Error doing multipart upload

[lmars]

I am getting the following error trying to upload a large (~1GB) box:

==> virtualbox-iso: Running post-processor: vagrant-s3
==> virtualbox-iso (vagrant-s3): Preparing to upload box for 'virtualbox' provider to S3 bucket 'vagrant.lmars.net'
    virtualbox-iso (vagrant-s3): Box to upload: packer_virtualbox-iso_virtualbox.box (1060157666 bytes)
    virtualbox-iso (vagrant-s3): Fetching latest manifest
    virtualbox-iso (vagrant-s3): Generating checksum
    virtualbox-iso (vagrant-s3): Checksum is fe24c05d9aac09a1d2f81965a3c25e01519192091c6f233a3f160a0a898dae48
    virtualbox-iso (vagrant-s3): Adding virtualbox 20141008.0 box to manifest
    virtualbox-iso (vagrant-s3): Uploading box to S3: vagrant/boxes/20141008.0/packer_virtualbox-iso_virtualbox.box
    virtualbox-iso (vagrant-s3): File size > 100MB. Initiating multipart upload
Build 'virtualbox-iso' errored: 1 error(s) occurred:

* Post-processor failed: Your socket connection to the server was not read from or written to within the timeout period. Idle connections will be closed.

[lmars]

The underlying error response from S3 has code RequestTimeout, so I tried making that a retriable error (i.e. here) which worked 90% of the time, but then the other 10% I got another error:

Put [URL]: http: Request.ContentLength=5242880 with Body length 5238784

AFAICS when retrying in putPart, the reader is seeked back to the start here so I'm not sure why it is getting the content length wrong in a small number of cases.

[Jalle19]

Would probably be fixed if #25 gets fixed, I had a quick look at the docs and it seems the SDK has a built-in fairly sophisticated file uploader.