diff --git a/src/Pure/Tools/build_log.scala b/src/Pure/Tools/build_log.scala
--- a/src/Pure/Tools/build_log.scala
+++ b/src/Pure/Tools/build_log.scala
@@ -1,357 +1,362 @@
 /*  Title:      Pure/Tools/build_log.scala
     Author:     Makarius
 
 Build log parsing for historic versions, back to "build_history_base".
 */
 
 package isabelle
 
 
 import java.time.ZonedDateTime
 import java.time.format.{DateTimeFormatter, DateTimeParseException}
 
 import scala.collection.mutable
 import scala.util.matching.Regex
 
 
 object Build_Log
 {
   /** settings **/
 
   object Settings
   {
     val build_settings = List("ISABELLE_BUILD_OPTIONS")
     val ml_settings = List("ML_PLATFORM", "ML_HOME", "ML_SYSTEM", "ML_OPTIONS")
     val all_settings = build_settings ::: ml_settings
 
     type Entry = (String, String)
     type T = List[Entry]
 
     object Entry
     {
       def unapply(s: String): Option[Entry] =
         s.indexOf('=') match {
           case -1 => None
           case i =>
             val a = s.substring(0, i)
             val b = Library.perhaps_unquote(s.substring(i + 1))
             Some((a, b))
         }
       def apply(a: String, b: String): String = a + "=" + quote(b)
       def getenv(a: String): String = apply(a, Isabelle_System.getenv(a))
     }
 
     def show(): String =
       cat_lines(
         build_settings.map(Entry.getenv(_)) ::: List("") ::: ml_settings.map(Entry.getenv(_)))
   }
 
 
   /** log file **/
 
   object Log_File
   {
     def apply(path: Path): Log_File =
     {
       val (path_name, ext) = path.expand.split_ext
       val text =
         if (ext == "gz") File.read_gzip(path)
         else if (ext == "xz") File.read_xz(path)
         else File.read(path)
       apply(path_name.base.implode, text)
     }
 
     def apply(name: String, lines: List[String]): Log_File =
       new Log_File(name, lines)
 
     def apply(name: String, text: String): Log_File =
       Log_File(name, Library.trim_split_lines(text))
   }
 
   class Log_File private(val name: String, val lines: List[String])
   {
     log_file =>
 
     override def toString: String = name
 
     def text: String = cat_lines(lines)
 
     def err(msg: String): Nothing =
       error("Error in log file " + quote(name) + ": " + msg)
 
 
     /* inlined content */
 
     def find[A](f: String => Option[A]): Option[A] =
       lines.iterator.map(f).find(_.isDefined).map(_.get)
 
     def find_match(regex: Regex): Option[String] =
       lines.iterator.map(regex.unapplySeq(_)).find(res => res.isDefined && res.get.length == 1).
         map(res => res.get.head)
 
 
     /* settings */
 
     def get_setting(a: String): Settings.Entry =
       Settings.Entry.unapply(
         lines.find(_.startsWith(a + "=")) getOrElse err("missing " + a)).get
 
     def get_settings(as: List[String]): Settings.T = as.map(get_setting(_))
 
 
     /* properties (YXML) */
 
     val xml_cache = new XML.Cache()
 
     def parse_props(text: String): Properties.T =
       xml_cache.props(XML.Decode.properties(YXML.parse_body(text)))
 
     def filter_props(prefix: String): List[Properties.T] =
       for (line <- lines; s <- Library.try_unprefix(prefix, line)) yield parse_props(s)
 
     def find_line(prefix: String): Option[String] =
       find(Library.try_unprefix(prefix, _))
 
     def find_props(prefix: String): Option[Properties.T] =
       find_line(prefix).map(parse_props(_))
 
 
     /* parse various formats */
 
     def parse_session_info(
         default_name: String = "",
         command_timings: Boolean = false,
         ml_statistics: Boolean = false,
         task_statistics: Boolean = false): Session_Info =
       Build_Log.parse_session_info(
         log_file, default_name, command_timings, ml_statistics, task_statistics)
 
     def parse_header(): Header = Build_Log.parse_header(log_file)
 
     def parse_build_info(): Build_Info = Build_Log.parse_build_info(log_file)
   }
 
 
   /* session log: produced by "isabelle build" */
 
   sealed case class Session_Info(
     session_name: String,
     session_timing: Properties.T,
     command_timings: List[Properties.T],
     ml_statistics: List[Properties.T],
     task_statistics: List[Properties.T])
 
