[2.x] feat: Support --autostart=false and --no-server in sbtn client (#8895)

**Problem**
When sbtn (native thin client) can't find a running sbt server, it prompts
to start one. There was no way to opt out of server auto-start from the
client side, which is needed for CI environments and scripting (sbt/sbt#7079).

**Solution**
Reuse the existing sbt.server.autostart system property in NetworkClient
to control whether sbtn should attempt to start a server. When no server is
running and sbt.server.autostart=false, sbtn exits with an error instead
of prompting.

Support setting this property via:
- sbtn --no-server compile (sets sbt.server.autostart=false)
- sbtn --autostart=false compile (new flag following sbt conventions)
- sbtn -Dsbt.server.autostart=false compile (direct system property)
- sbt --autostart=false compile (bash runner and sbtw)

This follows sbt's naming conventions for properties/options: positive
property names with =false opt-out (like --color=false, --supershell=false).

Fixes sbt/sbt#7079

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

main-command/src/main/scala/sbt/internal/client/NetworkClient.scala

@@ -151,6 +151,8 @@ class NetworkClient(
   private lazy val noStdErr = arguments.completionArguments.contains("--no-stderr") &&
     !sys.env.contains("SBTN_AUTO_COMPLETE") && !sys.env.contains("SBTC_AUTO_COMPLETE")
   private def shutdownOnly = arguments.commandArguments == Seq(Shutdown)
+  private lazy val serverAutoStart: Boolean =
+    sys.props.get("sbt.server.autostart").forall(_.toLowerCase == "true")
 
   private def mkSocket(file: File): (Socket, Option[String]) = ClientSocket.socket(file, useJNI)
 
@@ -189,6 +191,9 @@ class NetworkClient(
         if (shutdownOnly) {
           console.appendLog(Level.Info, "no sbt server is running. ciao")
           System.exit(0)
+        } else if (!serverAutoStart) {
+          console.appendLog(Level.Error, "no sbt server is running (sbt.server.autostart=false)")
+          System.exit(1)
         } else if (promptCompleteUsers) {
           val msg = if (noTab) "" else "No sbt server is running. Press <tab> to start one..."
           errorStream.print(s"\n$msg")
@@ -1187,7 +1192,7 @@ object NetworkClient {
         completionArguments,
         sbtScript,
         bsp,
-        sbtLaunchJar
+        sbtLaunchJar,
       )
   }
   private[client] val completions = "--completions"
@@ -1246,8 +1251,6 @@ object NetworkClient {
     "--timings",
     "-traces",
     "--traces",
-    "-no-server",
-    "--no-server",
     "-no-share",
     "--no-share",
     "-no-global",
@@ -1264,6 +1267,8 @@ object NetworkClient {
     "-supershell=",
     "--color=",
     "-color=",
+    "--autostart=",
+    "-autostart=",
   )
   private[client] def parseArgs(args: Array[String]): Arguments = {
     val defaultSbtScript = if (Properties.isWin) "sbt.bat" else "sbt"
@@ -1302,6 +1307,12 @@ object NetworkClient {
           i += 1
           launchJar = Option(sanitized(i).replace("%20", " "))
         case "-bsp" | "--bsp" => bsp = true
+        case "-no-server" | "--no-server" =>
+          System.setProperty("sbt.server.autostart", "false")
+        case a if a.startsWith("--autostart=") =>
+          System.setProperty("sbt.server.autostart", a.stripPrefix("--autostart="))
+        case a if a.startsWith("-autostart=") =>
+          System.setProperty("sbt.server.autostart", a.stripPrefix("-autostart="))
         case a if launcherValueFlags.contains(a) =>
           if (i + 1 < sanitized.length) i += 1
         case a if launcherNoValueFlags.contains(a)                => ()
@@ -1329,7 +1340,7 @@ object NetworkClient {
       completionArguments.toSeq,
       sbtScript.getOrElse(defaultSbtScript).replace("%20", " "),
       bsp,
-      launchJar
+      launchJar,
     )
   }
 

main-command/src/test/scala/sbt/internal/client/NetworkClientParseArgsTest.scala

@@ -111,6 +111,38 @@ object NetworkClientParseArgsTest extends BasicTestSuite:
     val result = parse("-bsp")
     assert(result.bsp)
 
