feat: add an API to list uninstalled themes (#2586)

#### What type of PR is this?
/kind feature
/milestone 2.0
/area core
/kind api-change

#### What this PR does / why we need it:
新增 API 用于查询未安装的主题

#### Which issue(s) this PR fixes:

Fixes #2554

#### Special notes for your reviewer:
how to test it?
1. 安装几个主题
2. 直接解压几个主题到 work dir 的 themes 目录
3. 使用以下 endpoint 查询未安装的主题，期望获得所有未安装主题的 themes.yaml 信息
```
/apis/api.console.halo.run/v1alpha1/themes?uninstalled=true
```

/cc @halo-dev/sig-halo 
#### Does this PR introduce a user-facing change?

```release-note
支持扫描主题目录下未安装的主题
```

src/main/java/run/halo/app/core/extension/endpoint/ThemeEndpoint.java

@@ -14,8 +14,10 @@
 import java.nio.file.Path;
 import java.nio.file.Paths;
 import java.util.ArrayList;
+import java.util.Comparator;
 import java.util.List;
 import java.util.function.Predicate;
+import java.util.stream.BaseStream;
 import java.util.zip.ZipEntry;
 import java.util.zip.ZipInputStream;
 import lombok.extern.slf4j.Slf4j;
@@ -28,6 +30,7 @@
 import org.springframework.http.MediaType;
 import org.springframework.http.codec.multipart.FilePart;
 import org.springframework.http.codec.multipart.Part;
+import org.springframework.lang.NonNull;
 import org.springframework.lang.Nullable;
 import org.springframework.stereotype.Component;
 import org.springframework.util.Assert;
@@ -46,8 +49,11 @@
 import run.halo.app.core.extension.Setting;
 import run.halo.app.core.extension.Theme;
 import run.halo.app.extension.ConfigMap;
+import run.halo.app.extension.ListResult;
 import run.halo.app.extension.ReactiveExtensionClient;
 import run.halo.app.extension.Unstructured;
+import run.halo.app.extension.router.IListRequest;
+import run.halo.app.extension.router.QueryParamBuildUtil;
 import run.halo.app.infra.exception.ThemeInstallationException;
 import run.halo.app.infra.properties.HaloProperties;
 import run.halo.app.infra.utils.DataBufferUtils;
@@ -106,9 +112,65 @@ public RouterFunction<ServerResponse> endpoint() {
                     .response(responseBuilder()
                         .implementation(Theme.class))
             )
+            .GET("themes", this::listThemes,
+                builder -> {
+                    builder.operationId("ListThemes")
+                        .description("List themes.")
+                        .tag(tag)
+                        .response(responseBuilder()
+                            .implementation(ListResult.generateGenericClass(Theme.class)));
+                    QueryParamBuildUtil.buildParametersFromType(builder, ThemeQuery.class);
+                }
+            )
             .build();
     }
 
+    public static class ThemeQuery extends IListRequest.QueryListRequest {
+
+        public ThemeQuery(MultiValueMap<String, String> queryParams) {
+            super(queryParams);
+        }
+
+        @NonNull
+        public Boolean getUninstalled() {
+            return Boolean.parseBoolean(queryParams.getFirst("uninstalled"));
+        }
+    }
+
+    Mono<ServerResponse> listThemes(ServerRequest request) {
+        MultiValueMap<String, String> queryParams = request.queryParams();
+        ThemeQuery query = new ThemeQuery(queryParams);
+        return Mono.defer(() -> {
+            if (query.getUninstalled()) {
+                return listUninstalled(query);
+            }
+            return client.list(Theme.class, null, null, query.getPage(), query.getSize());
+        }).flatMap(extensions -> ServerResponse.ok().bodyValue(extensions));
+    }
+
+    Mono<ListResult<Theme>> listUninstalled(ThemeQuery query) {
+        Path path = themePathPolicy.themesDir();
+        return ThemeUtils.listAllThemesFromThemeDir(path)
+            .collectList()
+            .flatMap(this::filterUnInstalledThemes)
+            .map(themes -> {
+                Integer page = query.getPage();
+                Integer size = query.getSize();
+                List<Theme> subList = ListResult.subList(themes, page, size);
+                return new ListResult<>(page, size, themes.size(), subList);
+            });
+    }
+
+    private Mono<List<Theme>> filterUnInstalledThemes(@NonNull List<Theme> allThemes) {
+        return client.list(Theme.class, null, null)
+            .map(theme -> theme.getMetadata().getName())
+            .collectList()
+            .map(installed -> allThemes.stream()
+                .filter(theme -> !installed.contains(theme.getMetadata().getName()))
+                .toList()
+            );
+    }
+
     Mono<ServerResponse> reloadSetting(ServerRequest request) {
         String name = request.pathVariable("name");
         return client.fetch(Theme.class, name)
@@ -238,12 +300,29 @@ private Predicate<Unstructured> hasConfigYaml(Theme theme) {
 
