warn on duplicate cache-to targets

bake/bake.go

@@ -32,6 +32,7 @@ import (
 	"github.com/moby/buildkit/client/llb"
 	"github.com/moby/buildkit/frontend/dockerfile/dfgitutil"
 	"github.com/pkg/errors"
+	"github.com/sirupsen/logrus"
 	"github.com/zclconf/go-cty/cty"
 	"github.com/zclconf/go-cty/cty/convert"
 )
@@ -1281,7 +1282,25 @@ func (t *Target) GetName(ectx *hcl.EvalContext, block *hcl.Block, loadDeps func(
 	return value.AsString(), nil
 }
 
+func warnDuplicateCacheExports(m map[string]*Target) {
+	seen := map[string][]string{}
+	for _, name := range slices.Sorted(maps.Keys(m)) {
+		t := m[name]
+		for _, entry := range t.CacheTo {
+			key := entry.String()
+			seen[key] = append(seen[key], name)
+		}
+	}
+	for _, key := range slices.Sorted(maps.Keys(seen)) {
+		names := seen[key]
+		if len(names) > 1 {
+			logrus.Warnf("multiple targets write to the same cache export %q: %s", key, strings.Join(names, ", "))
+		}
+	}
+}
+
 func TargetsToBuildOpt(m map[string]*Target, inp *Input) (map[string]build.Options, error) {
+	warnDuplicateCacheExports(m)
 	m2 := make(map[string]build.Options, len(m))
 	for k, v := range m {
 		bo, err := toBuildOpt(v, inp)

bake/bake_test.go

@@ -11,6 +11,7 @@ import (
 
 	"github.com/docker/buildx/util/buildflags"
 	"github.com/moby/buildkit/util/entitlements"
+	"github.com/sirupsen/logrus"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -2491,3 +2492,46 @@ func stringify[V fmt.Stringer](values []V) []string {
 	sort.Strings(s)
 	return s
 }
+
+func TestWarnDuplicateCacheExports(t *testing.T) {
+	var buf strings.Builder
+	logrus.SetOutput(&buf)
+	logrus.SetLevel(logrus.WarnLevel)
+	t.Cleanup(func() { logrus.SetOutput(os.Stderr) })
+
+	t.Run("duplicate cache-to", func(t *testing.T) {
+		buf.Reset()
+		m := map[string]*Target{
+			"app": {
+				CacheTo: buildflags.CacheOptions{
+					{Type: "registry", Attrs: map[string]string{"ref": "registry.example.com/cache/shared"}},
+				},
+			},
+			"worker": {
+				CacheTo: buildflags.CacheOptions{
+					{Type: "registry", Attrs: map[string]string{"ref": "registry.example.com/cache/shared"}},
+				},
+			},
+		}
+		warnDuplicateCacheExports(m)
+		require.Contains(t, buf.String(), "registry.example.com/cache/shared")
+	})
+
+	t.Run("no duplicate", func(t *testing.T) {
+		buf.Reset()
+		m := map[string]*Target{
+			"app": {
+				CacheTo: buildflags.CacheOptions{
+					{Type: "registry", Attrs: map[string]string{"ref": "registry.example.com/cache/app"}},
+				},
+			},
+			"worker": {
+				CacheTo: buildflags.CacheOptions{
+					{Type: "registry", Attrs: map[string]string{"ref": "registry.example.com/cache/worker"}},
+				},
+			},
+		}
+		warnDuplicateCacheExports(m)
+		require.Empty(t, buf.String())
+	})
+}