Enable bootstrap flow testing in CI (#15774)

eng/pipelines/templates/jobs/vmr-build.yml

@@ -53,6 +53,7 @@ parameters:
   type: boolean
   default: false
 
+# Use the previous version's SDK to build the current one
 - name: withPreviousSDK
   type: boolean
   default: false
@@ -160,22 +161,28 @@ jobs:
   - script: |
       set -x
 
-      if [[ -z '${{ parameters.reuseBuildArtifactsFrom }}' && '${{ parameters.withPreviousSDK }}' != 'True' ]]; then
-        docker run --rm -v "$(sourcesPath):/vmr" -w /vmr ${{ parameters.container }} ./prep.sh
-      else
+      customPrepArgs=""
+      prepSdk=true
+      if [[ '${{ parameters.withPreviousSDK }}' == 'True' ]]; then
+        # Source-built artifacts are from CentOS 8 Stream. We want to download them without
+        # downloading portable versions from the internet.
+        customPrepArgs="${customPrepArgs} --no-sdk --no-bootstrap"
+        prepSdk=false
+      elif [[ -n '${{ parameters.reuseBuildArtifactsFrom }}' ]]; then
+        customPrepArgs="${customPrepArgs} --no-sdk --no-artifacts"
+        prepSdk=false
+      fi
+      
+      if [[ "$prepSdk" == "false" ]]; then
         mkdir $(sourcesPath)/.dotnet
         previousSdkPath="$(sourcesPath)/prereqs/packages/archive/dotnet-sdk-*.tar.gz"
         eval tar -ozxf "$previousSdkPath" -C "$(sourcesPath)/.dotnet"
         eval rm -f "$previousSdkPath"
 
-        if [[ '${{ parameters.withPreviousSDK }}' == 'True' ]]; then
-          # Source-built artifacts are from CentOS 8 Stream. We want to download them without
-          # downloading portable versions from the internet.
-          docker run --rm -v "$(sourcesPath):/vmr" -w /vmr ${{ parameters.container }} ./prep.sh --no-bootstrap
-        fi
-
         echo "##vso[task.setvariable variable=additionalBuildArgs]--with-sdk /vmr/.dotnet"
       fi
+
+      docker run --rm -v "$(sourcesPath):/vmr" -w /vmr ${{ parameters.container }} ./prep.sh ${customPrepArgs}
     displayName: Prep the Build
 
   - script: |

src/SourceBuild/content/prep.sh

@@ -10,11 +10,17 @@ usage() {
     echo "  Prepares the environment to be built by downloading Private.SourceBuilt.Artifacts.*.tar.gz and"
     echo "  installing the version of dotnet referenced in global.json"
     echo "options:"
-    echo "  --no-bootstrap Don't replace portable packages in the download source-built artifacts"
+    echo "  --no-artifacts    Exclude the download of the previously source-built artifacts archive."
+    echo "  --no-bootstrap    Don't replace portable packages in the download source-built artifacts"
+    echo "  --no-prebuilts    Exclude the download of the prebuilts archive."
+    echo "  --no-sdk          Exclude the download of the .NET SDK."
     echo ""
 }
 
 buildBootstrap=true
+downloadArtifacts=true
+downloadPrebuilts=true
+installDotnet=true
 positional_args=()
 while :; do
     if [ $# -le 0 ]; then
@@ -29,6 +35,15 @@ while :; do
         --no-bootstrap)
             buildBootstrap=false
             ;;
+        --no-artifacts)
+            downloadArtifacts=false
+            ;;
+        --no-prebuilts)
+            downloadPrebuilts=false
+            ;;
+        --no-sdk)
+            installDotnet=false
+            ;;
         *)
             positional_args+=("$1")
             ;;
@@ -37,9 +52,12 @@ while :; do
     shift
 done
 
-downloadArtifacts=true
-downloadPrebuilts=true
-installDotnet=true
+# Attempting to bootstrap without an SDK will fail. So either the --no-sdk flag must be passed
+# or a pre-existing .dotnet SDK directory must exist.
+if [[ "$buildBootstrap" == "true" && "$installDotnet" == "false" && ! -d $SCRIPT_ROOT/.dotnet ]]; then
+    echo "  ERROR: --no-sdk requires --no-bootstrap or a pre-existing .dotnet SDK directory.  Exiting..."
+    exit -1
+fi
 
 # Check to make sure curl exists to download the archive files
 if ! command -v curl &> /dev/null
@@ -51,20 +69,20 @@ fi
 # Check if Private.SourceBuilt artifacts archive exists
 artifactsBaseFileName="Private.SourceBuilt.Artifacts"
 packagesArchiveDir="$SCRIPT_ROOT/prereqs/packages/archive/"
-if [ -f ${packagesArchiveDir}${artifactsBaseFileName}.*.tar.gz ]; then
+if [[ "$downloadArtifacts" == "true" && -f ${packagesArchiveDir}${artifactsBaseFileName}.*.tar.gz ]]; then
     echo "  Private.SourceBuilt.Artifacts.*.tar.gz exists...it will not be downloaded"
     downloadArtifacts=false
 fi
 
 # Check if Private.SourceBuilt prebuilts archive exists
 prebuiltsBaseFileName="Private.SourceBuilt.Prebuilts"
-if [ -f ${packagesArchiveDir}${prebuiltsBaseFileName}.*.tar.gz ]; then
+if [[ "$downloadPrebuilts" == "true" && -f ${packagesArchiveDir}${prebuiltsBaseFileName}.*.tar.gz ]]; then
     echo "  Private.SourceBuilt.Prebuilts.*.tar.gz exists...it will not be downloaded"
     downloadPrebuilts=false
 fi
 
 # Check if dotnet is installed
-if [ -d $SCRIPT_ROOT/.dotnet ]; then
+if [[ "$installDotnet" == "true" && -d $SCRIPT_ROOT/.dotnet ]]; then
     echo "  ./.dotnet SDK directory exists...it will not be installed"
     installDotnet=false;
 fi