Tolerate decommissioned package sources during restore

[akoeplinger]

Ports the mitigation from #17099 / #17118 to main.

Problem

Servicing builds can fail when a transport feed referenced in NuGet.config has been decommissioned after a release (e.g. dotnet/runtime#129694, the darc-pub-dotnet-emsdk feed). NuGet aborts the whole restore when any configured source is unavailable.

Changes

• ProjectDefaults.props: default RestoreIgnoreFailedSources=true (conditional, overridable) so every project's MSBuild-based restore falls back to the remaining sources instead of failing. This is the same change as [release/10.0] Tolerate decommissioned package sources during restore #17118.
• tools.ps1 / tools.sh: on main, InitializeToolset was refactored to acquire the Arcade SDK via dotnet package download instead of an MSBuild restore: Use dotnet package download for toolset initialization dotnet#5909. That command does not honor RestoreIgnoreFailedSources and hard-fails on a dead source, so [release/10.0] Tolerate decommissioned package sources during restore #17118's /p:RestoreIgnoreFailedSources=true argument has no equivalent here. Instead, when the config-driven download fails, retry once against the public dotnet-eng feed directly (via --source, which ignores the other sources) — the Arcade SDK is always published there for public builds. Implemented through a new ignore-failure parameter on the DotNet helper so the primary attempt can fall back instead of terminating the script.

Notes / limitations

• Internal official builds whose Arcade SDK isn't on the public feed would still fail if their internal feed is the one that's down — unchanged from today (no regression), since a public fallback can't serve an internal-only package.
• Verified: with runtime's real v9.0.11 NuGet.config (no source mapping, dead transport feed listed first), the config-driven download fails and the dotnet-eng fallback successfully downloads the Arcade SDK.

[Copilot]

Pull request overview

This PR mitigates build/restore failures caused by decommissioned or unavailable NuGet sources by making restores tolerate failed sources by default and adding a fallback path for acquiring the Arcade SDK when dotnet package download fails due to a dead feed.

Changes:

• Default RestoreIgnoreFailedSources=true (overridable) for MSBuild-based restores via Arcade SDK defaults.
• Add retry logic in InitializeToolset to re-attempt Arcade SDK acquisition from the public dotnet-eng feed when config-driven download fails.
• Extend the DotNet helper wrappers to optionally return failure to callers (to enable fallback logic) instead of terminating immediately.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/Microsoft.DotNet.Arcade.Sdk/tools/ProjectDefaults.props	Defaults restores to ignore failed sources so decommissioned feeds don’t hard-fail restore.
eng/common/tools.sh	Adds fallback download from dotnet-eng when dotnet package download fails due to an unavailable configured source.
eng/common/tools.ps1	Adds the same fallback logic and introduces an ignore-failure mode in the DotNet wrapper (currently with blocking issues noted in PR comments).

Comments suppressed due to low confidence (1)

eng/common/tools.ps1:866

• The new DotNet([switch]$ignoreFailure) signature makes the first positional argument bind to ignoreFailure, which will shift/lose the first dotnet argument (e.g. package) and can unintentionally flip the failure behavior. Make ignoreFailure a named-only switch by adding a positional dotnetArgs parameter that captures the remaining arguments, and iterate that parameter instead of $args.

function DotNet([switch]$ignoreFailure) {
  $dotnetRoot = InitializeDotNetCli -install:$restore
  $dotnetPath = Join-Path $dotnetRoot (GetExecutableFileName 'dotnet')

  $cmdArgs = ""
  foreach ($arg in $args) {

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[mmitche]

@copilot Create an issue in dotnet/sdk to support skipping unavailable sources when downloading a package.