[2.x] feat: Per-channel project cursor for multi-client server mode (#8649)

**Problem**
When multiple clients connect to the same sbt server, they all share the same "current project" state. When one client switches projects with `project X`, all other clients see that change.

**Solution**
Store per-channel project cursors in State attributes. Each client maintains its own cursor that tracks which project it has selected.

main/src/main/scala/sbt/Keys.scala

@@ -716,6 +716,7 @@ object Keys {
   private[sbt] val taskCancelStrategy = settingKey[State => TaskCancellationStrategy]("Experimental task cancellation handler.").withRank(DTask)
   private[sbt] val cacheStoreFactoryFactory = AttributeKey[CacheStoreFactoryFactory]("cache-store-factory-factory")
   private[sbt] val bootServerSocket = AttributeKey[BootServerSocket]("boot-server-socket")
+  private[sbt] val channelProjectCursors = AttributeKey[Map[String, ProjectRef]]("channel-project-cursors", "Per-channel project cursors for multi-client server mode.")
   val fileCacheSize = settingKey[String]("The approximate maximum size in bytes of the cache used to store previous task results. For example, it could be set to \"256M\" to make the maximum size 256 megabytes.")
 
   // Experimental in sbt 0.13.2 to enable grabbing semantic compile failures.

main/src/main/scala/sbt/MainLoop.scala

@@ -254,6 +254,7 @@ private[sbt] object MainLoop:
          * Dropping (FastTrackCommands.evaluate ... getOrElse) should be functionally identical
          * but slower.
          */
+        val prevSessionCurrent = termState.get(Keys.sessionSettings).map(_.current)
         val newState =
           try {
             var errorMsg: Option[String] = None
@@ -267,7 +268,7 @@ private[sbt] object MainLoop:
                 )
               case None => currentCmdProgress.foreach(_.afterCommand(exec.commandLine, Right(res)))
             }
-            res
+            syncChannelCursor(res, prevSessionCurrent, channelName)
           } catch {
             case _: RejectedExecutionException =>
               val cancelled = new Cancelled(exec.commandLine)
@@ -376,6 +377,21 @@ private[sbt] object MainLoop:
     then ExitCode(ErrorCodes.UnknownError)
     else ExitCode.Success
 
+  private def syncChannelCursor(
+      state: State,
+      prevSessionCurrent: Option[ProjectRef],
+      channelName: Option[String]
+  ): State =
+    channelName match
+      case Some(cn) =>
+        val newSessionCurrent = state.get(Keys.sessionSettings).map(_.current)
+        (prevSessionCurrent, newSessionCurrent) match
+          case (prev, Some(curr)) if prev != Some(curr) =>
+            val cursors = state.get(Keys.channelProjectCursors).getOrElse(Map.empty)
+            state.put(Keys.channelProjectCursors, cursors.updated(cn, curr))
+          case _ => state
+      case None => state
+
 end MainLoop
 
 // No stack trace since this is just to notify the user which command they cancelled

main/src/main/scala/sbt/ProjectExtra.scala

@@ -237,8 +237,12 @@ trait ProjectExtra extends Scoped.Syntax:
     def isProjectLoaded(state: State): Boolean =
       (state has Keys.sessionSettings) && (state has Keys.stateBuildStructure)
 
-    def extract(state: State): Extracted =
-      Project.extract(Project.session(state), Project.structure(state))
+    def extract(state: State): Extracted = {
+      val se = Project.session(state)
+      val st = Project.structure(state)
+      val currentRef = internal.ProjectNavigation.effectiveCurrentRef(state)
+      Extracted(st, se, currentRef)(using Project.showContextKey2(se))
+    }
 
     private[sbt] def extract(se: SessionSettings, st: BuildStructure): Extracted =
       Extracted(st, se, se.current)(using Project.showContextKey2(se))

main/src/main/scala/sbt/internal/ProjectNavigation.scala

@@ -11,21 +11,49 @@ package internal
 
 import java.net.URI
 import sbt.internal.util.complete, complete.{ DefaultParsers, Parser }, DefaultParsers.*
-import Keys.sessionSettings
+import Keys.{ channelProjectCursors, sessionSettings }
 import sbt.ProjectExtra.{ extract, updateCurrent }
 
