Stop vendored runtimes leaking into a workspace's shared node_modules

millbun/src/mill/bun/BunVendoredNodeModules.scala

@@ -44,9 +44,19 @@ object BunVendoredNodeModules:
           FileVisitResult.CONTINUE
     )
 
-  /** Merge vendored node_modules from a classpath entry into a destination. */
+  /** Merge vendored node_modules from a classpath entry into a destination.
+    *
+    * Refuses a symlinked destination: in workspace mode `node_modules` is a link into the shared
+    * workspace install's `Task.dest`, and merging through it would silently mutate another task's
+    * output — shared by every other member of the workspace.
+    */
   def mergeFromClasspathEntry(entry: os.Path, destNodeModules: os.Path): Boolean =
-    if os.isDir(entry) then mergeFromDir(entry, destNodeModules, entry.toString)
+    if os.isLink(destNodeModules) then
+      throw new RuntimeException(
+        s"Refusing to merge vendored Bun runtime into $destNodeModules: it is a symlink to " +
+          s"${os.readLink.absolute(destNodeModules)}, which belongs to another task."
+      )
+    else if os.isDir(entry) then mergeFromDir(entry, destNodeModules, entry.toString)
     else if os.exists(entry) && entry.ext == "jar" then mergeFromJar(entry, destNodeModules)
     else false
 

millbun/src/mill/scalajslib/bun/BunScalaJSModule.scala

@@ -254,8 +254,21 @@ trait BunScalaJSModule extends ScalaJSModule with BunToolchainModule with BunPac
         transitiveUnmanagedDeps().nonEmpty ||
         bunPackageJsonExtras().value.nonEmpty
 
+    val ownResourceRoots = resources().map(_.path).toSet
+    val vendoredEntries = runClasspath().map(_.path).filterNot(ownResourceRoots.contains)
+
     bunWorkspaceInstall() match
       case Some(workspaceInstall) =>
+        // Vendored trees must not be merged here: node_modules is a link into the workspace
+        // install's dest, so merging would mutate a directory shared by every workspace member.
+        val vendored = vendoredEntries.filter(BunVendoredNodeModules.hasVendoredNodeModules)
+        if vendored.nonEmpty then
+          Task.fail(
+            s"Bun workspace members cannot consume vendored runtime dependencies: " +
+              s"${vendored.map(_.last).mkString(", ")}. Depend on the manifest-only artifact, or " +
+              "install this module outside the workspace."
+          )
+
         val installed = workspaceInstall.path
         if os.exists(installed / "node_modules") then
           os.symlink(dest / "node_modules", installed / "node_modules")
@@ -281,9 +294,7 @@ trait BunScalaJSModule extends ScalaJSModule with BunToolchainModule with BunPac
             env = bunEnv()
           )
 
-    val ownResourceRoots = resources().map(_.path).toSet
-    val vendoredEntries = runClasspath().map(_.path).filterNot(ownResourceRoots.contains)
-    mergeVendoredNodeModules(vendoredEntries, dest / "node_modules")
+        mergeVendoredNodeModules(vendoredEntries, dest / "node_modules")
 
     PathRef(dest)
   }

millbun/test/src/mill/bun/BunVendoredNodeModulesTests.scala

@@ -100,6 +100,25 @@ object BunVendoredNodeModulesTests extends TestSuite {
 
       assert(err.getMessage.contains("Vendored Bun bundle conflict"))
     }
+
+    test("merging through a symlinked destination is refused") {
+      // In workspace mode node_modules links into the shared install's Task.dest. Merging
+      // through it would silently mutate a directory every workspace member depends on.
+      val source = os.temp.dir()
+      writeVendoredPackage(source, "react", "19.1.1")
+
+      val shared = os.temp.dir() / "shared-node-modules"
+      os.makeDir.all(shared)
+      val member = os.temp.dir()
+      os.symlink(member / "node_modules", shared)
+
+      val err = intercept[RuntimeException] {
+        BunVendoredNodeModules.mergeFromClasspathEntry(source, member / "node_modules")
+      }
+      assert(err.getMessage.contains("belongs to another task"))
+      // The critical assertion: nothing leaked into the shared directory.
+      assert(os.list(shared).isEmpty)
+    }
   }
 
   private def writeVendoredPackage(root: os.Path, name: String, version: String): Unit = {