don't start background jobs if tenant has not timelines

Before this change, test_pageserver_with_empty_tenants was failing at:

  assert loaded_tenant["state"] == {
        "Active": {"background_jobs_running": False}
    }, "Tenant {tenant_with_empty_timelines_dir} with empty timelines dir should be active and ready for timeline creation"

because background_jobs_running was True instead of False.

Personally I think we should simply always start the background loops
and not bother, but let's punt this until after we've merged this PR.

pageserver/src/tenant.rs

@@ -809,6 +809,9 @@ impl Tenant {
         // FIXME original collect_timeline_files contained one more check:
         //    1. "Timeline has no ancestor and no layer files"
 
+        // XXX get rid of enable_background_jobs
+        let enable_background_jobs = sorted_timelines.len() > 0;
+
         for (timeline_id, metadata) in sorted_timelines {
             // FIXME should we fail load of whole tenant if one timeline failed?
             //   consider branch hierarchy. Maybe set one to broken and others to Paused or something
@@ -818,7 +821,7 @@ impl Tenant {
         // We're ready for business.
         // Change to active state under the hood spawns background loops
         // The loops will shut themselves down when they notice that the tenant is inactive.
-        self.activate(true);
+        self.activate(enable_background_jobs);
 
         info!("Done");
 

test_runner/regress/test_tenants.py

@@ -217,8 +217,9 @@ def test_pageserver_with_empty_tenants(
     env.pageserver.allowed_errors.append(
         ".*marking .* as locally complete, while it doesnt exist in remote index.*"
     )
-    env.pageserver.allowed_errors.append(".*Tenant .* has no timelines directory.*")
-    env.pageserver.allowed_errors.append(".*No timelines to attach received.*")
+    env.pageserver.allowed_errors.append(
+        ".*could not load tenant.*Failed to list timelines directory.*"
+    )
 
     client = env.pageserver.http_client()