Enable parallel builds across repos of VMR (#18824)

src/SourceBuild/content/Directory.Build.props

@@ -146,7 +146,6 @@
     <EnableDefaultItems Condition="'$(MSBuildProjectExtension)' == '.proj'">false</EnableDefaultItems>
 
     <LogVerbosity Condition="'$(LogVerbosity)'==''">minimal</LogVerbosity>
-    <BuildInParallel Condition="'$(BuildInParallel)'==''">false</BuildInParallel>
 
     <ShellExtension Condition="'$(BuildOS)' == 'windows'">.cmd</ShellExtension>
     <ShellExtension Condition="'$(BuildOS)' != 'windows'">.sh</ShellExtension>

src/SourceBuild/content/repo-projects/Directory.Build.props

@@ -5,6 +5,12 @@
   <PropertyGroup>
     <RepositoryName>$(MSBuildProjectName)</RepositoryName>
     <GitInfoRepoPropsFile>$(GitInfoDir)$(RepositoryName).props</GitInfoRepoPropsFile>
+    <BuildInParallel Condition="'$(BuildInParallel)' == ''">true</BuildInParallel>
+    <RunEachTargetSeparately>false</RunEachTargetSeparately>
+    <!-- If this is a dev build with parallelism enabled, ensure that RunEachTargetSeparately is true so that StopOnFirstFailure will be honored.
+         This improves the dev experience because it will fail the build as soon as an error occurs rather than waiting for all parallel projects
+         to finish. This isn't enabled in CI builds because it has a slight perf impact. -->
+    <RunEachTargetSeparately Condition="'$(BuildInParallel)' == 'true' and '$(ContinuousIntegrationBuild)' != 'true'">true</RunEachTargetSeparately>
   </PropertyGroup>
 
   <Import Project="$(GitInfoRepoPropsFile)" Condition="Exists('$(GitInfoRepoPropsFile)')" />

src/SourceBuild/content/repo-projects/Directory.Build.targets

@@ -297,7 +297,8 @@
     <MSBuild Projects="@(_DependentProject)"
              Targets="Build"
              BuildInParallel="$(BuildInParallel)"
-             StopOnFirstFailure="true" />
+             StopOnFirstFailure="true"
+             RunEachTargetSeparately="$(RunEachTargetSeparately)" />
   </Target>
 
   <Target Name="SetSourceBuiltSdkOverrides"
@@ -460,9 +461,13 @@
       <_InnerPackageCacheFiles Include="$(ProjectDirectory)artifacts/sb/package-cache/**/*" />
     </ItemGroup>
 
+    <!-- Retries can be necessary due to file locking when parallel builds are enabled and multiple repos
+         are trying to write their restored packages to this location.
+         See https://github.com/dotnet/installer/pull/18824#issuecomment-1986371287 -->
     <Copy SourceFiles="@(_InnerPackageCacheFiles)"
           DestinationFiles="$(NuGetPackageRoot)%(RecursiveDir)%(Filename)%(Extension)"
           SkipUnchangedFiles="true"
+          Retries="5"
           Condition="'@(_InnerPackageCacheFiles)' != ''" />
   </Target>