Add generator meta into head (halo-dev#4821)

#### What type of PR is this?

/kind feature
/area core
/milestone 2.11.x

#### What this PR does / why we need it:

Please see https://html.spec.whatwg.org/multipage/semantics.html#meta-generator for more.

This PR add the generator meta into head, so that we can know what sites are using Halo.

```bash
http localhost:8090/ | grep generator

<meta name="generator" content="Halo 2.11.0-SNAPSHOT"/>
```

If someone want to disable the generator meta, they can configure the property `halo.theme.generator-meta-disabled` to `true`.

#### Does this PR introduce a user-facing change?

```release-note
添加 Generator 元数据标识
```

application/src/main/java/run/halo/app/infra/properties/ThemeProperties.java

@@ -9,6 +9,11 @@ public class ThemeProperties {
     @Valid
     private final Initializer initializer = new Initializer();
 
+    /**
+     * Indicates whether the generator meta needs to be disabled.
+     */
+    private boolean generatorMetaDisabled;
+
     @Data
     public static class Initializer {
 

application/src/main/java/run/halo/app/theme/ThemeConfiguration.java

@@ -8,7 +8,10 @@
 import java.nio.file.Path;
 import java.time.Instant;
 import org.apache.commons.lang3.StringUtils;
+import org.springframework.beans.factory.ObjectProvider;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
 import org.springframework.boot.autoconfigure.web.WebProperties;
+import org.springframework.boot.info.BuildProperties;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.core.io.FileSystemResource;
@@ -21,8 +24,10 @@
 import reactor.core.publisher.Mono;
 import run.halo.app.infra.ThemeRootGetter;
 import run.halo.app.infra.utils.FileUtils;
+import run.halo.app.theme.dialect.GeneratorMetaProcessor;
 import run.halo.app.theme.dialect.HaloSpringSecurityDialect;
 import run.halo.app.theme.dialect.LinkExpressionObjectDialect;
+import run.halo.app.theme.dialect.TemplateHeadProcessor;
 
 /**
  * @author guqing
@@ -86,4 +91,12 @@ SpringSecurityDialect springSecurityDialect(
         ServerSecurityContextRepository securityContextRepository) {
         return new HaloSpringSecurityDialect(securityContextRepository);
     }
+
+    @Bean
+    @ConditionalOnProperty(name = "halo.theme.generator-meta-disabled",
+        havingValue = "false",
+        matchIfMissing = true)
+    TemplateHeadProcessor generatorMetaProcessor(ObjectProvider<BuildProperties> buildProperties) {
+        return new GeneratorMetaProcessor(buildProperties);
+    }
 }

application/src/main/java/run/halo/app/theme/dialect/GeneratorMetaProcessor.java

@@ -0,0 +1,43 @@
+package run.halo.app.theme.dialect;
+
+import static org.thymeleaf.model.AttributeValueQuotes.DOUBLE;
+
+import java.util.Map;
+import org.springframework.beans.factory.ObjectProvider;
+import org.springframework.boot.info.BuildProperties;
+import org.springframework.core.annotation.Order;
+import org.thymeleaf.context.ITemplateContext;
+import org.thymeleaf.model.IModel;
+import org.thymeleaf.processor.element.IElementModelStructureHandler;
+import reactor.core.publisher.Mono;
+
+/**
+ * Processor for generating generator meta.
+ * Set the order to 0 for removing the meta in later TemplateHeadProcessor.
+ *
+ * @author johnniang
+ */
+@Order(0)
+public class GeneratorMetaProcessor implements TemplateHeadProcessor {
+
+    private final String generatorValue;
+
+    public GeneratorMetaProcessor(ObjectProvider<BuildProperties> buildProperties) {
+        this.generatorValue = "Halo " + buildProperties.stream().findFirst()
+            .map(BuildProperties::getVersion)
+            .orElse("Unknown");
+    }
+
+    @Override
+    public Mono<Void> process(ITemplateContext context, IModel model,
+        IElementModelStructureHandler structureHandler) {
+        return Mono.fromRunnable(() -> {
+            var modelFactory = context.getModelFactory();
+            var generatorMeta = modelFactory.createStandaloneElementTag("meta",
+                Map.of("name", "generator", "content", generatorValue),
+                DOUBLE, false, true);
+            model.add(generatorMeta);
+        });
+    }
+
+}

application/src/test/java/run/halo/app/theme/dialect/GeneratorMetaProcessorTest.java

@@ -0,0 +1,59 @@
+package run.halo.app.theme.dialect;
+
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.when;
+
+import java.io.FileNotFoundException;
+import java.net.URISyntaxException;
+import java.nio.file.Path;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.autoconfigure.web.reactive.AutoConfigureWebTestClient;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.boot.test.mock.mockito.MockBean;
+import org.springframework.test.web.reactive.server.WebTestClient;
+import org.springframework.util.ResourceUtils;
+import org.springframework.web.server.ServerWebExchange;
+import reactor.core.publisher.Mono;
+import run.halo.app.infra.InitializationStateGetter;
+import run.halo.app.theme.ThemeContext;
+import run.halo.app.theme.ThemeResolver;
+
+@SpringBootTest
+@AutoConfigureWebTestClient
+class GeneratorMetaProcessorTest {
+
+    @Autowired
+    WebTestClient webClient;
+
+    @MockBean
+    InitializationStateGetter initializationStateGetter;
+
+    @MockBean
+    ThemeResolver themeResolver;
+
+    @BeforeEach
+    void setUp() throws FileNotFoundException, URISyntaxException {
+        when(initializationStateGetter.userInitialized()).thenReturn(Mono.just(true));
+        var themeContext = ThemeContext.builder()
+            .name("default")
+            .path(Path.of(ResourceUtils.getURL("classpath:themes/default").toURI()))
+            .active(true)
+            .build();
+        when(themeResolver.getTheme(any(ServerWebExchange.class)))
+            .thenReturn(Mono.just(themeContext));
+    }
+
+    @Test
+    void requestIndexPage() {
+        webClient.get().uri("/")
+            .exchange()
+            .expectStatus().isOk()
+            .expectBody()
+            .consumeWith(System.out::println)
+            .xpath("/html/head/meta[@name=\"generator\"][starts-with(@content, \"Halo \")]")
+            .exists();
+    }
+
+}

application/src/test/java/run/halo/app/theme/message/ThemeMessageResolverIntegrationTest.java

