It doesn't actually say in the docs that you MUST set ExpectContinueTimeout in order for the Expect: 100-continue mechanism to work. See later for test programs which make me think this is so.

Here are what the docs say at the moment.

    // ExpectContinueTimeout, if non-zero, specifies the amount of
    // time to wait for a server's first response headers after fully
    // writing the request headers if the request has an
    // "Expect: 100-continue" header. Zero means no timeout.
    // This time does not include the time to send the request header.
    ExpectContinueTimeout time.Duration

What version of Go are you using (go version)?

go version devel +3c6b668 Thu Jun 2 00:22:03 2016 +0000 linux/amd64

What operating system and processor architecture are you using?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/ncw/Code/Go"
GORACE=""
GOROOT="/home/ncw/Code/go"
GOTOOLDIR="/home/ncw/Code/go/pkg/tool/linux_amd64"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build375906555=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"

What did you do?
What did you expect to see?
What did you see instead?

Here are some examples demonstrating that you need to set ExpectContinueTimeout

• ExpectContinueTimeout set
  • reads a single byte from the reader (shouldn't really read any - see net/http: client Expect: 100-continue reads first byte of input on failure #16002)
• ExpectContinueTimeout unset
  • reads the whole reader