Provide default JVM version to be used by Mill server process, when `…

….mill-jvm-version` and system `java` are not provided (#4597)

This PR makes Mill's server use a fixed JVM version by default in the
case where no other Java version is available:

1. No `.mill-jvm-version` file present
2. No `java` command available environmentally
3. The Mill client has no `java.home` set (i.e. it is a graal native
image)

This makes running Mill on a clean environment without a JVM installed
work by default. Previously, it either needed you to define a
`.mill-jvm-version` or install a JVM, either of which is a manual step
we now can now avoid. Now it instead uses the version of Java configured
in Mill's own `.mill-jvm-version` file

We still allow any system `java` command to take precedence over the
default, since that's what most people would expect when using the JVM.

Added an integration test in CI `no-jvm-bootstrap` that runs Mill with
the native launcher with no `.mill-jvm-version` file and no system
`java` installed to verify that it falls back to the Mill default JVM
version. Needed to add some hacky test hooks to make sure we exercise
the code path in question

.github/workflows/post-build-selective.yml

@@ -11,7 +11,8 @@ on:
         type: string
 
       coursierarchive:
-        default: ''
+        default: "/tmp"
+        required: false
         type: string
       java-version:
         required: true

.github/workflows/publish-artifacts.yml

@@ -27,22 +27,22 @@ jobs:
       matrix:
         include:
         - os: ubuntu-latest
-          coursierarchive: ""
+          coursierarchive: "/tmp"
           publishartifacts: __.publishArtifacts
           uploadgithub: true
 
         - os: ubuntu-24.04-arm
-          coursierarchive: ""
+          coursierarchive: "/tmp"
           publishartifacts: dist.native.publishArtifacts
           uploadgithub: false
 
         - os: macos-13
-          coursierarchive: ""
+          coursierarchive: "/tmp"
           publishartifacts: dist.native.publishArtifacts
           uploadgithub: false
 
         - os: macos-latest
-          coursierarchive: ""
+          coursierarchive: "/tmp"
           publishartifacts: dist.native.publishArtifacts
           uploadgithub: false
 

.github/workflows/run-tests.yml

@@ -1,5 +1,5 @@
 # Uncommment this to replace the rest of the file when you want to debug stuff in CI
-#
+
 #
 #name: Run Debug
 #
@@ -9,18 +9,19 @@
 #  workflow_dispatch:
 #
 #jobs:
-#  debug-windows:
+#  debug:
 ##    runs-on: ubuntu-latest
 #    runs-on: windows-latest
 #    steps:
 #      - uses: actions/checkout@v4
 #        with: { fetch-depth: 1 }
 #
-#      - run: ./mill scalalib.test mill.scalalib.LauncherTests.launcher
+#      - run: ./mill 'integration.feature[no-java-bootstrap].native.server.test'
 #        env:
 #          COURSIER_ARCHIVE_CACHE: "C:/coursier-arc"
 #
 
+
 # We run full CI on push builds to main and on all pull requests
 #
 # To maximize bug-catching changes while keeping CI times reasonable, we run
@@ -114,7 +115,7 @@ jobs:
       - uses: ./.github/actions/post-build-selective
         with:
           millargs: ${{ matrix.millargs }}
-          coursierarchive: ""
+          coursierarchive: "/tmp"
           install-android-sdk: false
           shell: bash
 
@@ -184,6 +185,12 @@ jobs:
             millargs: "'integration.{failure,feature,ide}.__.packaged.server.test'"
             install-android-sdk: false
 
+            # run this specifically in `native` mode to make sure our non-JVM native image
+            # launcher is able to bootstrap everything necessary without a JVM installed
+          - java-version: 17
+            millargs: "'integration.bootstrap[no-java-bootstrap].native.server.test'"
+            install-android-sdk: false
+
             # These invalidation tests need to be exercised in both execution modes
             # to make sure they work with and without -i/--no-server being passed
           - java-version: 17
@@ -225,6 +232,9 @@ jobs:
           - java-version: 11 # Run this with Mill native launcher as a smoketest
             millargs: "'integration.invalidation.__.native.server.test'"
 
+          - java-version: 17
+            millargs: "'integration.bootstrap[no-java-bootstrap].native.server.test'"
+
     uses: ./.github/workflows/post-build-selective.yml
     with:
       os: windows-latest

