Reading from multiple segments doesn't work

[recht]

I have an idl like this:

using Go = import "/go.capnp";

@0xb87bcd826779968d;
$Go.package("test");
struct Document {
  assoc @0 :List(Assoc);

  struct Assoc {
    kind @0 :UInt32;
  }
}

struct GeneratorData {
  documents @0 :List(Document);
}

Given this test:

func TestMulti(t *testing.T) {
	a := capnp.MultiSegment(nil)
	msg, seg, err := capnp.NewMessage(a)
	require.NoError(t, err)

	root, err := NewRootGeneratorData(seg)
	require.NoError(t, err)

	count := 10000
	docs, err := root.NewDocuments(int32(count))
	require.NoError(t, err)

	for i := 0; i < count; i++ {
		d := docs.At(i)
		_, err := d.NewAssoc(100)
		require.NoError(t, err)
	}

	root, err = ReadRootGeneratorData(msg)
	require.NoError(t, err)

	_, err = root.Documents()
	require.NoError(t, err)
}

There will be an error on the last line where it tries to read out the value. It seems that this happens when objects cross segments somehow?

I have my own MultiSegment implementation, which I assumed was the issue, but it looks as if there's a problem in general?

[zombiezen]

Very possible/probable. The multi-segment codepaths aren't as well-tested as single-segment. I'll take a look.

[zombiezen]

@recht, I'm unable to reproduce a failure using the test case you gave me. Can you give me what error message you saw on the last line? Please also include output from go env GOOS GOARCH and go version.

[recht]

--- FAIL: TestMulti (0.91s)
	Error Trace:	capnp_store_test.go:296_memorystore
	Error:		Received unexpected error capnp: invalid pointer address

My go env:

OARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/joakim/gocode"
GORACE=""
GOROOT="/pack/go"
GOTOOLDIR="/pack/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build105259280=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
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"

And go version: go version go1.9.1 linux/amd64

[zombiezen]

While doing some entirely unrelated work, I managed to reproduce this issue and I have a smaller test case that I can dig into. I'll work on a fix.