   private def parse_session_info(
     log_file: Log_File,
     default_name: String,
     command_timings: Boolean,
     ml_statistics: Boolean,
     task_statistics: Boolean): Session_Info =
   {
     val xml_cache = new XML.Cache()
 
     val session_name =
       log_file.find_line("\fSession.name = ") match {
         case None => default_name
         case Some(name) if default_name == "" || default_name == name => name
         case Some(name) => log_file.err("log from different session " + quote(name))
       }
     val session_timing = log_file.find_props("\fTiming = ") getOrElse Nil
     val command_timings_ =
       if (command_timings) log_file.filter_props("\fcommand_timing = ") else Nil
     val ml_statistics_ =
       if (ml_statistics) log_file.filter_props("\fML_statistics = ") else Nil
     val task_statistics_ =
       if (task_statistics) log_file.filter_props("\ftask_statistics = ") else Nil
 
     Session_Info(session_name, session_timing, command_timings_, ml_statistics_, task_statistics_)
   }
 
 
   /* header and meta data */
 
   object Header_Kind extends Enumeration
   {
     val ISATEST = Value("isatest")
     val AFP_TEST = Value("afp-test")
     val JENKINS = Value("jenkins")
   }
 
   sealed case class Header(
     kind: Header_Kind.Value, props: Properties.T, settings: List[(String, String)])
 
   object Field
   {
     val build_host = "build_host"
     val build_start = "build_start"
     val build_end = "build_end"
     val isabelle_version = "isabelle_version"
     val afp_version = "afp_version"
   }
 
   object AFP
   {
     val Date_Format =
       Date.Format.make_patterns(List("EEE MMM d HH:mm:ss VV yyyy", "EEE MMM d HH:mm:ss O yyyy"),
         // workaround for jdk-8u102
         s => Word.implode(Word.explode(s).map({ case "CEST" => "GMT+2" case a => a })))
 
     val Test_Start = new Regex("""^Start test for .+ at (.+), (\w+)$""")
     val Test_End = new Regex("""^End test on (.+), \w+, elapsed time:.*$""")
     val Isabelle_Version = new Regex("""^Isabelle version: .* -- hg id (\w+)$""")
     val AFP_Version = new Regex("""^AFP version: .* -- hg id (\w+)$""")
   }
 
   private def parse_header(log_file: Log_File): Header =
   {
     log_file.lines match {
       case AFP.Test_Start(start, hostname) :: _ =>
         (start, log_file.lines.last) match {
           case (AFP.Date_Format(start_date), AFP.Test_End(AFP.Date_Format(end_date))) =>
             val isabelle_version =
               log_file.find_match(AFP.Isabelle_Version) getOrElse
                 log_file.err("missing Isabelle version")
             val afp_version =
               log_file.find_match(AFP.AFP_Version) getOrElse
                 log_file.err("missing AFP version")
 
             Header(Header_Kind.AFP_TEST,
               List(
                 Field.build_host -> hostname,
                 Field.build_start -> start_date.toString,
                 Field.build_end -> end_date.toString,
                 Field.isabelle_version -> isabelle_version,
                 Field.afp_version -> afp_version),
               log_file.get_settings(Settings.all_settings))
 
           case _ => log_file.err("cannot detect start/end date in afp-test log")
         }
       case _ => log_file.err("cannot detect log header format")
     }
   }
 
 
   /* build info: produced by isabelle build */
 
   object Session_Status extends Enumeration
   {
     val EXISTING = Value("existing")
     val FINISHED = Value("finished")
     val FAILED = Value("failed")
     val CANCELLED = Value("cancelled")
   }
 
   sealed case class Session_Entry(
     chapter: String,
     groups: List[String],
     threads: Option[Int],
     timing: Option[Timing],
     ml_timing: Option[Timing],
     status: Session_Status.Value)
   {
-    def ok: Boolean = status == Session_Status.EXISTING || status == Session_Status.FINISHED
     def finished: Boolean = status == Session_Status.FINISHED
   }
 
   sealed case class Build_Info(sessions: Map[String, Session_Entry])
   {
     def session(name: String): Session_Entry = sessions(name)
     def get_session(name: String): Option[Session_Entry] = sessions.get(name)
 
     def finished(name: String): Boolean =
       get_session(name) match {
         case Some(entry) => entry.finished
         case None => false
       }
 
