Ensure deterministic execution, for tests

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

tsdb/postings_for_matchers_cache.go

@@ -93,6 +93,11 @@ func (p *postingsForMatcherPromise) result(ctx context.Context) (index.Postings,
 	case <-ctx.Done():
 		return nil, ctx.Err()
 	case <-p.done:
+		// Checking context error is necessary for deterministic tests,
+		// as channel selection order is random
+		if ctx.Err() != nil {
+			return nil, ctx.Err()
+		}
 		if p.err != nil {
 			return nil, p.err
 		}
@@ -107,12 +112,18 @@ func (c *PostingsForMatchersCache) postingsForMatchersPromise(ix IndexPostingsRe
 	key := matchersKey(ms)
 	oldPromise, loaded := c.calls.LoadOrStore(key, promise)
 	if loaded {
-		promise = oldPromise.(*postingsForMatcherPromise)
+		// promise was not stored, we return a previously stored promise, that's possibly being fulfilled in another goroutine
-		return promise.result
+		close(promise.done)
+		return oldPromise.(*postingsForMatcherPromise).result
 	}
+
+	// promise was stored, close its channel after fulfilment
 	defer close(promise.done)
 
-	// FIXME: do we need to cancel the call to postingsForMatchers if all the callers waiting for the result have cancelled their context?
+	// Don't let context cancellation fail the promise, since it may be used by multiple goroutines, each with
+	// its own context. Also, keep the call independent of this particular context, since the promise will be reused.
+	// FIXME: do we need to cancel the call to postingsForMatchers if all the callers waiting for the result have
+	// cancelled their context?
 	if postings, err := c.postingsForMatchers(context.Background(), ix, ms...); err != nil {
 		promise.err = err
 	} else {

tsdb/postings_for_matchers_cache_test.go

@@ -352,7 +352,7 @@ func TestPostingsForMatchersCache(t *testing.T) {
 		ctx1, cancel := context.WithCancel(context.Background())
 		cancel()
 		_, err := c.PostingsForMatchers(ctx1, indexForPostingsMock{}, true, matchers...)
-		require.Equal(t, context.Canceled, err)
+		require.ErrorIs(t, err, context.Canceled)
 
 		ctx2 := context.Background()
 		actualPostings, err := c.PostingsForMatchers(ctx2, indexForPostingsMock{}, true, matchers...)