2 is known to be
empty, so the call is skipped entirely.
A comprehensive test is added that uses a strict_memory_source
which fails on post-EOS get(), reproducing the exact code
path that caused the production deadlock.
aa9da87e97