.mill-jvm-version

@@ -1 +1 @@
-zulu:17.0.13
+zulu:17.0.14

build.mill

@@ -321,6 +321,8 @@ def millBinPlatform: T[String] = Task {
 
 def baseDir = build.millSourcePath
 
+def millJvmVersion = Task.Source(Task.workspace / ".mill-jvm-version")
+
 val essentialBridgeScalaVersions =
   Seq(Deps.scalaVersion, Deps.scala2Version, Deps.workerScalaVersion212)
 // published compiler bridges

ci/mill-bootstrap.patch

@@ -1,5 +1,5 @@
 diff --git a/build.mill b/build.mill
-index 36bf4e9a593..84fd76e6645 100644
+index 5c48e1e4e43..538e4b86e0c 100644
 --- a/build.mill
 +++ b/build.mill
 @@ -1,16 +1,16 @@
@@ -54,18 +54,18 @@ index 36bf4e9a593..84fd76e6645 100644
 -def baseDir = build.millSourcePath
 +def baseDir = build.moduleDir
 
- val essentialBridgeScalaVersions =
-   Seq(Deps.scalaVersion, Deps.scala2Version, Deps.workerScalaVersion212)
-@@ -467,7 +467,7 @@ trait MillPublishJavaModule extends MillJavaModule with PublishModule {
+ def millJvmVersion = Task.Source(Task.workspace / ".mill-jvm-version")
+
+@@ -469,7 +469,7 @@ trait MillPublishJavaModule extends MillJavaModule with PublishModule {
  /**
   * Some custom scala settings and test convenience
   */
 -trait MillScalaModule extends ScalaModule with MillJavaModule with ScalafixModule { outer =>
-+trait MillScalaModule extends ScalaModule with MillJavaModule /*with ScalafixModule */{ outer =>
++trait MillScalaModule extends ScalaModule with MillJavaModule/* with ScalafixModule*/ { outer =>
    def scalaVersion = Deps.scalaVersion
    def scalapVersion: T[String] = Deps.scala2Version
    def scalafixScalaBinaryVersion = T {
-@@ -524,8 +524,8 @@ trait MillScalaModule extends ScalaModule with MillJavaModule with ScalafixModul
+@@ -526,8 +526,8 @@ trait MillScalaModule extends ScalaModule with MillJavaModule with ScalafixModul
      val binaryVersion = ZincWorkerUtil.scalaBinaryVersion(sv)
      val hasModuleDefs = binaryVersion == "2.13" || binaryVersion == "3"
      super.scalacPluginIvyDeps() ++
@@ -76,7 +76,7 @@ index 36bf4e9a593..84fd76e6645 100644
    }
 
    def mandatoryIvyDeps = T {
-@@ -533,13 +533,13 @@ trait MillScalaModule extends ScalaModule with MillJavaModule with ScalafixModul
+@@ -535,13 +535,13 @@ trait MillScalaModule extends ScalaModule with MillJavaModule with ScalafixModul
      val binaryVersion = ZincWorkerUtil.scalaBinaryVersion(sv)
      val hasModuleDefs = binaryVersion == "2.13" || binaryVersion == "3"
      super.mandatoryIvyDeps() ++
@@ -92,7 +92,7 @@ index 36bf4e9a593..84fd76e6645 100644
      def scalafixConfig = T { Some(T.workspace / ".scalafix.conf") }
      def forkArgs = super.forkArgs() ++ outer.testArgs()
      def moduleDeps = outer.testModuleDeps
-@@ -579,7 +579,8 @@ trait MillBaseTestsModule extends TestModule {
+@@ -581,7 +581,8 @@ trait MillBaseTestsModule extends TestModule {
  trait MillPublishScalaModule extends MillScalaModule with MillPublishJavaModule
 
  /** Publishable module which contains strictly handled API. */
@@ -102,7 +102,7 @@ index 36bf4e9a593..84fd76e6645 100644
    import com.github.lolgab.mill.mima._
    override def mimaBinaryIssueFilters: T[Seq[ProblemFilter]] = Seq(
      // (5x) MIMA doesn't properly ignore things which are nested inside other private things
-@@ -709,7 +710,7 @@ trait MillStableScalaModule extends MillPublishScalaModule with Mima {
+@@ -711,7 +712,7 @@ trait MillStableScalaModule extends MillPublishScalaModule with Mima {
    def skipPreviousVersions: T[Seq[String]] = T {
      T.log.info("Skipping mima for previous versions (!!1000s of errors due to Scala 3)")
      mimaPreviousVersions() // T(Seq.empty[String])
@@ -489,7 +489,7 @@ index 62190fd6c1e..9d4f176c915 100644
      val (repoPath, repoHash) = repoInfo(crossValue)
      def repoSlug = repoPath.split("/").last
 diff --git a/integration/package.mill b/integration/package.mill
-index af00b40d3f0..e43343cabf1 100644
+index 53cdc15a9f6..9271663b81e 100644
 --- a/integration/package.mill
 +++ b/integration/package.mill
 @@ -3,13 +3,12 @@ package build.integration
@@ -530,7 +530,7 @@ index af00b40d3f0..e43343cabf1 100644
        def scalaVersion = build.Deps.scalaVersion
 
