chore: Implement database limits from v1 (#4669)

Under scale if there wasn't a PostgreSQL connection available,
an error was occurring instead of blocking for a new connection.

This fixes it!

coderd/database/db.go

@@ -42,6 +42,25 @@ type DBTX interface {
 // New creates a new database store using a SQL database connection.
 func New(sdb *sql.DB) Store {
 	dbx := sqlx.NewDb(sdb, "postgres")
+
+	// The default is 0 but the request will fail with a 500 if the DB
+	// cannot accept new connections, so we try to limit that here.
+	// Requests will wait for a new connection instead of a hard error
+	// if a limit is set.
+	dbx.SetMaxOpenConns(40)
+	// Allow a max of 3 idle connections at a time. Lower values end up
+	// creating a lot of connection churn. Since each connection uses about
+	// 10MB of memory, we're allocating 30MB to Postgres connections per
+	// replica, but is better than causing Postgres to spawn a thread 15-20
+	// times/sec. PGBouncer's transaction pooling is not the greatest so
+	// it's not optimal for us to deploy.
+	//
+	// This was set to 10 before we started doing HA deployments, but 3 was
+	// later determined to be a better middle ground as to not use up all
+	// of PGs default connection limit while simultaneously avoiding a lot
+	// of connection churn.
+	dbx.SetMaxIdleConns(3)
+
 	return &sqlQuerier{
 		db:  dbx,
 		sdb: dbx,