String in scalar dataset does not appear to work

[darkstar007]

Hi,

When I try to run this code:

package main

import (
	"fmt"
	"gonum.org/v1/hdf5"
)

func main() {
	hf, err := hdf5.CreateFile("my_string.h5", hdf5.F_ACC_TRUNC)
	if err != nil {
		fmt.Println("Failed to open file", err)
		return
	}

	dSpaceScalar, err := hdf5.CreateDataspace(hdf5.S_SCALAR)
	if err != nil {
		fmt.Println("Failed to create dataset", err)
		return
	}

	dSet, err := hf.CreateDataset("string_dataset", hdf5.T_GO_STRING, dSpaceScalar)
	if err != nil {
		fmt.Println("Failed to create dataset", err)
		return
	}
	someText := "once upon a time"
	err = dSet.Write(someText)
	if err != nil {
		fmt.Println("Failed to write dataset:")
		return
	}
	dSet.Close()
	dSpaceScalar.Close()
	hf.Close()
	fmt.Println("All good. Closed everything")

	return
}

I get this error:

./teststring
panic: reflect.Value.UnsafeAddr of unaddressable value

goroutine 1 [running]:
reflect.Value.UnsafeAddr(...)
reflect/value.go:1972
gonum.org/v1/hdf5.(*Dataset).WriteSubset(0xc000098360, 0x4be3c0, 0xc000098370, 0x0, 0x0, 0x0, 0x0)
gonum.org/v1/hdf5@v0.0.0-20200504100616-496fefe91614/h5d_dataset.go:130 +0x38b
gonum.org/v1/hdf5.(*Dataset).Write(...)
gonum.org/v1/hdf5@v0.0.0-20200504100616-496fefe91614/h5d_dataset.go:154
main.main()

This may be related to this comment in your readme:

Known problems
     support for structs with slices and strings as fields is broken

but this obviously isn't a field of a structure.

Thanks,

Matt

[kortschak]

I'm not sure how to deal with this. If &someText is passed in we can get the address of the reflect value, but then we crash with a SEGentering into the C code a few lines down. It's deeper than that too, this is not just a string issue; if I replace someText with the equivalent byte slice we get the same issues.

/cc @sbinet

[darkstar007]

It may or may not be useful to know, but it seems that writing attributes as strings seems to work OK, only writing them as dataset fails.