        def forkEnv =
-@@ -87,11 +86,11 @@ object `package` extends RootModule {
+@@ -87,13 +86,13 @@ object `package` extends RootModule {
      }
    }
 
@@ -543,6 +543,9 @@ index af00b40d3f0..e43343cabf1 100644
 -  object ide extends Cross[IdeIntegrationCrossModule](build.listIn(millSourcePath / "ide"))
 +      extends Cross[IntegrationCrossModule](build.listIn(moduleDir / "invalidation"))
 +  object ide extends Cross[IdeIntegrationCrossModule](build.listIn(moduleDir / "ide"))
+   object bootstrap
+-      extends Cross[IdeIntegrationCrossModule](build.listIn(millSourcePath / "bootstrap"))
++      extends Cross[IdeIntegrationCrossModule](build.listIn(moduleDir / "bootstrap"))
    trait IntegrationCrossModule extends build.MillScalaModule with IntegrationTestModule {
      override def moduleDeps = super[IntegrationTestModule].moduleDeps
      def forkEnv = super.forkEnv() ++ Seq(

integration/bootstrap/no-java-bootstrap/resources/bar/src/bar/Bar.java

@@ -0,0 +1,6 @@
+package bar;
+public class Bar {
+    public static void main(String[] args) {
+        System.out.println("Hello World! " + System.getProperty("java.version"));
+    }
+}

integration/bootstrap/no-java-bootstrap/resources/build.mill

@@ -0,0 +1,8 @@
+package build
+import mill._, javalib._
+
+def foo = Task {
+  println(System.getProperty("java.version"))
+}
+
+object bar extends JavaModule

integration/bootstrap/no-java-bootstrap/src/NoJavaBootstrapTests.scala

@@ -0,0 +1,39 @@
+package mill.integration
+
+import mill.testkit.UtestIntegrationTestSuite
+
+import utest._
+
+object NoJavaBootstrapTests extends UtestIntegrationTestSuite {
+  // Don't propagate `JAVA_HOME` to this test suite, because we want to exercise
+  // the code path where `JAVA_HOME` is not present during bootstrapping
+  override def propagateJavaHome = false
+  val tests: Tests = Tests {
+    test - integrationTest { tester =>
+      import tester._
+      os.remove(tester.workspacePath / ".mill-jvm-version")
+      // The Mill server process should use the default Mill Java version,
+      // even without the `.mill-jvm-version` present
+      //
+      // Force Mill client to ignore any system `java` installation, to make sure
+      // this tests works reliably regardless of what is installed on the system
+      val res1 = eval(
+        "foo",
+        env = Map("MILL_TEST_SUITE_IGNORE_SYSTEM_JAVA" -> "true"),
+        stderr = os.Inherit
+      )
+
+      assert(res1.out == System.getProperty("java.version"))
+
+      // Any `JavaModule`s run from the Mill server should also inherit
+      // the default Mill Java version from it
+      val res2 = eval(
+        "bar.run",
+        env = Map("MILL_TEST_SUITE_IGNORE_SYSTEM_JAVA" -> "true"),
+        stderr = os.Inherit
+      )
+
+      assert(res2.out == s"Hello World! ${System.getProperty("java.version")}")
+    }
+  }
+}

integration/ide/bsp-server/src/BspServerTests.scala

@@ -4,7 +4,7 @@ import ch.epfl.scala.{bsp4j => b}
 import mill.api.BuildInfo
 import mill.bsp.Constants
 import mill.integration.BspServerTestUtil._
-import mill.testkit.{UtestIntegrationTestSuite, IntegrationTester}
+import mill.testkit.UtestIntegrationTestSuite
 import utest._
 
 import scala.jdk.CollectionConverters._
@@ -48,7 +48,7 @@ object BspServerTests extends UtestIntegrationTestSuite {
 
       withBspServer(
         workspacePath,
-        IntegrationTester.millTestSuiteEnv
+        millTestSuiteEnv
       ) { (buildServer, initRes) =>
         val scala2Version = sys.props.getOrElse("TEST_SCALA_2_13_VERSION", ???)
         val scala3Version = sys.props.getOrElse("MILL_SCALA_3_NEXT_VERSION", ???)

