client/file: *File.ReadAt doesn't return io.EOF when file is read to the end

[rschio]

*File.ReadAt() returns nil error when the file is read to the end and doesn't completly fill the []byte it is reading to.

I expected to see io.EOF.

According to the io.ReaderAt interface:

When ReadAt returns n < len(p), it returns a non-nil error
explaining why more bytes were not returned. In this respect,
ReadAt is stricter than Read.

Here is a test explaning better:

package main

import (
	"io"
	"log"
	"testing"

	"upspin.io/client"
	"upspin.io/config"
	"upspin.io/path"
	"upspin.io/transports"
	"upspin.io/upbox"
	"upspin.io/upspin"
)

func TestReadAt(t *testing.T) {
	c, cfg, cleanup := newClient()
	t.Cleanup(cleanup)

	root := upspin.PathName(cfg.UserName())
	if _, err := c.MakeDirectory(root); err != nil {
		t.Fatalf("failed to create root dir: %v", err)
	}

	name := path.Join(root, "file.txt")
	data := []byte("hello")
	if _, err := c.Put(name, data); err != nil {
		t.Fatalf("failed to create file.txt: %v", err)
	}

	f, err := c.Open(name)
	if err != nil {
		t.Fatalf("failed to open file: %v", err)
	}
	defer f.Close()

	// When ReadAt returns n < len(p), it returns a non-nil error
	// explaining why more bytes were not returned. In this respect,
	// ReadAt is stricter than Read.
	p := make([]byte, len(data))
	n, err := f.ReadAt(p, 1)
	if n < len(p) && err == nil {
		t.Fatalf("should return (%d, io.EOF), got (%d, %v)", len(data)-1, n, err)
	}
}

const schema = `
users:
  - name: user

servers:
  - name: keyserver
  - name: storeserver
  - name: dirserver

domain: example.com
`

func newClient() (upspin.Client, upspin.Config, func()) {
	sc, err := upbox.SchemaFromYAML(schema)
	if err != nil {
		log.Fatalf("failed to parse schema: %v", err)
	}

	if err := sc.Start(); err != nil {
		log.Fatalf("failed to start schema: %v", err)
	}
	cleanup := func() { sc.Stop() }

	cfg, err := config.FromFile(sc.Config(sc.Users[0].Name))
	if err != nil {
		cleanup()
		log.Fatalf("failed to parse config: %v", err)
	}

	transports.Init(cfg)
	c := client.New(cfg)

	return c, cfg, cleanup
}

[n2vi]

Rodrigo,

Given my inexperience with our new review system, it seems I can't add you as an official reviewer for #655 but I would welcome your comments there before I ask Rob to review.

Thanks, Eric