net: no way to use sendfile with offset and count

[jbardin]

A go TCP server can efficiently server files to clients using the ReadFrom method, but
if the protocol allows partial files, e.g. HTTP Range requests, we have to fallback to
copying all the data again for each connection.

Currently sendfile is only accessible by calling *TCPConn.ReadFrom(r io.Reader), with no
way to set the offset and count. Internally, sendfile uses to the netFD writelock and
file descriptor. Doing this outside of the net package is awkward and requires putting
the connection into blocking mode.

One option is to special case an io.SectionReader in the sendfile implementation, though
this requires modifying SectionReader in a way to expose the underlying io.Reader,
offset, and count fields.

[ianlancetaylor]

Comment 1:

Labels changed: added repo-main, release-go1.5.

[rsc]

Comment 2:

There is a way.
f, _ := os.Open(filename)
f.Seek(offset, 0)
c.ReadFrom(io.LimitReader(f, count))

Status changed to Invalid.

[jbardin]

Comment 3:

That doesn't currently work either, because an io.LimitedReader isn't an *io.File, and
fails the type assertion in sendfile_linux.go. It would be nice if that worked too.
Using Seek though still doesn't quite cover the sendfile api. If you call sendfile with
an offset, it does not modify the current file offset.

[jbardin]

Comment 4:

(Sorry, I missed the re-assignment of r from the LimitedReader)
Though using Seek though still doesn't quite cover the sendfile api. If you call
sendfile with an offset, it does not modify the current file offset.

[rsc]

Comment 5:

We're not trying to cover the sendfile API. We're trying to make it possible to write
meaningful portable code that uses the sendfile optimization automatically. And we have.
ReadFrom changes the file offset. You can't avoid that and still have ReadFrom. So if
you really can't modify the file offset you're asking us to introduce a completely
different way to get to sendfile on top of what's already there. And I'd really rather
not. It's easy to open a file multiple times.