embed: allow seeking past EOF to match os.File behavior

[dajiaohuang]

Summary

Fixes #81330

embed.FS was returning 'invalid argument' when seeking past EOF, while os.File returns nil (and subsequent Read returns io.EOF).

Problem

Seek in embed.FS checks offset > int64(len(f.f.data)) and returns an error for offsets beyond EOF. This differs from os.File behavior where seeking past EOF is allowed and subsequent reads return io.EOF.

Solution

Remove the upper bound check in Seek, allowing offsets beyond the file size. This matches the behavior of os.File and io.SectionReader.

Changes

src/embed/embed.go line 371:

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

Behavior After Fix

f.Seek(100000, io.SeekStart)  // Returns nil instead of 'invalid argument'
f.Read(buf)                   // Returns io.EOF

This matches os.File, bytes.Reader, and io.SectionReader behavior.

Assisted-by: Claude Opus 4.6

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.