DependencyResolver: offline restore destructively removes committed NuGet DLLs (incomplete keep-closure) + misleading 'All packages up to date'

[IvanMurzak]

Summary

When NuGetPackageRestorer.Restore() runs while package downloads fail (offline / registry unreachable), NuGetPackageInstaller.RemoveUnnecessaryPackages() still executes its removal sweep with an essentially empty InstalledThisSession keep-closure. Result: it deletes committed, previously-valid DLLs from Assets/Plugins/NuGet/ (case 2: "DLLs now provided by Unity ... and package ID not in the closure", NuGetPackageInstaller.cs:293-307) and strips their entries from .nuget-installed.json.

Observed 2026-06-05 in the ai-game-developer-infra Unity-Test-Project testbed (Unity 6000.5.0b10, plugin source at McpPlugin 6.7.0, committed DLLs at 6.5.2/ReflectorNet 5.2.0):

• All 14 Install() calls failed with [NuGet] Failed to download <pkg>: An error occurred while sending the request (cold Library = empty NuGetCache, no fallback).
• The sweep then deleted 14 DLLs (Microsoft.Bcl.AsyncInterfaces, MS.Extensions.*.Abstractions, System.Buffers/Memory/Text.Json/...) that the previous committed state depended on.
• The manifest is left in a state that permanently fails AllPackagesInstalled() -> full restore re-runs on every domain reload (churn), and the deletions get reverted by the first ONLINE restore anyway - so the offline sweep is pure destruction with no steady state.
• The pass ends with [NuGet] All packages up to date. because anyChanged == false after total failure - misleading when 14/14 downloads errored.

Proposed fix

1. Guard the sweep: skip RemoveUnnecessaryPackages (or at minimum its case 2) when any configured top-level Install() failed this session - the keep-closure is incomplete, so removal decisions are unsound.
2. Do not log "All packages up to date." when installs failed; log a clear restore incomplete: N package(s) failed to download warning instead.

Repro

1. Commit a valid Assets/Plugins/NuGet state for plugin version X.
2. Bump plugin source to version Y (newer deps).
3. Delete Library/, block network access to nuget registries, open the project.
4. Observe: compile errors (expected), then the resolver deletes previously-committed DLLs and reports "All packages up to date."

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