publish current log file, e.g. for easy error detection;

--- a/src/Pure/Admin/isabelle_cronjob.scala	Tue Mar 13 21:22:02 2018 +0100
+++ b/src/Pure/Admin/isabelle_cronjob.scala	Tue Mar 13 21:54:48 2018 +0100
@@ -7,6 +7,8 @@
 package isabelle
 
 
+import java.nio.file.Files
+
 import scala.annotation.tailrec
 import scala.collection.mutable
 
@@ -54,6 +56,9 @@
         Isabelle_System.bash(
           """rsync -a --include="*/" --include="plain_identify*" --exclude="*" """ +
             Bash.string(backup + "/log") + " " + File.bash_path(main_dir)).check
+
+        if (!Isabelle_Devel.cronjob_log.is_file)
+          Files.createSymbolicLink(Isabelle_Devel.cronjob_log.file.toPath, current_log.file.toPath)
       })
 
   val exit =

--- a/src/Pure/Admin/isabelle_devel.scala	Tue Mar 13 21:22:02 2018 +0100
+++ b/src/Pure/Admin/isabelle_devel.scala	Tue Mar 13 21:54:48 2018 +0100
@@ -9,11 +9,13 @@
 
 object Isabelle_Devel
 {
-  val root = Path.explode("~/html-data/devel")
-
   val RELEASE_SNAPSHOT = "release_snapshot"
   val BUILD_LOG_DB = "build_log.db"
   val BUILD_STATUS = "build_status"
+  val CRONJOB_LOG = "cronjob-main.log"
+
+  val root = Path.explode("~/html-data/devel")
+  val cronjob_log = root + Path.basic(CRONJOB_LOG)
 
 
   /* index */
@@ -40,7 +42,9 @@
             HTML.text(" information (e.g. for ") :::
             List(HTML.link("http://sqlitebrowser.org",
               List(HTML.code(HTML.text("sqlitebrowser"))))) :::
-            HTML.text(")")))))
+            HTML.text(")"),
+
+            HTML.text("Cronjob ") ::: List(HTML.link(CRONJOB_LOG, HTML.text("log file")))))))
   }