Prepare source-build for prebuiltless build

eng/Versions.props

@@ -159,6 +159,19 @@
          removed.  See https://github.com/dotnet/source-build/issues/2295 -->
     <MicrosoftBuildFrameworkVersion>15.7.179</MicrosoftBuildFrameworkVersion>
     <MicrosoftBuildUtilitiesCoreVersion>15.7.179</MicrosoftBuildUtilitiesCoreVersion>
+    <!--
+      Building .NET from source depends on one or two tar.gz files depending on the branch's current
+      source-buildability status.
+
+      PrivateSourceBuiltArtifactsPackageVersion is a tar.gz of .NET build outputs from a previous
+      build needed to build the current version of .NET. This is always defined, because .NET needs
+      to be bootstrappable at any point in time.
+
+      PrivateSourceBuiltPrebuiltsPackageVersion is a tar.gz of assets downloaded from the internet
+      that are needed to build the current version of .NET. Early in the lifecycle of a .NET major
+      or minor release, prebuilts may be needed. When the release is mature, prebuilts are not
+      necessary, and this property is removed from the file.
+    -->
     <PrivateSourceBuiltArtifactsPackageVersion>0.1.0-6.0.100-bootstrap.29</PrivateSourceBuiltArtifactsPackageVersion>
     <PrivateSourceBuiltPrebuiltsPackageVersion>0.1.0-6.0.100-36</PrivateSourceBuiltPrebuiltsPackageVersion>
   </PropertyGroup>

src/SourceBuild/Arcade/tools/SourceBuildArcadeTarball.targets

@@ -292,7 +292,9 @@
 
     <ItemGroup>
       <ArtifactUrls Include="$(SourceBuiltArtifactsTarballUrl)$(SourceBuiltArtifactsTarballName).$(PrivateSourceBuiltArtifactsPackageVersion).tar.gz" />
-      <ArtifactUrls Include="$(SourceBuiltArtifactsTarballUrl)$(SourceBuiltPrebuiltsTarballName).$(PrivateSourceBuiltPrebuiltsPackageVersion).tar.gz" />
+      <ArtifactUrls
+        Include="$(SourceBuiltArtifactsTarballUrl)$(SourceBuiltPrebuiltsTarballName).$(PrivateSourceBuiltPrebuiltsPackageVersion).tar.gz"
+        Condition="'$(PrivateSourceBuiltPrebuiltsPackageVersion)' != ''" />
     </ItemGroup>
 
     <WriteLinesToFile

src/SourceBuild/tarball/content/build.proj

@@ -133,7 +133,8 @@
   </Target>
 
   <Target Name="CreatePrebuiltsTarball"
-          AfterTargets="Build">
+          AfterTargets="Build"
+          Condition="Exists('$(ResultingPrebuiltPackagesDir)')">
 
     <PropertyGroup>
       <TarballFileVersion>$(PrivateSourceBuiltPrebuiltsPackageVersionPrefix)$([MSBuild]::Add($(PrivateSourceBuiltPrebuiltsPackageVersionSuffix), 1))</TarballFileVersion>

src/SourceBuild/tarball/content/tools-local/init-build.proj

@@ -46,7 +46,14 @@
           WorkingDirectory="$(PrebuiltSourceBuiltPackagesPath)"
           Condition="'$(CustomPrebuiltSourceBuiltPackagesPath)' == ''" />
 
-    <Exec Command="tar -xzf $(ExternalTarballsDir)$(SourceBuiltPrebuiltsTarballName).*.tar.gz"
+    <!--
+      Check for a prebuilt dependency tarball and extract if exists. If there isn't one, we expect
+      the build to be working without prebuilts.
+    -->
+    <ItemGroup>
+      <SourceBuiltPrebuiltsTarballFile Include="$(ExternalTarballsDir)$(SourceBuiltPrebuiltsTarballName).*.tar.gz" />
+    </ItemGroup>
+    <Exec Command="tar -xzf %(SourceBuiltPrebuiltsTarballFile.FullPath)"
           WorkingDirectory="$(PrebuiltPackagesPath)" />
 
     <!-- Move SBRP packages to reference packages location -->