refactor: describe asset completion results

backend/app/services/story_service.py

@@ -1,6 +1,8 @@
 """Story business logic service."""
 
 import asyncio
+from dataclasses import dataclass
+from typing import Literal
 
 from fastapi import HTTPException
 from sqlalchemy import desc, select
@@ -40,6 +42,25 @@ from app.tasks.achievements import extract_story_achievements
 
 logger = get_logger(__name__)
 
+AssetCompletionKind = Literal["cover_image", "storybook_images", "audio"]
+
+
+@dataclass(frozen=True)
+class AssetCompletionResult:
+    """Service-level result for a generated asset completion attempt."""
+
+    asset: AssetCompletionKind
+    status: StoryAssetStatus
+    value: str | bytes | None = None
+    error: str | None = None
+    blocks_main_result: bool = False
+
+    @property
+    def succeeded(self) -> bool:
+        """Whether the asset reached a usable ready state."""
+
+        return self.status == StoryAssetStatus.READY and self.error is None
+
 
 def _build_storybook_error_message(
     *,
@@ -281,7 +302,7 @@ async def _complete_cover_image_asset(
     raise_on_failure: bool = False,
     last_error_prefix: str | None = None,
     log_event: str = "cover_asset_generation_failed",
-) -> tuple[str | None, str | None]:
+) -> AssetCompletionResult:
     """Generate or retry a text story cover through one asset workflow."""
 
     if not story.cover_prompt:
@@ -295,7 +316,12 @@ async def _complete_cover_image_asset(
         story.image_url = image_url
         sync_story_status(story, image_status=StoryAssetStatus.READY)
         await db.commit()
-        return image_url, None
+        return AssetCompletionResult(
+            asset="cover_image",
+            status=StoryAssetStatus.READY,
+            value=image_url,
+            blocks_main_result=raise_on_failure,
+        )
     except Exception as exc:
         provider_error = str(exc)
         last_error = (
@@ -317,7 +343,12 @@ async def _complete_cover_image_asset(
                 detail=f"Image generation failed: {provider_error}",
             ) from exc
 
-        return None, provider_error
+        return AssetCompletionResult(
+            asset="cover_image",
+            status=StoryAssetStatus.FAILED,
+            error=provider_error,
+            blocks_main_result=raise_on_failure,
+        )
 
 
 def _get_storybook_pages_data(story: Story) -> list[dict]:
@@ -329,7 +360,7 @@ def _get_storybook_pages_data(story: Story) -> list[dict]:
 async def _complete_storybook_image_assets(
     story: Story,
     db: AsyncSession,
-) -> None:
+) -> AssetCompletionResult:
     """Complete missing cover/page images for a persisted storybook."""
 
     pages_data = _get_storybook_pages_data(story)
@@ -374,20 +405,28 @@ async def _complete_storybook_image_assets(
             )
 
     story.pages = pages_data
+    error_message = _build_storybook_error_message(
+        cover_failed=cover_failed,
+        failed_pages=failed_pages,
+    )
+    image_status = _resolve_storybook_image_status(
+        generate_images=True,
+        cover_prompt=story.cover_prompt,
+        cover_url=story.image_url,
+        pages_data=pages_data,
+    )
     sync_story_status(
         story,
-        image_status=_resolve_storybook_image_status(
-            generate_images=True,
-            cover_prompt=story.cover_prompt,
-            cover_url=story.image_url,
-            pages_data=pages_data,
-        ),
-        last_error=_build_storybook_error_message(
-            cover_failed=cover_failed,
-            failed_pages=failed_pages,
-        ),
+        image_status=image_status,
+        last_error=error_message,
     )
     await db.commit()
+    return AssetCompletionResult(
+        asset="storybook_images",
+        status=image_status,
+        value=story.image_url,
+        error=error_message,
+    )
 
 
 async def _read_cached_audio_asset(story: Story, db: AsyncSession) -> bytes | None:
@@ -418,7 +457,7 @@ async def _complete_audio_asset(
     db: AsyncSession,
     *,
     raise_on_failure: bool = True,
-) -> bytes | None:
+) -> AssetCompletionResult:
     """Complete TTS audio generation through one asset workflow."""
 
     if not story.story_text:
@@ -426,7 +465,12 @@ async def _complete_audio_asset(
 
     cached_audio = await _read_cached_audio_asset(story, db)
     if cached_audio is not None:
-        return cached_audio
+        return AssetCompletionResult(
+            asset="audio",
+            status=StoryAssetStatus.READY,
+            value=cached_audio,
+            blocks_main_result=raise_on_failure,
+        )
 
     from app.services.provider_router import text_to_speech
 
@@ -441,7 +485,12 @@ async def _complete_audio_asset(
             audio_status=StoryAssetStatus.READY,
         )
         await db.commit()
-        return audio_data
+        return AssetCompletionResult(
+            asset="audio",
+            status=StoryAssetStatus.READY,
+            value=audio_data,
+            blocks_main_result=raise_on_failure,
+        )
     except Exception as exc:
         provider_error = str(exc)
         story.audio_path = None
@@ -459,7 +508,12 @@ async def _complete_audio_asset(
                 detail=f"Audio generation failed: {provider_error}",
             ) from exc
 
-        return None
+        return AssetCompletionResult(
+            asset="audio",
+            status=StoryAssetStatus.FAILED,
+            error=provider_error,
+            blocks_main_result=raise_on_failure,
+        )
 
 
 async def validate_profile_and_universe(
@@ -550,13 +604,15 @@ async def generate_full_story_service(
     errors: dict[str, str | None] = {}
 
     if story.cover_prompt:
-        image_url, image_error = await _complete_cover_image_asset(
+        image_result = await _complete_cover_image_asset(
             story,
             db,
             log_event="image_generation_failed",
         )
-        if image_error:
-            errors["image"] = image_error
+        if image_result.succeeded and isinstance(image_result.value, str):
+            image_url = image_result.value
+        if image_result.error:
+            errors["image"] = image_result.error
 
     return FullStoryResponse(
         id=story.id,
@@ -854,14 +910,14 @@ async def generate_story_cover(
     """Generate cover image for an existing story."""
     story = await get_story_detail(story_id, user_id, db)
 
-    image_url, _ = await _complete_cover_image_asset(
+    image_result = await _complete_cover_image_asset(
         story,
         db,
         raise_on_failure=True,
         log_event="cover_generation_failed",
     )
-    if image_url is not None:
-        return image_url
+    if image_result.succeeded and isinstance(image_result.value, str):
+        return image_result.value
 
     raise HTTPException(status_code=500, detail="Image generation failed")
 
@@ -874,9 +930,9 @@ async def generate_story_audio(
     """Generate audio for a story."""
     story = await get_story_detail(story_id, user_id, db)
 
-    audio_data = await _complete_audio_asset(story, db, raise_on_failure=True)
-    if audio_data is not None:
-        return audio_data
+    audio_result = await _complete_audio_asset(story, db, raise_on_failure=True)
+    if audio_result.succeeded and isinstance(audio_result.value, bytes):
+        return audio_result.value
 
     raise HTTPException(status_code=500, detail="Audio generation failed")