fix(language-detection): 최초 감지 캐시 경합을 직렬화한다

src/main/kotlin/kr/co/vividnext/sodalive/content/LanguageDetectionCacheService.kt

@@ -25,18 +25,22 @@ class LanguageDetectionCacheService(
         )
         if (cached != null) return cached.detectedLanguage
 
-        val detectedLanguage = detector()?.takeIf { it.isNotBlank() } ?: return null
-        languageDetectionResultRepository.save(
-            LanguageDetectionResult(
+        val detectedLanguage = detector()?.takeIf { it.isNotBlank() }?.lowercase() ?: return null
+        languageDetectionResultRepository.insertIfAbsent(
+            sourceHash = sourceHash,
+            sourceTextSample = normalizedQuery.take(MAX_SAMPLE_LENGTH),
+            detectedLanguage = detectedLanguage,
+            provider = provider,
+            confidence = null,
+            normalizationVersion = SourceTextNormalizer.NORMALIZATION_VERSION
+        )
+        return requireNotNull(
+            languageDetectionResultRepository.findDetectedLanguageForUpdate(
                 sourceHash = sourceHash,
-                sourceTextSample = normalizedQuery.take(MAX_SAMPLE_LENGTH),
-                detectedLanguage = detectedLanguage.lowercase(),
                 provider = provider,
-                confidence = null,
                 normalizationVersion = SourceTextNormalizer.NORMALIZATION_VERSION
             )
         )
-        return detectedLanguage.lowercase()
     }
 
     companion object {

src/main/kotlin/kr/co/vividnext/sodalive/content/LanguageDetectionResultRepository.kt

@@ -1,6 +1,9 @@
 package kr.co.vividnext.sodalive.content
 
 import org.springframework.data.jpa.repository.JpaRepository
+import org.springframework.data.jpa.repository.Modifying
+import org.springframework.data.jpa.repository.Query
+import org.springframework.data.repository.query.Param
 
 interface LanguageDetectionResultRepository : JpaRepository<LanguageDetectionResult, Long> {
     fun findBySourceHashAndProviderAndNormalizationVersion(
@@ -8,4 +11,55 @@ interface LanguageDetectionResultRepository : JpaRepository<LanguageDetectionRes
         provider: String,
         normalizationVersion: String
     ): LanguageDetectionResult?
+
+    @Modifying
+    @Query(
+        value = """
+            INSERT INTO language_detection_result (
+                source_hash,
+                source_text_sample,
+                detected_language,
+                provider,
+                confidence,
+                normalization_version,
+                created_at,
+                updated_at
+            ) VALUES (
+                :sourceHash,
+                :sourceTextSample,
+                :detectedLanguage,
+                :provider,
+                :confidence,
+                :normalizationVersion,
+                CURRENT_TIMESTAMP,
+                CURRENT_TIMESTAMP
+            ) ON DUPLICATE KEY UPDATE id = id
+        """,
+        nativeQuery = true
+    )
+    fun insertIfAbsent(
+        @Param("sourceHash") sourceHash: String,
+        @Param("sourceTextSample") sourceTextSample: String,
+        @Param("detectedLanguage") detectedLanguage: String,
+        @Param("provider") provider: String,
+        @Param("confidence") confidence: Double?,
+        @Param("normalizationVersion") normalizationVersion: String
+    ): Int
+
+    @Query(
+        value = """
+            SELECT detected_language
+            FROM language_detection_result
+            WHERE source_hash = :sourceHash
+              AND provider = :provider
+              AND normalization_version = :normalizationVersion
+            FOR UPDATE
+        """,
+        nativeQuery = true
+    )
+    fun findDetectedLanguageForUpdate(
+        @Param("sourceHash") sourceHash: String,
+        @Param("provider") provider: String,
+        @Param("normalizationVersion") normalizationVersion: String
+    ): String?
 }