Start task scheduler at end of startup. (#22649)

* Start task scheduler at end of startup.

The sync db task does a lot of db work to be addressed in a later
commit. But it also begins firing tasks _during startup_. Lets prevent
syncs and other tasks while we are starting up until we have finished.

Scheduler can be set to standby mode while we wait on finishing up
initializing and then start after.

* docstring cleanup and comments

src/metabase/core.clj

@@ -104,8 +104,8 @@
     (events/initialize-events!)
     (init-status/set-progress! 0.7)
 
-    ;; Now start the task runner
-    (task/start-scheduler!)
+    ;; Now initialize the task runner
+    (task/init-scheduler!)
     (init-status/set-progress! 0.8)
 
     (when new-install?
@@ -123,6 +123,8 @@
       ;; otherwise update if appropriate
       (sample-data/update-sample-database-if-needed!)))
 
+  ;; start scheduler at end of init!
+  (task/start-scheduler!)
   (init-status/set-complete!)
   (log/info (trs "Metabase Initialization COMPLETE")))
 

src/metabase/task.clj

@@ -146,8 +146,9 @@
   (when (= (mdb/db-type) :postgres)
     (System/setProperty "org.quartz.jobStore.driverDelegateClass" "org.quartz.impl.jdbcjobstore.PostgreSQLDelegate")))
 
-(defn start-scheduler!
-  "Start our Quartzite scheduler which allows jobs to be submitted and triggers to begin executing."
+(defn init-scheduler!
+  "Initialize our Quartzite scheduler which allows jobs to be submitted and triggers to scheduled. Puts scheduler in
+  standby mode. Call [[start-scheduler!]] to begin running scheduled tasks."
   []
   (classloader/the-classloader)
   (when-not @quartz-scheduler
@@ -155,9 +156,19 @@
     (let [new-scheduler (qs/initialize)]
       (when (compare-and-set! quartz-scheduler nil new-scheduler)
         (find-and-load-task-namespaces!)
-        (qs/start new-scheduler)
+        (qs/standby new-scheduler)
+        (log/info (trs "Task scheduler initialized into standby mode."))
         (init-tasks!)))))
 
+(defn start-scheduler!
+  "Start an initialized scheduler. Tasks do not run before calling this function. It is an error to call this function
+  when [[quartz-scheduler]] has not been set. The function [[init-scheduler!]] will initialize this correctly."
+  []
+  (if-let [scheduler @quartz-scheduler]
+    (do (qs/start scheduler)
+        (log/info (trs "Task scheduler started")))
+    (throw (trs "Scheduler not initialized but `start-scheduler!` called. Please call `init-scheduler!` before attempting to start."))))
+
 (defn stop-scheduler!
   "Stop our Quartzite scheduler and shutdown any running executions."
   []