Add --longRunning option to link incrementally in a long living process

cli/src/org/scalajs/cli/Scalajsld.scala

@@ -53,7 +53,8 @@ object Scalajsld {
       stdLib: Seq[File] = Nil,
       jsHeader: String = "",
       logLevel: Level = Level.Info,
-      importMap: Option[File] = None
+      importMap: Option[File] = None,
+      longRunning: Boolean = false
   )
 
   private def moduleInitializer(
@@ -234,6 +235,9 @@ object Scalajsld {
       opt[String]("jsHeader")
         .action { (jsHeader, c) => c.copy(jsHeader = jsHeader) }
         .text("A header that will be added at the top of generated .js files")
+      opt[Unit]("longRunning")
+        .action { (_, c) => c.copy(longRunning = true) }
+        .text("Run linking incrementally every time a line is printed to stdin")
       opt[Unit]('d', "debug")
         .action { (_, c) => c.copy(logLevel = Level.Debug) }
         .text("Debug mode: Show full log")
@@ -315,39 +319,53 @@ object Scalajsld {
       val logger = new ScalaConsoleLogger(options.logLevel)
       val cache = StandardImpl.irFileCache().newCache
 
-      val result = PathIRContainer
-        .fromClasspath(classpath)
-        .flatMap(containers => cache.cached(containers._1))
-        .flatMap { irFiles: Seq[IRFile] =>
+      val stdinLinesIterator = scala.io.Source.stdin.getLines()
 
-          val irImportMappedFiles = options.importMap match {
-            case None => irFiles
-            case Some(importMap) => ImportMapJsonIr.remapImports(importMap, irFiles)
-          }
+      while({
+        val result = PathIRContainer
+          .fromClasspath(classpath)
+          .flatMap(containers => cache.cached(containers._1))
+          .flatMap { irFiles: Seq[IRFile] =>
+
+            val irImportMappedFiles = options.importMap match {
+              case None => irFiles
+              case Some(importMap) => ImportMapJsonIr.remapImports(importMap, irFiles)
+            }
 
-          (options.output, options.outputDir) match {
-            case (Some(jsFile), None) =>
-              (DeprecatedLinkerAPI: DeprecatedLinkerAPI).link(
-                linker,
-                irImportMappedFiles.toList,
-                moduleInitializers,
-                jsFile,
-                logger
-              )
-            case (None, Some(outputDir)) =>
-              linker.link(
-                irImportMappedFiles,
-                moduleInitializers,
-                PathOutputDirectory(outputDir.toPath()),
-                logger
-              )
-            case _ =>
-              throw new AssertionError(
-                "Either output or outputDir have to be defined."
-              )
+            (options.output, options.outputDir) match {
+              case (Some(jsFile), None) =>
+                (DeprecatedLinkerAPI: DeprecatedLinkerAPI).link(
+                  linker,
+                  irImportMappedFiles.toList,
+                  moduleInitializers,
+                  jsFile,
+                  logger
+                )
+              case (None, Some(outputDir)) =>
+                linker.link(
+                  irImportMappedFiles,
+                  moduleInitializers,
+                  PathOutputDirectory(outputDir.toPath()),
+                  logger
+                )
+              case _ =>
+                throw new AssertionError(
+                  "Either output or outputDir have to be defined."
+                )
+            }
           }
-        }
-      Await.result(result, Duration.Inf)
+        Await.result(result, Duration.Inf)
+
+        if (options.longRunning) {
+          // print SCALA_JS_LINKING_DONE\n everytime one linking succeeds
+          println("SCALA_JS_LINKING_DONE")
+
+          if (stdinLinesIterator.hasNext) {
+            stdinLinesIterator.next()
+            true
+          } else false
+        } else false
+      }) {}
     }
   }
 

tests/test/src/org/scalajs/cli/tests/Tests.scala

@@ -113,6 +113,112 @@ class Tests extends munit.FunSuite {
     assert(splitRunOutput == "asdf 2")
   }
 
+  test("longRunning") {
+    val dir = os.temp.dir()
+    def writePrintlnMain(stringToPrint: String) = {
+      os.write.over(
+        dir / "foo.scala",
+        s"""object Foo {
+          |  def main(args: Array[String]): Unit = {
+          |    println("$stringToPrint")
+          |  }
+          |}
+          |""".stripMargin
+      )
+    }
+
+    val scalaJsLibraryCp = getScalaJsLibraryCp(dir)
+
+    os.makeDir.all(dir / "bin")
+
+    def compile() = {
+      val compileCommand = os.proc(
+        "cs",
+        "launch",
+        "scalac:2.13.14",
+        "--",
+        "-classpath",
+        scalaJsLibraryCp,
+        s"-Xplugin:${getScalaJsCompilerPlugin(dir)}",
+        "-d",
+        "bin",
+        "foo.scala"
+      )
+      compileCommand.call(cwd = dir, stdin = os.Inherit, stdout = os.Inherit)
+    }
+
+    val command = os
+      .proc(
+        launcher,
+        "--stdlib",
+        scalaJsLibraryCp,
+        "-s",
+        "-o",
+        "test.js",
+        "-mm",
+        "Foo.main",
+        "--longRunning",
+        "bin",
+      )
+
+    writePrintlnMain("first version")
+    compile()
+    val process = command.spawn(cwd = dir)
+
+    def waitForLinkingToFinish() = {
+      while({
+        val line = process.stdout.readLine()
+        assert(line != null, "Got null from reading stdout")
+        line != "SCALA_JS_LINKING_DONE"
+      }) {}
+    }
+
+    try {
+      locally {
+        waitForLinkingToFinish()
+        val testJsSize = os.size(dir / "test.js")
+        val testJsMapSize = os.size(dir / "test.js.map")
+        assert(testJsSize > 0)
+        assert(testJsMapSize > 0)
+
+        val runRes = os.proc("node", "test.js").call(cwd = dir)
+        val runOutput = runRes.out.trim()
+        assert(runOutput == "first version")
+      }
+
+      writePrintlnMain("second version")
+      compile()
+
+      // trigger new linking
+      process.stdin.writeLine("")
+      process.stdin.flush()
+
+      waitForLinkingToFinish()
+
+      locally {
+        val testJsSize = os.size(dir / "test.js")
+        val testJsMapSize = os.size(dir / "test.js.map")
+        assert(testJsSize > 0)
+        assert(testJsMapSize > 0)
+
+        val runRes = os.proc("node", "test.js").call(cwd = dir)
+        val runOutput = runRes.out.trim()
+        assertEquals(runOutput, "second version")
+      }
+
+      // close stdin to allow the process to terminate gracefully
+      process.stdin.close()
+
+      // wait some time for the process to terminate
+      Thread.sleep(100)
+
+      assert(!process.isAlive(), "process did not terminate gracefully")
+    } finally {
+
+      process.close()
+    }
+  }
+
   test("fullLinkJs mode does not throw") {
     val dir = os.temp.dir()
     os.write(