@@ -68,21 +68,9 @@ void messageResolverWhenDefaultTheme() {
             .exchange()
             .expectStatus()
             .isOk()
-            .expectBody(String.class)
-            .isEqualTo("""
-                <!DOCTYPE html>
-                <html lang="en">
-                <head>
-                    <meta charset="UTF-8">
-                    <title>Title</title>
-                </head>
-                <body>
-                index
-                <div>zh</div>
-                <div>欢迎来到首页</div>
-                </body>
-                </html>
-                """);
+            .expectBody()
+            .xpath("/html/body/div[1]").isEqualTo("zh")
+            .xpath("/html/body/div[2]").isEqualTo("欢迎来到首页");
     }
 
     @Test
@@ -92,21 +80,9 @@ void messageResolverForEnLanguageWhenDefaultTheme() {
             .exchange()
             .expectStatus()
             .isOk()
-            .expectBody(String.class)
-            .isEqualTo("""
-                <!DOCTYPE html>
-                <html lang="en">
-                <head>
-                    <meta charset="UTF-8">
-                    <title>Title</title>
-                </head>
-                <body>
-                index
-                <div>en</div>
-                <div>Welcome to the index</div>
-                </body>
-                </html>
-                """);
+            .expectBody()
+            .xpath("/html/body/div[1]").isEqualTo("en")
+            .xpath("/html/body/div[2]").isEqualTo("Welcome to the index");
     }
 
     @Test
@@ -116,21 +92,9 @@ void shouldUseDefaultWhenLanguageNotSupport() {
             .exchange()
             .expectStatus()
             .isOk()
-            .expectBody(String.class)
-            .isEqualTo("""
-                <!DOCTYPE html>
-                <html lang="en">
-                <head>
-                    <meta charset="UTF-8">
-                    <title>Title</title>
-                </head>
-                <body>
-                index
-                <div>foo</div>
-                <div>欢迎来到首页</div>
-                </body>
-                </html>
-                """);
+            .expectBody()
+            .xpath("/html/body/div[1]").isEqualTo("foo")
+            .xpath("/html/body/div[2]").isEqualTo("欢迎来到首页");
     }
 
     @Test
@@ -140,57 +104,28 @@ void switchTheme() throws URISyntaxException {
             .exchange()
             .expectStatus()
             .isOk()
-            .expectBody(String.class)
-            .isEqualTo("""
-                <!DOCTYPE html>
-                <html lang="en">
-                <head>
-                    <meta charset="UTF-8">
-                    <title>Title</title>
-                </head>
-                <body>
-                index
-                <div>zh</div>
-                <div>欢迎来到首页</div>
-                </body>
-                </html>
-                """);
+            .expectBody()
+            .xpath("/html/head/title").isEqualTo("Title")
+            .xpath("/html/body/div[1]").isEqualTo("zh")
+            .xpath("/html/body/div[2]").isEqualTo("欢迎来到首页")
+        ;
 
         // For other theme
         when(themeResolver.getTheme(any(ServerWebExchange.class)))
             .thenReturn(Mono.just(createOtherContext()));
         webTestClient.get()
             .uri("/index?language=zh")
             .exchange()
-            .expectBody(String.class)
-            .isEqualTo("""
-                <!DOCTYPE html>
-                <html lang="en">
-                <head>
-                    <meta charset="UTF-8">
-                    <title>Other theme title</title>
-                </head>
-                <body>
-                <p>Other 首页</p>
-                </body>
-                </html>
-                """);
+            .expectBody()
+            .xpath("/html/head/title").isEqualTo("Other theme title")
+            .xpath("/html/body/p").isEqualTo("Other 首页");
+
         webTestClient.get()
             .uri("/index?language=en")
             .exchange()
-            .expectBody(String.class)
-            .isEqualTo("""
-                <!DOCTYPE html>
-                <html lang="en">
-                <head>
-                    <meta charset="UTF-8">
-                    <title>Other theme title</title>
-                </head>
-                <body>
-                <p>other index</p>
-                </body>
-                </html>
-                """);
+            .expectBody()
+            .xpath("/html/head/title").isEqualTo("Other theme title")
+            .xpath("/html/body/p").isEqualTo("other index");
     }
 
     ThemeContext createDefaultContext() throws URISyntaxException {

application/src/test/resources/themes/default/templates/index.html

@@ -1,7 +1,7 @@
 <!DOCTYPE html>
 <html lang="en">
 <head>
-    <meta charset="UTF-8">
+    <meta charset="UTF-8"/>
     <title>Title</title>
 </head>
 <body>

application/src/test/resources/themes/other/templates/index.html

@@ -1,7 +1,7 @@
 <!DOCTYPE html>
 <html lang="en">
 <head>
-    <meta charset="UTF-8">
+    <meta charset="UTF-8"/>
     <title>Other theme title</title>
 </head>
 <body>