virtiofs: Fix issue with getdents

[damanm24]

In WSL we have the following testcase which has been failing when running against our virtiofs implementation: https://github.com/microsoft/WSL/blob/116f8d6bf5257b9c5ddef141d4ae6946ef38a4e3/test/linux/unit_tests/lxtfs.c#L807.
The test case does the following:

1. Creates a directory with 500 files.
2. Calls getdents with a buffer of 500 bytes
3. Deletes all the entries returned
4. Return to step 2 until there are no more entries (i.e. all files have been deleted)
5. Check that all files have been deleted

The test case fails in the following manner:
Initial state: 500 files (positions 0-499)
First getdents64: Reads entries 0-99, returns offset 100
Guest deletes files at positions 0-99
Second getdents64 with offset 100: Windows seeks to position 100, but this is now what was originally position 200 (since 100 files were deleted)
Result: Files 100-199 are skipped entirely.

The solution introduced in this PR is to cache 32 dirent entries (which is roughly 4kB worth of data matching what Linux requests). Each entry is assigned a stable offset that doesn't change as files are deleted.

[Copilot]

Pull request overview

This pull request fixes a critical bug in virtiofs directory enumeration where files could be skipped when deleted between getdents calls. It implements a caching mechanism with stable offsets and also fixes a reference counting issue with hard links.

Changes:

• Implements a sliding window cache (32 entries) for directory enumeration with stable offsets that survive file deletions
• Adds inc_lookup() method to VirtioFsInode for incrementing lookup count without path updates
• Fixes hard link reference counting by incrementing lookup count when returning existing inode

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
vm/devices/virtio/virtiofs/src/file.rs	Implements DirEntryCursor with stable offset caching, refactors read_dir to use caching mechanism, adds comprehensive unit tests
vm/devices/virtio/virtiofs/src/inode.rs	Adds inc_lookup() method for manual lookup count increment
vm/devices/virtio/virtiofs/src/lib.rs	Fixes hard link handling to increment lookup count for existing inodes
vm/devices/virtio/virtiofs/Cargo.toml	Adds arrayvec dependency for fixed-size cache
Cargo.lock	Updates lock file for arrayvec dependency

[github-actions]

At least one Petri test failed.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 5 changed files in this pull request and generated no new comments.

[github-actions]

⚠️ Unsafe Code Detected

This PR modifies files containing unsafe Rust code. Extra scrutiny is required during review.

For more on why we check whole files, instead of just diffs, check out the Rustonomicon