     static class ThemeUtils {
         private static final String THEME_TMP_PREFIX = "halo-theme-";
-        private static final String[] themeManifests = {"theme.yaml", "theme.yml"};
+        public static final String[] THEME_MANIFESTS = {"theme.yaml", "theme.yml"};
 
         private static final String[] THEME_CONFIG = {"config.yaml", "config.yml"};
 
         private static final String[] THEME_SETTING = {"settings.yaml", "settings.yml"};
 
+        static Flux<Theme> listAllThemesFromThemeDir(Path themesDir) {
+            return walkThemesFromPath(themesDir)
+                .filter(Files::isDirectory)
+                .map(themePath -> loadUnstructured(themePath, THEME_MANIFESTS))
+                .map(unstructured -> Unstructured.OBJECT_MAPPER.convertValue(unstructured,
+                    Theme.class))
+                .sort(Comparator.comparing(theme -> theme.getMetadata().getName()));
+        }
+
+        private static Flux<Path> walkThemesFromPath(Path path) {
+            return Flux.using(() -> Files.walk(path, 2),
+                    Flux::fromStream,
+                    BaseStream::close
+                )
+                .subscribeOn(Schedulers.boundedElastic());
+        }
+
         static List<Unstructured> loadThemeSetting(Path themePath) {
             return loadUnstructured(themePath, THEME_SETTING);
         }
@@ -329,7 +408,7 @@ static Unstructured loadThemeManifest(Path themeManifestPath) {
 
         @Nullable
         private static Path resolveThemeManifest(Path tempDirectory) {
-            for (String themeManifest : themeManifests) {
+            for (String themeManifest : THEME_MANIFESTS) {
                 Path path = tempDirectory.resolve(themeManifest);
                 if (Files.exists(path)) {
                     return path;

src/main/java/run/halo/app/extension/ListResult.java

@@ -3,6 +3,7 @@
 import com.fasterxml.jackson.annotation.JsonIgnore;
 import com.fasterxml.jackson.annotation.JsonProperty;
 import io.swagger.v3.oas.annotations.media.Schema;
+import java.util.ArrayList;
 import java.util.Collections;
 import java.util.Iterator;
 import java.util.List;
@@ -128,4 +129,21 @@ public static <T> Class<?> generateGenericClass(Class<T> type) {
     public static <T> ListResult<T> emptyResult() {
         return new ListResult<>(List.of());
     }
+
+    /**
+     * Manually paginate the List collection.
+     */
+    public static <T> List<T> subList(List<T> list, int page, int size) {
+        if (page < 1) {
+            return list;
+        }
+        List<T> listSort = new ArrayList<>();
+        int total = list.size();
+        int pageStart = page == 1 ? 0 : (page - 1) * size;
+        int pageEnd = Math.min(total, page * size);
+        if (total > pageStart) {
+            listSort = list.subList(pageStart, pageEnd);
+        }
+        return listSort;
+    }
 }

src/main/java/run/halo/app/theme/ThemePathPolicy.java

@@ -23,6 +23,10 @@ public ThemePathPolicy(Path workDir) {
     public Path generate(Theme theme) {
         Assert.notNull(theme, "The theme must not be null.");
         String name = theme.getMetadata().getName();
-        return workDir.resolve(THEME_WORK_DIR).resolve(name);
+        return themesDir().resolve(name);
+    }
+
+    public Path themesDir() {
+        return workDir.resolve(ThemePathPolicy.THEME_WORK_DIR);
     }
 }

src/main/resources/extensions/role-template-theme.yaml

@@ -18,7 +18,7 @@ rules:
     resources: [ "themes", "themes/reload-setting" ]
     verbs: [ "*" ]
   - nonResourceURLs: [ "/apis/api.console.halo.run/themes/install" ]
-    verbs: [ "post" ]
+    verbs: [ "create" ]
 ---
 apiVersion: v1alpha1
 kind: "Role"
@@ -36,5 +36,6 @@ rules:
     resources: [ "themes" ]
     verbs: [ "get", "list" ]
   - apiGroups: [ "api.console.halo.run" ]
-    resources: [ "singlepages" ]
+    resources: [ "themes" ]
     verbs: [ "get", "list" ]
+