bake: avoid evaluating parent targets before child LLB loaded

Because of the way buildkit cache works if you have request
with external cache, if some vertices from the request have
already been evaluated and are available in the shared graph
BuildKit will not load cache keys from external source for such
vertices. This may mean that children of such vertices will
not load cache because there isn't a cache path through the parent.

To work around it, wait before child definition is loaded before
evaluating the parent.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

build/build.go

@@ -759,6 +759,11 @@ func BuildWithResultHandler(ctx context.Context, nodes []builder.Node, opt map[s
 						results.Set(rKey, res)
 
 						if children, ok := childTargets[rKey]; ok && len(children) > 0 {
+							// wait for the child targets to register their LLB before evaluating
+							_, err := results.Get(ctx, children...)
+							if err != nil {
+								return nil, err
+							}
 							// we need to wait until the child targets have completed before we can release
 							eg, ctx := errgroup.WithContext(ctx)
 							eg.Go(func() error {