Fix "Can't clean external worker" (#6559)

Fixes #6549

Author: lihaoyi

libs/util/src/mill/util/MainModule.scala

@@ -243,7 +243,8 @@ trait MainModule extends RootModule0, MainModuleApi, JdkCommandsModule {
           val allPaths = ts.flatMap { segments =>
             val evPaths = ExecutionPaths.resolve(rootDir, segments)
             val paths = Seq(evPaths.dest, evPaths.meta, evPaths.log)
-            val potentialModulePath = rootDir / segments.parts
+            val potentialModulePath =
+              rootDir / segments.parts.map(ExecutionPaths.sanitizePathSegment)
             if (os.exists(potentialModulePath)) {
               // this is either because of some pre-Mill-0.10 files lying around
               // or most likely because the segments denote a module but not a task

libs/util/test/src/mill/util/MainModuleTests.scala

@@ -4,7 +4,7 @@ import mill.api.{ExecResult, Result, Val}
 import mill.constants.{OutFolderMode}
 import mill.constants.OutFiles.OutFiles
 import mill.{Task, given}
-import mill.api.{Cross, DefaultTaskModule, Discover, Module, PathRef}
+import mill.api.{Cross, DefaultTaskModule, Discover, ExternalModule, Module, PathRef}
 import mill.testkit.UnitTester
 import mill.testkit.TestRootModule
 import mill.util.MainModule
@@ -15,6 +15,17 @@ import scala.collection.mutable
 import scala.concurrent.duration.DurationInt
 import scala.util.Properties
 
+// External module for testing clean command with external workers
+object TestExternalWorkerModule extends ExternalModule {
+  def externalWorker = Task.Worker {
+    new AutoCloseable {
+      def close() = ()
+      override def toString = "externalWorkerInstance"
+    }
+  }
+  lazy val millDiscover = Discover[this.type]
+}
+
 object MainModuleTests extends TestSuite {
 
   object mainModule extends TestRootModule with MainModule {
@@ -707,6 +718,31 @@ object MainModuleTests extends TestSuite {
           assert(workers.size == 2)
         }
       }
+
+      test("external-module") {
+        // Test cleaning external module workers (issue #6549)
+        // External modules have segments ending with '/' which need special handling
+        object externalModuleBuild extends TestRootModule with MainModule {
+          def useExternalWorker = Task {
+            TestExternalWorkerModule.externalWorker()
+            "done"
+          }
+          lazy val millDiscover = Discover[this.type]
+        }
+        UnitTester(externalModuleBuild, null).scoped { ev =>
+          // First run the task that uses the external worker
+          val r1 = ev.evaluator.execute(Seq(externalModuleBuild.useExternalWorker)).executionResults
+          assert(r1.transitiveFailing.size == 0)
+
+          // Now try to clean the external worker - this should not throw an error
+          // Previously this would fail with: "[mill.util.TestExternalWorkerModule/] is not a valid path segment"
+          val r2 = ev.evaluator.execute(Seq(externalModuleBuild.clean(
+            ev.evaluator,
+            "mill.util.TestExternalWorkerModule/externalWorker"
+          ))).executionResults
+          assert(r2.transitiveFailing.size == 0)
+        }
+      }
     }
   }
 }