     def timing(name: String): Timing =
       (for (entry <- get_session(name); t <- entry.timing) yield t) getOrElse Timing.zero
 
     def ml_timing(name: String): Timing =
       (for (entry <- get_session(name); t <- entry.ml_timing) yield t) getOrElse Timing.zero
   }
 
   private def parse_build_info(log_file: Log_File): Build_Info =
   {
     object Chapter_Name
     {
       def unapply(s: String): Some[(String, String)] =
         space_explode('/', s) match {
           case List(chapter, name) => Some((chapter, name))
           case _ => Some(("", s))
         }
     }
 
     val Session_No_Groups = new Regex("""^Session (\S+)$""")
     val Session_Groups = new Regex("""^Session (\S+) \((.*)\)$""")
     val Session_Finished1 =
       new Regex("""^Finished (\S+) \((\d+):(\d+):(\d+) elapsed time, (\d+):(\d+):(\d+) cpu time.*$""")
     val Session_Finished2 =
       new Regex("""^Finished (\S+) \((\d+):(\d+):(\d+) elapsed time.*$""")
     val Session_Timing =
       new Regex("""^Timing (\S+) \((\d) threads, (\d+\.\d+)s elapsed time, (\d+\.\d+)s cpu time, (\d+\.\d+)s GC time.*$""")
+    val Session_Started = new Regex("""^(?:Running|Building) (\S+) \.\.\.$""")
     val Session_Failed = new Regex("""^(\S+) FAILED""")
     val Session_Cancelled = new Regex("""^(\S+) CANCELLED""")
 
     var chapter = Map.empty[String, String]
     var groups = Map.empty[String, List[String]]
     var threads = Map.empty[String, Int]
     var timing = Map.empty[String, Timing]
     var ml_timing = Map.empty[String, Timing]
+    var started = Set.empty[String]
     var failed = Set.empty[String]
     var cancelled = Set.empty[String]
     def all_sessions: Set[String] =
       chapter.keySet ++ groups.keySet ++ threads.keySet ++
-      timing.keySet ++ ml_timing.keySet ++ failed ++ cancelled
+      timing.keySet ++ ml_timing.keySet ++ failed ++ cancelled ++ started
 
 
     for (line <- log_file.lines) {
       line match {
         case Session_No_Groups(Chapter_Name(chapt, name)) =>
           chapter += (name -> chapt)
           groups += (name -> Nil)
         case Session_Groups(Chapter_Name(chapt, name), grps) =>
           chapter += (name -> chapt)
           groups += (name -> Word.explode(grps))
+        case Session_Started(name) =>
+          started += name
         case Session_Finished1(name,
             Value.Int(e1), Value.Int(e2), Value.Int(e3),
             Value.Int(c1), Value.Int(c2), Value.Int(c3)) =>
           val elapsed = Time.hms(e1, e2, e3)
           val cpu = Time.hms(c1, c2, c3)
           timing += (name -> Timing(elapsed, cpu, Time.zero))
         case Session_Finished2(name,
             Value.Int(e1), Value.Int(e2), Value.Int(e3)) =>
           val elapsed = Time.hms(e1, e2, e3)
           timing += (name -> Timing(elapsed, Time.zero, Time.zero))
         case Session_Timing(name,
             Value.Int(t), Value.Double(e), Value.Double(c), Value.Double(g)) =>
           val elapsed = Time.seconds(e)
           val cpu = Time.seconds(c)
           val gc = Time.seconds(g)
           ml_timing += (name -> Timing(elapsed, cpu, gc))
           threads += (name -> t)
         case _ =>
       }
     }
 
     val sessions =
       Map(
         (for (name <- all_sessions.toList) yield {
           val status =
             if (failed(name)) Session_Status.FAILED
             else if (cancelled(name)) Session_Status.CANCELLED
-            else if (timing.isDefinedAt(name)) Session_Status.FINISHED
+            else if (timing.isDefinedAt(name) || ml_timing.isDefinedAt(name))
+              Session_Status.FINISHED
+            else if (started(name)) Session_Status.FAILED
             else Session_Status.EXISTING
           val entry =
             Session_Entry(
               chapter.getOrElse(name, ""),
               groups.getOrElse(name, Nil),
               threads.get(name),
               timing.get(name),
               ml_timing.get(name),
               status)
           (name -> entry)
         }):_*)
     Build_Info(sessions)
   }
 }