diff -rN -u old-smederee/modules/hub/src/main/scala/de/smederee/ssh/DarcsSshCommand.scala new-smederee/modules/hub/src/main/scala/de/smederee/ssh/DarcsSshCommand.scala
--- old-smederee/modules/hub/src/main/scala/de/smederee/ssh/DarcsSshCommand.scala	2025-02-02 06:54:30.859470922 +0000
+++ new-smederee/modules/hub/src/main/scala/de/smederee/ssh/DarcsSshCommand.scala	2025-02-02 06:54:30.859470922 +0000
@@ -39,11 +39,11 @@
 abstract class DarcsSshCommand extends Command with ServerSessionAware {
   private val log = LoggerFactory.getLogger(classOf[DarcsSshCommand])
 
-  protected var session: ServerSession = null
-  protected var stdin: InputStream     = null
-  protected var stdout: OutputStream   = null
-  protected var stderr: OutputStream   = null
-  protected var callback: ExitCallback = null
+  @volatile protected var session: ServerSession = null
+  @volatile protected var stdin: InputStream     = null
+  @volatile protected var stdout: OutputStream   = null
+  @volatile protected var stderr: OutputStream   = null
+  @volatile protected var callback: ExitCallback = null
 
   override def destroy(channel: ChannelSession): Unit            = ()
   override def setErrorStream(errorStream: OutputStream): Unit   = this.stderr = errorStream
@@ -63,18 +63,26 @@
   *   The username of the owner of the requested repository.
   * @param repository
   *   The name of the vcs repository.
+  * @param useDebugFlag
+  *   A flag indicating if we should use the `--debug` flag.
   */
-final class DarcsApply(darcsConfiguration: DarcsConfiguration, owner: Username, repository: VcsRepositoryName)
-    extends DarcsSshCommand {
+final class DarcsApply(
+    darcsConfiguration: DarcsConfiguration,
+    owner: Username,
+    repository: VcsRepositoryName,
+    useDebugFlag: Boolean
+) extends DarcsSshCommand {
   private val log = LoggerFactory.getLogger(classOf[DarcsApply])
 
   override def start(channel: ChannelSession, env: Environment): Unit = {
     log.debug(s"DarcsApply for $owner/$repository")
     val repoDir = Paths.get(owner.toString, repository.toString)
-    val cmd = os.proc(
-      darcsConfiguration.executable.toString,
-      List("apply", "--all", "--repodir", repoDir.toString)
-    )
+    val options =
+      if (useDebugFlag)
+        List("apply", "--all", "--debug", "--repodir", repoDir.toString)
+      else
+        List("apply", "--all", "--repodir", repoDir.toString)
+    val cmd = os.proc(darcsConfiguration.executable.toString, options)
     cmd.spawn(
       cwd = os.Path(darcsConfiguration.repositoriesDirectory.toPath),
       stdin = os.ProcessInput.makeSourceInput(this.stdin),
@@ -131,9 +139,25 @@
     log.debug(s"Requested SSH command: $command")
 
     command match {
-      case DarcsSshCommandFactory.FilterDarcsApplyCommand("apply", _, _, "--repodir", owner, repository) =>
+      case DarcsSshCommandFactory.FilterDarcsApplyCommand(
+            "apply",
+            _,
+            _,
+            _,
+            debugFlag,
+            "--repodir",
+            owner,
+            repository
+          ) =>
         (SshUsername.from(owner), VcsRepositoryName.from(repository))
-          .mapN((owner, repository) => new DarcsApply(darcsConfiguration, owner.toUsername, repository))
+          .mapN((owner, repository) =>
+            new DarcsApply(
+              darcsConfiguration,
+              owner.toUsername,
+              repository,
+              debugFlag === "--debug"
+            )
+          )
           .getOrElse(new UnknownCommand(command))
       case DarcsSshCommandFactory.FilterDarcsTransferModeCommand(
             "transfer-mode",
@@ -143,7 +167,8 @@
           ) =>
         (SshUsername.from(owner), VcsRepositoryName.from(repository))
           .mapN((owner, repository) =>
-            new DarcsTransferMode(darcsConfiguration, owner.toUsername, repository)
+            // new DarcsTransferMode(darcsConfiguration, owner.toUsername, repository)
+            new UnknownCommand(command) // FIXME Make transfer-mode work (stalls currently).
           )
           .getOrElse(new UnknownCommand(command))
       case DarcsSshCommandFactory.FilterScpCommand("-f", _, "--", path) =>
@@ -174,7 +199,7 @@
 object DarcsSshCommandFactory {
   // A regular expression which should match on our allowed darcs commands and their arguments.
   val FilterDarcsApplyCommand: Regex =
-    "^darcs (apply) ((--all)\\s)?\\s*(--repodir) '([a-z][a-z0-9]{1,31})/([a-zA-Z0-9][a-zA-Z0-9\\-_]{1,63})'$".r
+    "^darcs (apply) ((--all)\\s)?((--debug)\\s)?\\s*(--repodir) '([a-z][a-z0-9]{1,31})/([a-zA-Z0-9][a-zA-Z0-9\\-_]{1,63})'$".r
   val FilterDarcsTransferModeCommand: Regex =
     "^darcs (transfer-mode) (--repodir) ([a-z][a-z0-9]{1,31})/([a-zA-Z0-9][a-zA-Z0-9\\-_]{1,63})/?$".r
   // After the initial transfer-mode call darcs will fallback to using scp.