Don't use try_acquire alongside spawned task

We're hitting an issue that looks similar to
launchbadge/sqlx#622

I was able to work around it by disabling fairness, but we can just
avoid the lookup entirely if active workers is >= the pool size.

Signed-off-by: Joe Grund <jgrund@whamcloud.io>

iml-task-runner/src/main.rs

@@ -7,7 +7,7 @@ use iml_action_client::invoke_rust_agent;
 use iml_manager_env::get_pool_limit;
 use iml_postgres::{
     get_db_pool,
-    sqlx::{self, pool::PoolConnection, Done, Executor, PgPool, Postgres},
+    sqlx::{self, Done, Executor, PgPool},
 };
 use iml_tracing::tracing;
 use iml_wire_types::{
@@ -37,7 +37,7 @@ lazy_static! {
 }
 
 async fn available_workers(
-    conn: &mut PoolConnection<Postgres>,
+    pool: &PgPool,
     ids: Vec<i32>,
 ) -> Result<Vec<LustreClient>, error::ImlTaskRunnerError> {
     let clients = sqlx::query_as!(
@@ -53,7 +53,7 @@ async fn available_workers(
         &ids,
         max(*POOL_LIMIT as i64 - ids.len() as i64, 0),
     )
-    .fetch_all(conn)
+    .fetch_all(pool)
     .await?;
 
     Ok(clients)
@@ -332,29 +332,21 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
 
         tracing::debug!("Pool State: {:?}", pg_pool);
 
-        let mut conn = match pg_pool.try_acquire() {
-            Some(x) => x,
-            None => {
-                tracing::info!(
-                    "Could not acquire connection (pool full), will try again next tick"
-                );
-                continue;
-            }
-        };
-
-        let workers = {
+        let ids: Vec<i32> = {
             let xs = active_clients.lock().await;
-            let ids: Vec<i32> = xs.iter().copied().collect();
+            xs.iter().copied().collect()
+        };
 
-            tracing::debug!("checking workers for ids: {:?}", ids);
+        if ids.len() as u32 >= *POOL_LIMIT {
+            tracing::info!("No more capacity to service tasks. Active workers: {:?}, Connection Limit: {}. Will try again next tick.", ids, *POOL_LIMIT);
+            continue;
+        }
 
-            let workers = available_workers(&mut conn, ids).await?;
+        tracing::debug!("checking workers for ids: {:?}", ids);
 
-            tracing::debug!("got workers: {:?}", workers);
+        let workers = available_workers(&pg_pool, ids).await?;
 
-            drop(conn);
-            workers
-        };
+        tracing::debug!("got workers: {:?}", workers);
 
         {
             let mut x = active_clients.lock().await;