storage: add sync querier (#5570)

* storage: add fallback querier

* storage: add sync querier

* storage: add typed querier

* use synced querier

pkg/storage/querier_fallback.go

@@ -0,0 +1,49 @@
+package storage
+
+import (
+	"context"
+	"errors"
+
+	grpc "google.golang.org/grpc"
+
+	"github.com/pomerium/pomerium/pkg/grpc/databroker"
+)
+
+type fallbackQuerier []Querier
+
+// NewFallbackQuerier creates a new fallback-querier. The first call to Query that
+// does not return an error will be used.
+func NewFallbackQuerier(queriers ...Querier) Querier {
+	return fallbackQuerier(queriers)
+}
+
+// InvalidateCache invalidates the cache of all the queriers.
+func (q fallbackQuerier) InvalidateCache(ctx context.Context, req *databroker.QueryRequest) {
+	for _, qq := range q {
+		qq.InvalidateCache(ctx, req)
+	}
+}
+
+// Query returns the first querier's results that doesn't result in an error.
+func (q fallbackQuerier) Query(ctx context.Context, req *databroker.QueryRequest, opts ...grpc.CallOption) (*databroker.QueryResponse, error) {
+	if len(q) == 0 {
+		return nil, ErrUnavailable
+	}
+
+	var merr error
+	for _, qq := range q {
+		res, err := qq.Query(ctx, req, opts...)
+		if err == nil {
+			return res, nil
+		}
+		merr = errors.Join(merr, err)
+	}
+	return nil, merr
+}
+
+// Stop stops all the queriers.
+func (q fallbackQuerier) Stop() {
+	for _, qq := range q {
+		qq.Stop()
+	}
+}