This was a most surprising leak. It occurred in the process that is forked
off to feed data to gpg. That process was passed a lazy ByteString of
input, and ghc seemed to not GC the ByteString as it was lazily read
and consumed, so memory slowly leaked as the file was read and passed
through gpg to bup.
To fix it, I simply changed the feeder to take an IO action that returns
the lazy bytestring, and fed the result directly to hPut.
AFAICS, this should change nothing WRT buffering. But somehow it makes
ghc's GC do the right thing. Probably I triggered some weakness in ghc's
GC (version 6.12.1).
(Note that S3 still has this leak, and others too. Fixing it will involve
another dance with the type system.)
Update: One theory I have is that this has something to do with
the forking of the feeder process. Perhaps, when the ByteString
is produced before the fork, ghc decides it need to hold a pointer
to the start of it, for some reason -- maybe it doesn't realize that
it is only used in the forked process.
Forking a new process rather than relying on a thread to feed gpg.
The feeder thread was stalling, probably when the main thread got
to the point it was wait()ing on the gpg to exit.
