Stop all background workers for the duration of the upgrade

This commit stops bgws while upgrading the Promscale extension.
This prevents lock contention between jobs and upgrades and thus
seems safer.

build.rs

@@ -102,9 +102,13 @@ fn render_bootstrap(manifest_dir: &str) -> String {
     ext_schema_path.push("migration/bootstrap/001-create-ext-schema.sql");
     let ext_schema_sql = wrap(ext_schema_path.as_path(), &Incremental);
 
+    let stop_bgw_sql =
+        include_str!("migration/bootstrap/002-stop-bgw.sql").to_string();
+
     let mut result = String::from("");
     result.push_str(&migration_table_sql);
     result.push_str(&ext_schema_sql);
+    result.push_str(&stop_bgw_sql);
     result
 }
 

migration/bootstrap/002-stop-bgw.sql

@@ -0,0 +1,12 @@
+-- stop all background workers in the current db so they don't interfere
+-- with the upgrade (by e.g. holding locks).
+--
+-- Note: this stops the scheduler (and thus all jobs)
+-- and waits for the current txn (the one doing the upgrade) to finish
+-- before starting it up again. Thus, no jobs will be running during
+-- the upgrade.
+
+-- TimescaleDB itself does the same thing:
+-- https://github.com/timescale/timescaledb/blob/72d03e6f7d30cc4794c9263445f14199241e2eb5/sql/updates/pre-update.sql#L34
+SELECT _timescaledb_internal.restart_background_workers();
+