 object ProjectNavigation {
   def command(s: State): Parser[() => State] =
     if s.get(sessionSettings).isEmpty then failure("No project loaded")
     else (new ProjectNavigation(s)).command
+
+  private[sbt] def getChannelName(s: State): Option[String] =
+    s.currentCommand
+      .flatMap(_.source)
+      .map(_.channelName)
+      .orElse(StandardMain.exchange.currentExec.flatMap(_.source).map(_.channelName))
+
+  def getChannelCursor(s: State): Option[ProjectRef] =
+    for {
+      channelName <- getChannelName(s)
+      cursors <- s.get(channelProjectCursors)
+      ref <- cursors.get(channelName)
+    } yield ref
+
+  def setChannelCursor(s: State, ref: ProjectRef): State =
+    getChannelName(s) match {
+      case Some(channelName) =>
+        val cursors = s.get(channelProjectCursors).getOrElse(Map.empty)
+        s.put(channelProjectCursors, cursors.updated(channelName, ref))
+      case None => s
+    }
+
+  def effectiveCurrentRef(s: State): ProjectRef =
+    getChannelCursor(s).getOrElse(
+      s.get(sessionSettings).map(_.current).getOrElse(sys.error("Session not initialized"))
+    )
 }
 
 final class ProjectNavigation(s: State) {
   val extracted: Extracted = Project.extract(s)
-  import extracted.{ currentRef, structure, session }
+  import extracted.{ structure, session }
+
+  def effectiveCurrentRef: ProjectRef = ProjectNavigation.effectiveCurrentRef(s)
 
   def setProject(nuri: URI, nid: String): State = {
-    val neval = if (currentRef.build == nuri) session.currentEval else mkEval(nuri)
+    val neval = if (effectiveCurrentRef.build == nuri) session.currentEval else mkEval(nuri)
     Project.updateCurrent(s.put(sessionSettings, session.setCurrent(nuri, nid, neval)))
   }
 
@@ -38,11 +66,10 @@ final class ProjectNavigation(s: State) {
         show(); s
       case Some(BuildRef(uri))       => changeBuild(uri)
       case Some(ProjectRef(uri, id)) => selectProject(uri, id)
-      /*			else if(to.forall(_ == '.'))
-				if(to.length > 1) gotoParent(to.length - 1, nav, s) else s */ // semantics currently undefined
     }
 
-  def show(): Unit = s.log.info(s"${currentRef.project} (in build ${currentRef.build})")
+  def show(): Unit =
+    s.log.info(s"${effectiveCurrentRef.project} (in build ${effectiveCurrentRef.build})")
 
   def selectProject(uri: URI, to: String): State =
     if (structure.units(uri).defined.contains(to))
@@ -63,12 +90,13 @@ final class ProjectNavigation(s: State) {
   import Parser.*, complete.Parsers.*
 
   val parser: Parser[Option[ResolvedReference]] = {
-    val reference = Act.resolvedReference(structure.index.keyIndex, currentRef.build, success(()))
+    val reference =
+      Act.resolvedReference(structure.index.keyIndex, effectiveCurrentRef.build, success(()))
     val root = token('/' ^^^ rootRef)
     success(None) | some(token(Space) ~> (root | reference))
   }
 
-  def rootRef = ProjectRef(currentRef.build, getRoot(currentRef.build))
+  def rootRef = ProjectRef(effectiveCurrentRef.build, getRoot(effectiveCurrentRef.build))
 
   val command: Parser[() => State] = Command.applyEffect(parser)(apply)
 }

server-test/src/server-test/channel-cursor/build.sbt

@@ -0,0 +1,28 @@
+scalaVersion := "3.8.1"
+
+val printCurrentProject = taskKey[Unit]("Prints current project name")
+
+lazy val projectA = (project in file("projectA"))
+  .settings(
+    name := "project-a",
+    printCurrentProject := {
+      streams.value.log.info(s"CURRENT_PROJECT_IS:${name.value}")
+    }
+  )
+
+lazy val projectB = (project in file("projectB"))
+  .settings(
+    name := "project-b",
+    printCurrentProject := {
+      streams.value.log.info(s"CURRENT_PROJECT_IS:${name.value}")
+    }
+  )
+
+lazy val root = (project in file("."))
+  .aggregate(projectA, projectB)
+  .settings(
+    name := "root",
+    printCurrentProject := {
+      streams.value.log.info(s"CURRENT_PROJECT_IS:${name.value}")
+    }
+  )

