Move Mill process working directory to sandbox (#3367)

This makes `os.pwd` an empty directory: `out/mill-worker-*/sandbox/` for
Mill's server mode, `out/mill-no-server/sandbox/` otherwise

In general, code outside of tasks should use `millSourcePath`, while
code inside of tasks should use `T.dest` or the `PathRef`s given to it
by upstream tasks. Accessing project files via `os.pwd` can result in
tasks not invalidating when files changed.

Like #3347, this does not
prevent people intentionally walking the filesystem to work mischieve,
but it does help mitigate "accidental" usage of `os.pwd` to access
project files. The original `os.pwd` is still made available as
`mill.api.WorkspaceRoot.workspaceRoot` outside of tasks, or
`T.workspace` inside of tasks, for the occasional scenarios that need
it. But at least this adds some partial guardrails against accidentally
using `os.pwd` in a way that subverts Mill's expectations, and the APIs
that expose the workspace root can be appropriately documented to warn
people against doing the wrong thing

contrib/bloop/src/mill/contrib/bloop/Bloop.scala

@@ -1,8 +1,12 @@
 package mill.contrib.bloop
 
 import mill.eval.Evaluator
+import mill.api.WorkspaceRoot
 
 /**
  * Usage : `mill mill.contrib.bloop.Bloop/install`
  */
-object Bloop extends BloopImpl(() => Evaluator.allBootstrapEvaluators.value.value, os.pwd)
+object Bloop extends BloopImpl(
+      () => Evaluator.allBootstrapEvaluators.value.value,
+      WorkspaceRoot.workspaceRoot
+    )

example/thirdparty/acyclic/build.sc

@@ -1,5 +1,14 @@
 import mill._, scalalib._, publish._
 
+// acyclic test suite assumes files are on disk at specific paths relative to `os.pwd`.
+// To avoid changing the test code, we instead copy the necessary files into the `os.pwd`
+// when preparing the resources for test suite execution
+os.copy.over(
+  interp.watch(mill.api.WorkspaceRoot.workspaceRoot / "acyclic"),
+  os.pwd / "acyclic",
+  createFolders = true
+)
+
 object Deps {
   def acyclic = ivy"com.lihaoyi:::acyclic:0.3.6"
   def scalaCompiler(scalaVersion: String) = ivy"org.scala-lang:scala-compiler:$scalaVersion"

example/thirdparty/netty/build.sc

@@ -252,8 +252,8 @@ object common extends NettyModule{
     val shell = new groovy.lang.GroovyShell()
 
     val context = new java.util.HashMap[String, Object]
-    context.put("collection.template.dir", "common/src/main/templates")
-    context.put("collection.template.test.dir", "common/src/test/templates")
+    context.put("collection.template.dir", T.workspace + "/common/src/main/templates")
+    context.put("collection.template.test.dir", T.workspace + "/common/src/test/templates")
     context.put("collection.src.dir", (T.dest / "src").toString)
     context.put("collection.testsrc.dir", (T.dest / "testsrc").toString)
     shell.setProperty("properties", context)

integration/invalidation/watch-source-input/repo/build.sc

@@ -34,7 +34,7 @@ def lol = T{
 def writeCompletionMarker(name: String) = {
 
   Range(0, 10)
-    .map(i => os.pwd / "out" / s"$name$i")
+    .map(i => mill.api.WorkspaceRoot.workspaceRoot / "out" / s"$name$i")
     .find(!os.exists(_))
     .foreach(os.write(_, ""))
 }

integration/src/mill/integration/IntegrationTestSuite.scala

@@ -140,7 +140,7 @@ abstract class IntegrationTestSuite extends TestSuite {
 
   override def utestAfterEach(path: Seq[String]): Unit = {
     runnerState = RunnerState.empty
-    if (integrationTestMode == "server") {
+    if (integrationTestMode == "server" || integrationTestMode == "local") {
       // try to stop the server
       try {
         os.proc(millReleaseFileOpt.get, "shutdown").call(

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

@@ -48,7 +48,7 @@ static Process configureRunMillProcess(ProcessBuilder builder,
         builder.environment().put("MILL_WORKSPACE_ROOT", new File("").getCanonicalPath());
         File sandbox = new java.io.File(serverDir + "/" + ServerFiles.sandbox);
         sandbox.mkdirs();
-        // builder.directory(sandbox);
+        builder.directory(sandbox);
         return builder.start();
     }
 
@@ -88,7 +88,7 @@ static String javaExe() {
         return "java";
     }
 
-    static String[] millClasspath() {
+    static String[] millClasspath() throws Exception {
         String selfJars = "";
         List<String> vmOptions = new LinkedList<>();
         String millOptionsPath = System.getProperty("MILL_OPTIONS_PATH");
@@ -121,10 +121,14 @@ static String[] millClasspath() {
         if (selfJars == null || selfJars.trim().isEmpty()) {
             throw new RuntimeException("MILL_CLASSPATH is empty!");
         }
-        return selfJars.split("[,]");
+        String[] selfJarsArray = selfJars.split("[,]");
+        for(int i = 0; i < selfJarsArray.length; i++){
+            selfJarsArray[i] = new java.io.File(selfJarsArray[i]).getCanonicalPath();
+        }
+        return selfJarsArray;
     }
 
-    static List<String> millLaunchJvmCommand(boolean setJnaNoSys) {
+    static List<String> millLaunchJvmCommand(boolean setJnaNoSys) throws Exception {
         final List<String> vmOptions = new ArrayList<>();
 
         // Java executable