code review 11403043: io: let SectionReader seek past the end; document Seeke...

io: let SectionReader seek past the end; document Seeker semantics more

Be consistent with os.File, strings.Reader, bytes.Reader, etc,
which all allow seeks past the end.

[bradfitz, 2013-07-17 00:59:42]

Hello golang-dev@googlegroups.com,

I'd like you to review this change to
https://go.googlecode.com/hg/

[r, 2013-07-17 03:13:39]

What's the back story here? Who needs to seek past the end, and why? Without
further study it feels wrong.

[bradfitz, 2013-07-17 03:19:03]

$ man 2 lseek

       The lseek() function allows the file offset to be set beyond the end
of the file  (but  this  does
       not change the size of the file).  If data is later written at this
point, subsequent reads of the
       data in the gap (a "hole") return null bytes ('\0') until data is
actually written into the gap.

This is what *os.File.Seek calls (syscall.Seek ->  Syscall(SYS_LSEEK,
uintptr(fd), uintptr(offset), uintptr(whence))

And this is the behavior we've imitated in strings.Reader and bytes.Reader
(per the CL description), and in Google-internal io.Seekers.

The only Seeker in the standard library or Google where we're not
consistent is io.SectionReader.

And so I also documented it here, but I could remove that part.



On Wed, Jul 17, 2013 at 1:13 PM, <r@golang.org> wrote:

> What's the back story here? Who needs to seek past the end, and why?
> Without further study it feels wrong.
>
>
>
https://codereview.appspot.**com/11403043/<https://codereview.appspot.com/114...
>

[r, 2013-07-17 04:23:32]

You're answering what I asked but not what I meant to ask. My question is
about SectionReader. The seek docs rae fien.

Why should a section reader be allowed to jump outside the section it's
supposed to be reading? That feels very wrong.

-rob

[r, 2013-07-17 04:23:59]

wow, nice typos. "rea fien" is how a scotsman spells "are fine"

[bradfitz, 2013-07-17 04:43:53]

If you jump past the end of it, it just returns EOF on a subsequent Read.
 You can't cause any harm to yourself or others with it.

This is purely about consistency with everything else (Unix, other Go
types, other Google types).

For *os.File, we do want to allow seeks past the end, since we want to
allow writes past the end.  If we permit it there, and it's permitted
elsewhere, it feels like it should feel the same everywhere.




On Wed, Jul 17, 2013 at 2:23 PM, Rob Pike <r@golang.org> wrote:

> You're answering what I asked but not what I meant to ask. My question is
> about SectionReader. The seek docs rae fien.
>
> Why should a section reader be allowed to jump outside the section it's
> supposed to be reading? That feels very wrong.
>
> -rob
>
>
>
>

[r, 2013-07-17 05:27:57]

https://codereview.appspot.com/11403043/diff/6001/src/pkg/io/io.go
File src/pkg/io/io.go (right):

https://codereview.appspot.com/11403043/diff/6001/src/pkg/io/io.go#newcode97
src/pkg/io/io.go:97: // error.
I'm really not sure about this. I'm definitely not sure it's explained well.
When can an error actually arise? Perhaps it's:

Seeking to a negative offset 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.

[bradfitz, 2013-07-17 08:00:52]

PTAL

I like that wording.




On Wed, Jul 17, 2013 at 3:27 PM, <r@golang.org> wrote:

>
>
https://codereview.appspot.**com/11403043/diff/6001/src/**pkg/io/io.go<https:...
> File src/pkg/io/io.go (right):
>
> https://codereview.appspot.**com/11403043/diff/6001/src/**
>
pkg/io/io.go#newcode97<https://codereview.appspot.com/11403043/diff/6001/src/pkg/io/io.go#newcode97>
> src/pkg/io/io.go:97: // error.
> I'm really not sure about this. I'm definitely not sure it's explained
> well. When can an error actually arise? Perhaps it's:
>
> Seeking to a negative offset 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.
>
>
https://codereview.appspot.**com/11403043/<https://codereview.appspot.com/114...
>

[bradfitz, 2013-07-17 08:00:52]

Hello golang-dev@googlegroups.com, r@golang.org (cc:
golang-dev@googlegroups.com),

Please take another look.

[r, 2013-07-18 02:53:47]

LGTM

[bradfitz, 2013-07-18 03:03:37]

*** Submitted as https://code.google.com/p/go/source/detail?r=f2b6ec47a9e5 ***

io: let SectionReader seek past the end; document Seeker semantics more

Be consistent with os.File, strings.Reader, bytes.Reader, etc,
which all allow seeks past the end.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/11403043