[MicroVM] Update Nanvix to v0.15.4

[ppenna]

📖 Description

Updates the MicroVM (NanVix) backend to Nanvix 0.15.4 and switches the runner to NanVix's warm-start snapshot protocol, dropping the explicit guest Python command line in favor of a multi-binary python3.initrd. Cold-boot latency on the hot path drops by restoring a pre-generated WHP snapshot instead of re-booting the guest on every execution. Also expands the microvm_staging unit-test coverage to lock in the new single-file bootstrap layout and pre-existing path-rewriting / staging behavior.

Binary layout

• Replace the flat python3.12 sidecar with a single python3.initrd multi-binary archive (daemons + CPython) loaded by NanVix at warm start.
• Move kernel.elf into a bin/ subdirectory next to wxc-exec.exe so nanvixd can locate it via -bin-dir as it now expects.
• Add a snapshots/ subdirectory holding the WHP snapshot pair (kernel.vmem + kernel.whp.cbor).
• Refresh versions.json / checksums.json for the new release artifacts.

Build-time changes (nanvix_binaries/build.rs)

• Extend the download/extract path to populate the new bin/ subdir and verify its checksums alongside the flat binaries.
• After download, pre-generate the WHP snapshots locally via a one-shot nanvixd cold boot so even the first runtime execution is warm.
• Remove the legacy partial-binary cleanup since the new layout no longer ships those files.

Runtime changes (wxc_common/src/nanvix_runner.rs)

• Introduce ResolvedPaths and resolve_snapshot_home() that picks $NANVIX_HOME → OS-local data path (%LOCALAPPDATA%\nanvix or ~/.local/share/nanvix) → <exe> as a fallback.
• If no snapshot is present (fresh install or env override), generate one on demand via cold boot; subsequent runs restore from the snapshot.
• Spawn nanvixd with -snapshot snapshots/kernel.whp.cbor -bin-dir <exe>/bin -ramfs … -mount … -- python3.initrd, dropping the old python3.12 -S -B …; PYTHONHOME=/sysroot guest cmdline since warm-start auto-executes the initrd entry point.
• Pipe nanvixd stderr and drain it before wait() to surface guest diagnostics on failure without blocking the child on a full pipe.

Staging changes (wxc_common/src/microvm_staging.rs)

• Drop the separate .mxc-bootstrap.py + .mxc-script.py pair. The warm-start protocol executes a single bootstrap.py automatically, so the staging layer now writes one file consisting of a small sys.argv preamble followed by the (path-rewritten) user script.

Shared helpers (nanvix_common)

• Add BIN_SUBDIR, SNAPSHOTS_SUBDIR, BIN_SUBDIR_FILES, and SNAPSHOT_FILES constants describing the new on-disk layout.
• Add generate_snapshot() (shared by the build script and runtime fallback) and copy_artifacts_to_target() (used by wxc/build.rs to stage flat binaries, bin/, and snapshots/ next to the output exe).
• Add wxc_common → nanvix_common dependency (Windows-only) and simplify wxc/build.rs to delegate to the shared copy helper.

New microvm_staging tests

• Path validation: reject .. traversal components in staged paths.
• Mixed RW + RO staging lands files at the expected guest-relative locations.
• Staging overhead heuristic: near-zero for empty stagings, 30-second cap honored for oversized inputs.
• preserved_path() returns None when nothing was preserved.
• Host→guest path conversion strips trailing slashes and lowercases the drive letter.
• Script rewriting handles escaped backslash literals (C:\\Users\\…) and matches overlapping prefixes longest-first.
• build_guest_path() format check for both rw and ro categories.
• Empty user script produces a bootstrap consisting solely of the preamble.
• RW copy-back round-trips modified deep files and newly created files back to the host.
• sweep_orphaned_staging_dirs() is a no-op on a non-existent root.
• Concurrent stagings under the same root get unique directory names.

🔗 References

Related docs: docs/nanvix-microvm/nanvix.md, docs/nanvix-microvm/nanvix-integration-plan.md.

🔍 Validation

• cargo build --release --features microvm on x86_64-pc-windows-msvc — fetches the 0.15.4 artifacts, verifies checksums, generates the WHP snapshot at build time, and stages bin/ + snapshots/ next to wxc-exec.exe.
• cargo test -p wxc_common — new and existing microvm_staging unit tests pass.
• Manual smoke: examples/01_hello_world.json and a multi-file RW/RO config execute end-to-end on the warm-start path; cold-boot fallback exercised by deleting snapshots/ and rerunning.
• Verified $NANVIX_HOME override and the %LOCALAPPDATA%\nanvix default both resolve correctly and trigger on-demand snapshot generation when missing.

✅ Checklist

• Signed the Contributor License Agreement
• Linked to an issue
• Updated documentation (if applicable)
• Updated Copilot instructions (if build, architecture, or conventions changed)

📋 Issue Type

• Bug fix
• Feature
• Task

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR updates the Windows MicroVM (NanVix) backend to NanVix 0.15.4, switching execution to NanVix’s warm-start snapshot protocol and updating the on-disk artifact layout and staging layer accordingly.

Changes:

• Update NanVix artifact layout to use python3.initrd, add bin/ (for kernel.elf) and snapshots/, and refresh pinned versions/checksums.
• Generate WHP snapshots at build time (and on-demand at runtime if missing) and invoke nanvixd using -snapshot ... -bin-dir ... -- python3.initrd.
• Simplify MicroVM staging to produce a single bootstrap.py (preamble + rewritten user script) and expand unit tests.

Show a summary per file

File	Description
src/wxc/build.rs	Switch to shared helper for copying NanVix artifacts (flat + bin/ + snapshots/) next to wxc-exec.exe.
src/wxc_common/src/nanvix_runner.rs	Add snapshot-home resolution, warm-start invocation, and stderr piping/draining.
src/wxc_common/src/microvm_staging.rs	Replace two-file staging (.mxc-*) with single bootstrap.py; add extensive new tests.
src/wxc_common/Cargo.toml	Add nanvix_common dependency (Windows-only) for shared constants/helpers.
src/nanvix_common/src/lib.rs	Introduce new layout constants, snapshot generation helper, and artifact copy helper.
src/nanvix_binaries/build.rs	Update download/extract for new layout and generate snapshots at build time.
src/nanvix_binaries/versions.json	Bump NanVix release tag and update expected binary list.
src/nanvix_binaries/checksums.json	Refresh SHA256 hashes (including kernel.elf).
src/Cargo.lock	Add nanvix_common to the lockfile dependency graph.

Copilot's findings

• Files reviewed: 8/9 changed files
• Comments generated: 6