From c83e00e7022425e82027c7212383281c5658c37b Mon Sep 17 00:00:00 2001
From: Fabian Meumertzheim <fabian@meumertzhe.im>
Date: Sun, 11 Jun 2023 18:52:30 +0200
Subject: [PATCH] Add TypeAdapterFactory for Optional

---
 .../lib/bazel/bzlmod/GsonTypeAdapterUtil.java | 54 +++++++++++++++++++
 1 file changed, 54 insertions(+)

diff --git a/src/main/java/com/google/devtools/build/lib/bazel/bzlmod/GsonTypeAdapterUtil.java b/src/main/java/com/google/devtools/build/lib/bazel/bzlmod/GsonTypeAdapterUtil.java
index b1b047ba4d4a8b..f6d8d542f4decf 100644
--- a/src/main/java/com/google/devtools/build/lib/bazel/bzlmod/GsonTypeAdapterUtil.java
+++ b/src/main/java/com/google/devtools/build/lib/bazel/bzlmod/GsonTypeAdapterUtil.java
@@ -26,11 +26,17 @@
 import com.google.gson.GsonBuilder;
 import com.google.gson.JsonParseException;
 import com.google.gson.TypeAdapter;
+import com.google.gson.TypeAdapterFactory;
+import com.google.gson.reflect.TypeToken;
 import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonToken;
 import com.google.gson.stream.JsonWriter;
 import com.ryanharter.auto.value.gson.GenerateTypeAdapter;
 import java.io.IOException;
+import java.lang.reflect.ParameterizedType;
+import java.lang.reflect.Type;
 import java.util.List;
+import java.util.Optional;
 
 /**
  * Utility class to hold type adapters and helper methods to get gson registered with type adapters
@@ -88,6 +94,53 @@ public ModuleKey read(JsonReader jsonReader) throws IOException {
         }
       };
 
+  public static final TypeAdapterFactory OPTIONAL =
+      new TypeAdapterFactory() {
+        @Override
+        public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
+          if (typeToken.getRawType() != Optional.class) {
+            return null;
+          }
+          Type type = typeToken.getType();
+          if (!(type instanceof ParameterizedType)) {
+            return null;
+          }
+          Type elementType = ((ParameterizedType) typeToken.getType()).getActualTypeArguments()[0];
+          var elementTypeAdapter = gson.getAdapter(TypeToken.get(elementType));
+          if (elementTypeAdapter == null) {
+            return null;
+          }
+          return (TypeAdapter<T>) new OptionalTypeAdapter<>(elementTypeAdapter);
+        }
+      };
+
+  private static final class OptionalTypeAdapter<T> extends TypeAdapter<Optional<T>> {
+    private final TypeAdapter<T> elementTypeAdapter;
+
+    public OptionalTypeAdapter(TypeAdapter<T> elementTypeAdapter) {
+      this.elementTypeAdapter = elementTypeAdapter;
+    }
+
+    @Override
+    public void write(JsonWriter jsonWriter, Optional<T> t) throws IOException {
+      if (t == null || t.isEmpty()) {
+        jsonWriter.nullValue();
+      } else {
+        elementTypeAdapter.write(jsonWriter, t.get());
+      }
+    }
+
+    @Override
+    public Optional<T> read(JsonReader jsonReader) throws IOException {
+      if (jsonReader.peek() == JsonToken.NULL) {
+        jsonReader.nextNull();
+        return Optional.empty();
+      } else {
+        return Optional.of(elementTypeAdapter.read(jsonReader));
+      }
+    }
+  }
+
   public static final Gson LOCKFILE_GSON =
       new GsonBuilder()
           .setPrettyPrinting()
@@ -98,6 +151,7 @@ public ModuleKey read(JsonReader jsonReader) throws IOException {
           .registerTypeAdapterFactory(IMMUTABLE_LIST)
           .registerTypeAdapterFactory(IMMUTABLE_BIMAP)
           .registerTypeAdapterFactory(IMMUTABLE_SET)
+          .registerTypeAdapterFactory(OPTIONAL)
           .registerTypeAdapter(Version.class, VERSION_TYPE_ADAPTER)
           .registerTypeAdapter(ModuleKey.class, MODULE_KEY_TYPE_ADAPTER)
           .registerTypeAdapter(AttributeValues.class, new AttributeValuesAdapter())