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

$ go version
     go version go1.14.1 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOENV="/root/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/root/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build020686916=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I'm trying to return a tar.Reader from a function, but its Read() method has a strange behavior
if you write a tar using tar.Writer interface it points to a buffer, trying to create a reader with tar.NewReader(buf) gives the correct list of files from tr.Next() method, but its Read() method always points to end of buffer
Here's an example:
https://play.golang.org/p/GnCF_B1TWFE
here error is EOF, although we can iterate over tr.Next() subsequently (aren't readers streams?)

We could make sure this isn't about allocation using ioutil.ReadAll():
https://play.golang.org/p/DLFn_J0pgzN
here error is nil, but the []byte is empty

Things get a little more weird if you edit first example adding a Next() call before calling tr.Read()
https://play.golang.org/p/RYJY1cz02Ms
Now []byte is not empty but instead has the first file content, and read error: EOF 🤯

What did you expect to see?

A filled []byte when read is called

What did you see instead?

An empty []byte in return