server-test/src/test/scala/testpkg/ChannelCursorTest.scala

@@ -0,0 +1,151 @@
+/*
+ * sbt
+ * Copyright 2023, Scala center
+ * Copyright 2011 - 2022, Lightbend, Inc.
+ * Copyright 2008 - 2010, Mark Harrah
+ * Licensed under Apache License 2.0 (see LICENSE)
+ */
+
+package testpkg
+
+import java.io.IOException
+import java.net.Socket
+import java.util.concurrent.{ LinkedBlockingQueue, TimeUnit }
+import java.util.concurrent.atomic.AtomicBoolean
+import sbt.protocol.ClientSocket
+
+import scala.annotation.tailrec
+import scala.concurrent.duration.*
+
+class ChannelCursorTest extends AbstractServerTest {
+  override val testDirectory: String = "channel-cursor"
+
+  private def createSecondConnection()
+      : (Socket, java.io.OutputStream, LinkedBlockingQueue[String], AtomicBoolean) = {
+    val portfile = testPath.resolve("project/target/active.json").toFile
+    @tailrec
+    def connect(attempt: Int): Socket = {
+      val res =
+        try Some(ClientSocket.socket(portfile)._1)
+        catch { case _: IOException if attempt < 10 => None }
+      res match {
+        case Some(s) => s
+        case _ =>
+          Thread.sleep(100)
+          connect(attempt + 1)
+      }
+    }
+    val sk = connect(0)
+    val out = sk.getOutputStream
+    val in = sk.getInputStream
+    val lines = new LinkedBlockingQueue[String]
+    val running = new AtomicBoolean(true)
+    new Thread(
+      () => {
+        while (running.get) {
+          try lines.put(sbt.ReadJson(in, running))
+          catch { case _: Exception => running.set(false) }
+        }
+      },
+      "sbt-server-test-read-thread-2"
+    ) {
+      setDaemon(true)
+      start()
+    }
+    (sk, out, lines, running)
+  }
+
+  private def sendJsonRpc(out: java.io.OutputStream, message: String): Unit = {
+    def writeLine(s: String): Unit = {
+      val retByte: Byte = '\r'.toByte
+      val delimiter: Byte = '\n'.toByte
+      if (s != "") {
+        out.write(s.getBytes("UTF-8"))
+      }
+      out.write(retByte.toInt)
+      out.write(delimiter.toInt)
+      out.flush
+    }
+    writeLine(s"""Content-Length: ${message.size + 2}""")
+    writeLine("")
+    writeLine(message)
+  }
+
+  private def waitForString(lines: LinkedBlockingQueue[String], duration: FiniteDuration)(
+      f: String => Boolean
+  ): Boolean = {
+    val deadline = duration.fromNow
+    @tailrec def impl(): Boolean =
+      lines.poll(deadline.timeLeft.toMillis, TimeUnit.MILLISECONDS) match {
+        case null => false
+        case s    => if (!f(s) && !deadline.isOverdue) impl() else !deadline.isOverdue()
+      }
+    impl()
+  }
+
+  test("channel cursor - independent project cursors") {
+    val (sk2, out2, lines2, running2) = createSecondConnection()
+    try {
+      sendJsonRpc(
+        out2,
+        """{ "jsonrpc": "2.0", "id": 1, "method": "initialize", "params": { "initializationOptions": { "skipAnalysis": true } } }"""
+      )
+      waitForString(lines2, 10.seconds)(_.contains(""""capabilities":{"""))
+
+      svr.sendJsonRpc(
+        """{ "jsonrpc": "2.0", "id": 10, "method": "sbt/exec", "params": { "commandLine": "project projectA" } }"""
+      )
+      assert(
+        svr.waitForString(10.seconds) { s =>
+          println(s"[channel1] $s")
+          s.contains("projectA") || s.contains("\"execId\":10")
+        },
+        "Channel 1 should switch to projectA"
+      )
+
+      sendJsonRpc(
+        out2,
+        """{ "jsonrpc": "2.0", "id": 20, "method": "sbt/exec", "params": { "commandLine": "project projectB" } }"""
+      )
+      assert(
+        waitForString(lines2, 10.seconds) { s =>
+          println(s"[channel2] $s")
+          s.contains("projectB") || s.contains("\"execId\":20")
+        },
+        "Channel 2 should switch to projectB"
+      )
+
+      svr.sendJsonRpc(
+        """{ "jsonrpc": "2.0", "id": 11, "method": "sbt/exec", "params": { "commandLine": "printCurrentProject" } }"""
+      )
+      var foundProjectA = false
+      assert(
+        svr.waitForString(30.seconds) { s =>
+          println(s"[channel1 name] $s")
+          if (s.contains("CURRENT_PROJECT_IS:project-a")) foundProjectA = true
+          s.contains("\"execId\":11") && s.contains("\"status\":\"Done\"")
+        },
+        "First channel printCurrentProject command should complete"
+      )
+      assert(foundProjectA, "First channel should still be on projectA")
+
+      sendJsonRpc(
+        out2,
+        """{ "jsonrpc": "2.0", "id": 21, "method": "sbt/exec", "params": { "commandLine": "printCurrentProject" } }"""
+      )
+      var foundProjectB = false
+      assert(
+        waitForString(lines2, 30.seconds) { s =>
+          println(s"[channel2 name] $s")
+          if (s.contains("CURRENT_PROJECT_IS:project-b")) foundProjectB = true
+          s.contains("\"execId\":21") && s.contains("\"status\":\"Done\"")
+        },
+        "Second channel printCurrentProject command should complete"
+      )
+      assert(foundProjectB, "Second channel should still be on projectB")
+    } finally {
+      running2.set(false)
+      sk2.close()
+    }
+  }
+}