fc-common: implement deficit-based fair-share scheduling in list_pending

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: Ic1345cfdf712aa6ee6f0eeae45b3e62b6a6a6964

crates/common/src/repo/builds.rs

@@ -69,13 +69,29 @@ pub async fn list_for_evaluation(
   .map_err(CiError::Database)
 }
 
-pub async fn list_pending(pool: &PgPool, limit: i64) -> Result<Vec<Build>> {
+pub async fn list_pending(
+  pool: &PgPool,
+  limit: i64,
+  worker_count: i32,
+) -> Result<Vec<Build>> {
   sqlx::query_as::<_, Build>(
-    "SELECT b.* FROM builds b JOIN evaluations e ON b.evaluation_id = e.id \
-     JOIN jobsets j ON e.jobset_id = j.id WHERE b.status = 'pending' ORDER BY \
-     b.priority DESC, j.scheduling_shares DESC, b.created_at ASC LIMIT $1",
+    "WITH running_counts AS ( SELECT e.jobset_id, COUNT(*) AS running FROM \
+     builds b JOIN evaluations e ON b.evaluation_id = e.id WHERE b.status = \
+     'running' GROUP BY e.jobset_id ), active_shares AS ( SELECT j.id AS \
+     jobset_id, j.scheduling_shares, COALESCE(rc.running, 0) AS running, \
+     SUM(j.scheduling_shares) OVER () AS total_shares FROM jobsets j JOIN \
+     evaluations e2 ON e2.jobset_id = j.id JOIN builds b2 ON b2.evaluation_id \
+     = e2.id AND b2.status = 'pending' LEFT JOIN running_counts rc ON \
+     rc.jobset_id = j.id WHERE j.scheduling_shares > 0 GROUP BY j.id, \
+     j.scheduling_shares, rc.running ) SELECT b.* FROM builds b JOIN \
+     evaluations e ON b.evaluation_id = e.id JOIN active_shares ash ON \
+     ash.jobset_id = e.jobset_id WHERE b.status = 'pending' ORDER BY \
+     b.priority DESC, (ash.scheduling_shares::float / \
+     GREATEST(ash.total_shares, 1) - ash.running::float / GREATEST($2, 1)) \
+     DESC, b.created_at ASC LIMIT $1",
   )
   .bind(limit)
+  .bind(worker_count)
   .fetch_all(pool)
   .await
   .map_err(CiError::Database)

crates/common/tests/repo_tests.rs

@@ -293,7 +293,7 @@ async fn test_evaluation_and_build_lifecycle() {
   assert_eq!(build.system.as_deref(), Some("x86_64-linux"));
 
   // List pending
-  let pending = repo::builds::list_pending(&pool, 10)
+  let pending = repo::builds::list_pending(&pool, 10, 4)
     .await
     .expect("list pending");
   assert!(pending.iter().any(|b| b.id == build.id));