### Please describe the problem.

Uploading a 21GB file to an S3 special remote fails. It will generally fail somewhere at about 3-15%. I am using the new chunking feature, with chunks set to 25MiB.

### What steps will reproduce the problem?

    $ git annex copy my-big-file.tar.bz --to s3
    copy my-big-file.tar.bz (gpg) (checking s3...) (to s3...)
    13%       863.8KB/s 6h0m
      ErrorClosed
    failed
    git-annex: copy: 1 failed

### What version of git-annex are you using? On what operating system?

Running on Arch Linux.

    git-annex version: 5.20140818-g10bf03a
    build flags: Assistant Webapp Webapp-secure Pairing Testsuite S3 WebDAV Inotify DBus DesktopNotify XMPP DNS Feeds Quvi TDFA CryptoHash
    key/value backends: SHA256E SHA1E SHA512E SHA224E SHA384E SKEIN256E SKEIN512E SHA256 SHA1 SHA512 SHA224 SHA384 SKEIN256 SKEIN512 WORM URL
    remote types: git gcrypt S3 bup directory rsync web webdav tahoe glacier ddar hook external
    local repository version: 5
    supported repository version: 5
    upgrade supported from repository versions: 0 1 2 4

### Please provide any additional information below.

If I fire up the web app and open the log, the end looks like this:


[[!format sh """
...

3%       857.3KB/s 6h46m
3%       857.3KB/s 6h46m
3%       857.3KB/s 6h46m
3%       857.4KB/s 6h46m
3%       857.4KB/s 6h46m
3%       857.5KB/s 6h46m
3%       857.5KB/s 6h46m
3%       857.6KB/s 6h46m
3%       857.6KB/s 6h46m
3%       857.6KB/s 6h46m
3%       857.7KB/s 6h46m
3%       857.7KB/s 6h46m
3%       857.8KB/s 6h46m
3%       857.8KB/s 6h46m
3%       857.8KB/s 6h46m
3%       857.9KB/s 6h46m
3%       857.9KB/s 6h46m
3%       858.0KB/s 6h46m
3%       858.0KB/s 6h46m
3%       858.1KB/s 6h46m
3%       858.1KB/s 6h45m
3%       858.1KB/s 6h45mmux_client_request_session: read from master failed: Broken pipe

"""]]
