Fix nocache short-read tail initialization

[XavLimSG]

Fixes #59

Summary

In -o nocache mode, Read() can get a short read from Reader::Read when hitting sparse regions. The code shrinks dst to the unread tail and adds dst.size() to the return length, but the std::ranges::fill(dst, '\0') line that should zero-fill that tail is currently commented out. In release builds (no assert), this means uninitialized buffer contents get returned to the caller.

Changes

• fuse-archive.cc: Uncomment std::ranges::fill(dst, '\0') so unread tail bytes are explicitly zeroed before the return length is inflated.
• test/test.py: Add TestNocacheSparseReadIsZeroFilled regression test that generates a sparse tar archive, mounts with -o nocache,direct_io, reads the sparse region at multiple offsets/sizes, and asserts all returned bytes are NUL.

How to test

The new test covers the exact scenario. Run the existing test suite:

python3 test/test.py

[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.