fix: crash during /reload from nil connection race condition (#971)

Author: utafrali

query.go

@@ -125,6 +125,10 @@ func (q *Query) Collect(ctx context.Context, conn *sql.DB, ch chan<- Metric) {
 
 // run executes the query on the provided database, in the provided context.
 func (q *Query) run(ctx context.Context, conn *sql.DB) (*sql.Rows, errors.WithContext) {
+	if conn == nil {
+		return nil, errors.Errorf(q.logContext, "nil database connection")
+	}
+
 	if slog.Default().Enabled(ctx, slog.LevelDebug) {
 		start := time.Now()
 		defer func() {

target.go

@@ -136,12 +136,13 @@ func (t *target) Collect(ctx context.Context, ch chan<- Metric) {
 	var wg sync.WaitGroup
 	// Don't bother with the collectors if target is down.
 	if targetUp {
+		conn := t.conn
 		wg.Add(len(t.collectors))
 		for _, c := range t.collectors {
 			// If using a single DB connection, collectors will likely run sequentially anyway. But we might have more.
 			go func(collector Collector) {
 				defer wg.Done()
-				collector.Collect(ctx, t.conn, ch)
+				collector.Collect(ctx, conn, ch)
 			}(c)
 		}
 	}