+  // -- --no-server and --autostart= set sbt.server.autostart --
+
+  test("--no-server sets sbt.server.autostart=false"):
+    try
+      System.clearProperty("sbt.server.autostart")
+      parse("--no-server", "compile")
+      assert(System.getProperty("sbt.server.autostart") == "false")
+    finally System.clearProperty("sbt.server.autostart")
+
+  test("-no-server sets sbt.server.autostart=false"):
+    try
+      System.clearProperty("sbt.server.autostart")
+      parse("-no-server", "compile")
+      assert(System.getProperty("sbt.server.autostart") == "false")
+    finally System.clearProperty("sbt.server.autostart")
+
+  test("--autostart=false sets sbt.server.autostart=false"):
+    try
+      System.clearProperty("sbt.server.autostart")
+      val result = parse("--autostart=false", "compile")
+      assert(System.getProperty("sbt.server.autostart") == "false")
+      assert(!result.sbtArguments.exists(_.contains("autostart")))
+      assert(result.commandArguments.contains("compile"))
+    finally System.clearProperty("sbt.server.autostart")
+
+  test("--autostart=true sets sbt.server.autostart=true"):
+    try
+      System.clearProperty("sbt.server.autostart")
+      parse("--autostart=true", "compile")
+      assert(System.getProperty("sbt.server.autostart") == "true")
+    finally System.clearProperty("sbt.server.autostart")
+
   test("--sbt-launch-jar is preserved"):
     val result = parse("--sbt-launch-jar", "/path/to/sbt-launch.jar", "compile")
     assert(result.sbtLaunchJar.contains("/path/to/sbt-launch.jar"))

sbt

@@ -732,6 +732,8 @@ map_args () {
              --supershell=*) options=( "${options[@]}" "-Dsbt.supershell=${1:13}" ) && shift ;;
               -supershell=*) options=( "${options[@]}" "-Dsbt.supershell=${1:12}" ) && shift ;;
      -no-server|--no-server) options=( "${options[@]}" "-Dsbt.io.virtual=false" "-Dsbt.server.autostart=false" ) && shift ;;
+            --autostart=*) options=( "${options[@]}" "-Dsbt.server.autostart=${1:13}" ) && shift ;;
+             -autostart=*) options=( "${options[@]}" "-Dsbt.server.autostart=${1:12}" ) && shift ;;
                   --color=*) options=( "${options[@]}" "-Dsbt.color=${1:8}" ) && shift ;;
                    -color=*) options=( "${options[@]}" "-Dsbt.color=${1:7}" ) && shift ;;
        -no-share|--no-share) options=( "${options[@]}" "${noshare_opts[@]}" ) && shift ;;

sbtw/src/main/scala/sbtw/ArgParser.scala

@@ -41,6 +41,7 @@ object ArgParser:
         opt[Int]("mem").action((x, c) => c.copy(mem = Some(x))),
         opt[String]("supershell").action((x, c) => c.copy(supershell = Some(x))),
         opt[String]("color").action((x, c) => c.copy(color = Some(x))),
+        opt[String]("autostart").action((x, c) => c.copy(autostart = Some(x))),
         opt[Int]("jvm-debug").action((x, c) => c.copy(jvmDebug = Some(x))),
         opt[String]("java-home").action((x, c) => c.copy(javaHome = Some(x))),
         arg[String]("<arg>")

sbtw/src/main/scala/sbtw/LauncherOptions.scala

@@ -29,6 +29,7 @@ case class LauncherOptions(
     mem: Option[Int] = None,
     supershell: Option[String] = None,
     color: Option[String] = None,
+    autostart: Option[String] = None,
     jvmDebug: Option[Int] = None,
     javaHome: Option[String] = None,
     server: Boolean = false,

sbtw/src/main/scala/sbtw/Runner.scala

@@ -62,6 +62,7 @@ object Runner:
     opts.sbtCache.foreach(v => s = s :+ s"-Dsbt.global.localcache=$v")
     opts.ivy.foreach(v => s = s :+ s"-Dsbt.ivy.home=$v")
     opts.color.foreach(v => s = s :+ s"-Dsbt.color=$v")
+    opts.autostart.foreach(v => s = s :+ s"-Dsbt.server.autostart=$v")
     if opts.timings then
       s = s ++ Seq("-Dsbt.task.timings=true", "-Dsbt.task.timings.on.shutdown=true")
     if opts.traces then s = s :+ "-Dsbt.traces=true"