integration/invalidation/version-change/src/VersionChangeTests.scala

@@ -11,7 +11,7 @@ object VersionChangeTests extends UtestIntegrationTestSuite {
       val javaVersion1 = eval(("show", "javaVersion"))
       assert(!javaVersion1.out.contains("19.0.2"))
 
-      os.write(workspacePath / ".mill-jvm-version", "temurin:19.0.2")
+      os.write.over(workspacePath / ".mill-jvm-version", "temurin:19.0.2")
 
       val javaVersion2 = eval(("show", "javaVersion"))
       assert(javaVersion2.out == "\"19.0.2\"")

integration/package.mill

@@ -92,6 +92,8 @@ object `package` extends RootModule {
   object invalidation
       extends Cross[IntegrationCrossModule](build.listIn(millSourcePath / "invalidation"))
   object ide extends Cross[IdeIntegrationCrossModule](build.listIn(millSourcePath / "ide"))
+  object bootstrap
+      extends Cross[IdeIntegrationCrossModule](build.listIn(millSourcePath / "bootstrap"))
   trait IntegrationCrossModule extends build.MillScalaModule with IntegrationTestModule {
     override def moduleDeps = super[IntegrationTestModule].moduleDeps
     def forkEnv = super.forkEnv() ++ Seq(

runner/client/src/mill/runner/client/MillProcessLauncher.java

@@ -113,13 +113,24 @@ static boolean isWin() {
     return System.getProperty("os.name", "").startsWith("Windows");
   }
 
-  static String javaHome() throws IOException {
+  static String javaHome() throws Exception {
     String jvmId;
     Path millJvmVersionFile = millJvmVersionFile();
 
     String javaHome = null;
     if (Files.exists(millJvmVersionFile)) {
       jvmId = Files.readString(millJvmVersionFile).trim();
+    } else {
+      boolean systemJavaExists =
+          new ProcessBuilder(isWin() ? "where" : "which", "java").start().waitFor() == 0;
+      if (systemJavaExists && System.getenv("MILL_TEST_SUITE_IGNORE_SYSTEM_JAVA") == null) {
+        jvmId = null;
+      } else {
+        jvmId = mill.client.BuildInfo.defaultJvmId;
+      }
+    }
+
+    if (jvmId != null) {
 
       // Fast path to avoid calling `CoursierClient` and paying the classloading cost
       // when the `javaHome` JVM has already been initialized for the configured `jvmId`
@@ -144,7 +155,7 @@ static String javaHome() throws IOException {
     return javaHome;
   }
 
-  static String javaExe() throws IOException {
+  static String javaExe() throws Exception {
     String javaHome = javaHome();
     if (javaHome == null) return "java";
     else {

runner/server/client/package.mill

@@ -10,7 +10,12 @@ import mill.scalalib._
  */
 object `package` extends RootModule with build.MillPublishJavaModule with BuildInfo {
   def buildInfoPackageName = "mill.client"
-  def buildInfoMembers = Seq(BuildInfo.Value("millVersion", build.millVersion(), "Mill version."))
+  def millJvmVersion = Task.Source(Task.workspace / ".mill-jvm-version")
+
+  def buildInfoMembers = Seq(
+    BuildInfo.Value("millVersion", build.millVersion(), "Mill version."),
+    BuildInfo.Value("defaultJvmId", os.read(millJvmVersion().path).trim())
+  )
 
   def moduleDeps = Seq(build.core.constants)
 

testkit/src/mill/testkit/ExampleTester.scala

@@ -75,7 +75,8 @@ class ExampleTester(
     val workspaceSourcePath: os.Path,
     millExecutable: os.Path,
     bashExecutable: String = ExampleTester.defaultBashExecutable(),
-    val baseWorkspacePath: os.Path
+    val baseWorkspacePath: os.Path,
+    val propagateJavaHome: Boolean = true
 ) extends IntegrationTesterBase {
 
   def processCommandBlock(commandBlock: String): Unit = {
@@ -130,7 +131,7 @@ class ExampleTester(
       stderr = os.Inherit,
       cwd = workspacePath,
       mergeErrIntoOut = true,
-      env = IntegrationTester.millTestSuiteEnv ++ windowsPathEnv,
+      env = millTestSuiteEnv ++ windowsPathEnv,
       check = false
     )