tools/syz-testbed: support per-checkout syz-manager configs

Let the user of the tool specify individual syz-manager configs for
checkouts. A base config is specified as previously and then individual
parts of jsons are applied to that base config, e.g.

<...>
  "checkouts": [
    {
      "name": "first",
      "repo": "https://github.com/google/syzkaller.git",
    },
    {
      "name": "second",
      "repo": "https://github.com/google/syzkaller.git",
      "manager_config": {
        "kernel_obj": "/tmp/linux-stable2"
      }
    }
  ],
  "manager_config": {
	  "target": "linux/amd64",
	  "kernel_obj": "/tmp/linux-stable",
          <...>
  }
<...>

1 files changed, 57 insertions, 0 deletions

diff --git a/pkg/config/merge.go b/pkg/config/merge.go
new file mode 100644
index 000000000..49a7254de
--- /dev/null
+++ b/pkg/config/merge.go
@@ -0,0 +1,57 @@
+// Copyright 2021 syzkaller project authors. All rights reserved.
+// Use of this source code is governed by Apache 2 LICENSE that can be found in the LICENSE file.
+
+package config
+
+import (
+	"encoding/json"
+)
+
+// Unfortunately, if we want to apply a JSON patch to some configuration, we cannot just unmarshal
+// it twice - in that case json.RawMessage objects will be completely replaced, but not merged.
+func MergeJSONData(left, right []byte) ([]byte, error) {
+	vLeft, err := parseFragment(left)
+	if err != nil {
+		return nil, err
+	}
+	vRight, err := parseFragment(right)
+	if err != nil {
+		return nil, err
+	}
+	merged := mergeRecursive(vLeft, vRight)
+	return json.Marshal(merged)
+}
+
+func parseFragment(input []byte) (parsed interface{}, err error) {
+	if len(input) == 0 {
+		// For convenience, we allow empty strings to be passed to the function that merges JSONs.
+		return
+	}
+	err = json.Unmarshal(json.RawMessage(input), &parsed)
+	return
+}
+
+// If one of the elements is not a map, use the new one.
+// Otherwise, recursively merge map elements.
+func mergeRecursive(left, right interface{}) interface{} {
+	if left == nil {
+		return right
+	}
+	if right == nil {
+		return left
+	}
+	mLeft, okLeft := left.(map[string]interface{})
+	mRight, okRight := right.(map[string]interface{})
+	if !okLeft || !okRight {
+		return right
+	}
+	for key, val := range mRight {
+		valLeft, ok := mLeft[key]
+		if ok {
+			mLeft[key] = mergeRecursive(valLeft, val)
+		} else {
+			mLeft[key] = val
+		}
+	}
+	return mLeft
+}