store: Use a fdw connection for copy::is_source

store/postgres/src/copy.rs

@@ -37,7 +37,7 @@ use graph::{
         info, lazy_static, o, warn, BlockNumber, BlockPtr, CheapClone, Logger, StoreError, ENV_VARS,
     },
     schema::EntityType,
-    slog::error,
+    slog::{debug, error},
 };
 use itertools::Itertools;
 
@@ -113,16 +113,33 @@ table! {
 }
 
 /// Return `true` if the site is the source of a copy operation. The copy
-/// operation might be just queued or in progress already
-pub fn is_source(conn: &mut PgConnection, site: &Site) -> Result<bool, StoreError> {
+/// operation might be just queued or in progress already. This method will
+/// block until a fdw connection becomes available.
+pub fn is_source(logger: &Logger, pool: &ConnectionPool, site: &Site) -> Result<bool, StoreError> {
     use active_copies as ac;
 
+    // We use a fdw connection to check if the site is being copied. If we
+    // used an ordinary connection and there are many calls to this method,
+    // postgres_fdw might open an unmanageable number of connections into
+    // the primary, which makes the primary run out of connections
+    let mut last_log = Instant::now();
+    let mut conn = pool.get_fdw(&logger, || {
+        if last_log.elapsed() > LOG_INTERVAL {
+            last_log = Instant::now();
+            debug!(
+                logger,
+                "Waiting for fdw connection to check if site {} is being copied", site.namespace
+            );
+        }
+        false
+    })?;
+
     select(diesel::dsl::exists(
         ac::table
             .filter(ac::src.eq(site.id))
             .filter(ac::cancelled_at.is_null()),
     ))
-    .get_result::<bool>(conn)
+    .get_result::<bool>(&mut conn)
     .map_err(StoreError::from)
 }
 

store/postgres/src/deployment_store.rs

@@ -1235,8 +1235,7 @@ impl DeploymentStore {
             req: PruneRequest,
         ) -> Result<(), StoreError> {
             {
-                let mut conn = store.get_conn()?;
-                if copy::is_source(&mut conn, &site)? {
+                if copy::is_source(&logger, &store.pool, &site)? {
                     debug!(
                         logger,
                         "Skipping pruning since this deployment is being copied"