store: Improve conflicting root error message

Fixes #7806

```sh
$ opa eval -b b1.tar.gz -b b2.tar.gz data;  go run main.go eval -b b1.tar.gz -b b2.tar.gz data
{
  "errors": [
    {
      "message": "detected overlapping roots in bundle manifest with: [b1.tar.gz b2.tar.gz]"
    }
  ]
}
{
  "errors": [
    {
      "message": "bundles b2.tar.gz, b1.tar.gz have overlapping roots and cannot be activated simultaneously because bundles b1.tar.gz specify empty root paths ('') which overlap with any other bundle root"
    }
  ]
}
```

Signed-off-by: Charlie Egan <charlie@styra.com>

Author: Charlie Egan

v1/bundle/store.go

@@ -1104,10 +1104,29 @@ func hasRootsOverlap(ctx context.Context, store storage.Store, txn storage.Trans
 
 	if len(collisions) > 0 {
 		var bundleNames []string
+		var bundlesWithEmptyRoots []string
+
 		for name := range collisions {
 			bundleNames = append(bundleNames, name)
+
+			bundle, ok := bundles[name]
+			if !ok || bundle.Manifest.Roots == nil {
+				continue
+			}
+
+			for _, root := range *bundle.Manifest.Roots {
+				if root == "" {
+					bundlesWithEmptyRoots = append(bundlesWithEmptyRoots, name)
+					break
+				}
+			}
 		}
-		return fmt.Errorf("detected overlapping roots in bundle manifest with: %s", bundleNames)
+
+		if len(bundlesWithEmptyRoots) > 0 {
+			return fmt.Errorf("bundles %s have overlapping roots and cannot be activated simultaneously because bundles %s specify empty root paths ('') which overlap with any other bundle root", strings.Join(bundleNames, ", "), strings.Join(bundlesWithEmptyRoots, ", "))
+		}
+
+		return fmt.Errorf("detected overlapping roots in bundle manifest with: %s", strings.Join(bundleNames, ", "))
 	}
 	return nil
 }

v1/bundle/store_test.go

@@ -6668,6 +6668,18 @@ func TestHasRootsOverlap(t *testing.T) {
 			bundleRoots: map[string]*[]string{"bundle2": {"c", "a"}, "bundle3": {"a"}},
 			overlaps:    true,
 		},
+		{
+			note:        "overlap with empty root bundle - bundle without manifest",
+			storeRoots:  map[string]*[]string{"bundle1": {"a", "b"}},
+			bundleRoots: map[string]*[]string{"bundle2": {""}},
+			overlaps:    true,
+		},
+		{
+			note:        "overlap between multiple empty root bundles - bundles without manifests",
+			storeRoots:  map[string]*[]string{},
+			bundleRoots: map[string]*[]string{"bundle1": {""}, "bundle2": {""}},
+			overlaps:    true,
+		},
 	}
 
 	for _, tc := range cases {
@@ -6694,7 +6706,7 @@ func TestHasRootsOverlap(t *testing.T) {
 			err := hasRootsOverlap(ctx, mockStore, txn, bundles)
 			if !tc.overlaps && err != nil {
 				t.Fatalf("unepected error: %s", err)
-			} else if tc.overlaps && (err == nil || !strings.Contains(err.Error(), "detected overlapping roots in bundle manifest")) {
+			} else if tc.overlaps && (err == nil || (!strings.Contains(err.Error(), "detected overlapping roots in bundle manifest") && !strings.Contains(err.Error(), "have overlapping roots and cannot be activated simultaneously"))) {
 				t.Fatalf("expected overlapping roots error, got: %s", err)
 			}