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

go1.17

Does this issue reproduce with the latest release?

Yes.

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

N/A

What did you do?

Read https://pkg.go.dev/io@go1.17#Seeker. Compare the behavior it requires to the implementations of that interface in the standard library.

What did you expect to see?

Behavior matching the documentation (emphasis mine).

Seek sets the offset for the next Read or Write to offset, interpreted according to whence: SeekStart means relative to the start of the file, SeekCurrent means relative to the current offset, and SeekEnd means relative to the end. Seek returns the new offset relative to the start of the file and an error, if any.

The use of “and” here implies that implementations must return the current offset regardless of any error.

Seeking to an offset before the start of the file is an error. Seeking to any positive offset is legal, but the behavior of subsequent I/O operations on the underlying object is implementation-dependent.

To me, “is legal” implies “does not result in an error”.

What did you see instead?

Many implementations reject positive offsets that exceed the size of the entity being read:

• debug/elf.readSeekerFromReader:

  go/src/debug/elf/reader.go

  Line 80 in 2ebe77a

  case newOffset < 0, newOffset > r.size:

• embed.openFile:

  go/src/embed/embed.go

  Line 371 in 90c5660

  if offset < 0 || offset > int64(len(f.f.data)) {

• testing/fstest.openMapFile:

  go/src/testing/fstest/mapfs.go

  Line 194 in 213e157

  if offset < 0 || offset > int64(len(f.f.Data)) {

• io.SectionReader has a particularly subtle variation on this. It represents the offset relative to the base of the section being read and fails to detect overflow:

  go/src/io/io.go

  Line 519 in 90c5660

  if offset < s.base {

  • That means that it accepts most positive offsets, but still rejects very large ones (https://play.golang.org/p/Rkk3LNhg129).

It isn't obvious to me whether os.File.Seek ever rejects positive offsets, since it passes the offset through to a platform-specific system call.

On error (for example, when the offset overflows or is otherwise invalid, or when an underlying system call fails), most implementations return 0 instead of “the new offset relative to the start of the file”.

• bytes.Reader:

  go/src/bytes/reader.go

  Line 131 in 2ebe77a

  return 0, errors.New("bytes.Reader.Seek: negative position")

• debug/elf.readSeekerFromReader:

  go/src/debug/elf/reader.go

  Line 81 in 2ebe77a

  return 0, os.ErrInvalid

• embed.openFile:

  go/src/embed/embed.go

  Line 372 in 90c5660

  return 0, &fs.PathError{Op: "seek", Path: f.f.name, Err: fs.ErrInvalid}

• os.File:

  go/src/os/file.go

  Line 243 in a25c584

  return 0, f.wrapErr("seek", e)

• strings.Reader:

  go/src/strings/reader.go

  Line 130 in 2ebe77a

  return 0, errors.New("strings.Reader.Seek: negative position")

• testing/fstest.openMapFile:

  go/src/testing/fstest/mapfs.go

  Line 195 in 213e157

  return 0, &fs.PathError{Op: "seek", Path: f.path, Err: fs.ErrInvalid}