Please answer these questions before submitting your issue. Thanks!

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

go version go1.9.2 darwin/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/pstuart/go"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/qt/_9gpg7kx2zn0180w_tftkqgh0000gn/T/go-build893477021=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

What did you do?

Used a bufio reader to read small chunks from a large io stream

Example here: https://play.golang.org/p/ufs826X0l7

What did you expect to see?

I expected the read to fill my buffer completely

What did you see instead?

I only saw what was left in the buffer

This is fixed by adding the following at line 222 of bufio.go:

    // refill buffer if needed
    if len(p) > b.w-b.r {
            // Slide existing data to beginning.
            if b.r > 0 {
                    copy(b.buf, b.buf[b.r:b.w])
                    b.w -= b.r
                    b.r = 0
            }

            n, err := b.rd.Read(b.buf[b.w:])
            if n < 0 {
                    panic(errNegativeRead)
            }
            b.w += n
            if err != nil {
                    b.err = err
            }
    }