fix: bound large WebP lossless probe cost

docs/api.md

@@ -276,7 +276,7 @@ Idempotency-Key: <key>          # 建议
 | `output_format` | String | 否 | 输出格式：`png/jpeg/webp/avif/gif/bmp/tiff/ico`（默认保持原格式；ICO 自动等比缩至 256x256 边界） |
 | `max_width` | Integer | 否 | 大于 0 的最大宽度（等比缩放） |
 | `max_height` | Integer | 否 | 大于 0 的最大高度（等比缩放） |
-| `target_size_bytes` | Integer | 否 | 不小于 1024 的目标体积（字节），仅 `jpeg/webp/avif` 输出支持；不能与 `compression_rate` 同时指定 |
+| `target_size_bytes` | Integer | 否 | 不小于 1024 的最大输出体积（字节），仅 `jpeg/webp/avif` 输出支持；系统选择上限内的最高保真结果，不用填充凑到固定大小；不能与 `compression_rate` 同时指定 |
 | `preserve_metadata` | Boolean | 否 | 是否保留 EXIF/ICC（默认 `false`）；元数据输出仅支持 `jpeg/png/webp` |
 
 处理约束：

frontend/src/pages/HomePage.vue

@@ -24,6 +24,7 @@ interface UploadItem {
   status: ItemStatus
   result?: CompressResponse
   error?: string
+  targetSizeBytes?: number
 }
 
 const auth = useAuthStore()
@@ -135,6 +136,28 @@ function getTargetSizeBytes(): number | undefined {
   return Math.round(bytes)
 }
 
+function targetOutputFormat(file: File): OutputFormat {
+  const mime = file.type.trim().toLowerCase()
+  if (mime === 'image/jpeg' || mime === 'image/jpg') return 'jpeg'
+  if (mime === 'image/webp') return 'webp'
+  if (mime === 'image/avif') return 'avif'
+
+  const extension = file.name.split('.').pop()?.toLowerCase()
+  if (extension === 'jpg' || extension === 'jpeg') return 'jpeg'
+  if (extension === 'webp') return 'webp'
+  if (extension === 'avif') return 'avif'
+  return 'webp'
+}
+
+function targetResultHint(item: UploadItem): string | null {
+  if (!item.result || !item.targetSizeBytes) return null
+  const target = formatBytes(item.targetSizeBytes)
+  if (item.result.compressed_size * 4 < item.targetSizeBytes * 3) {
+    return `体积上限 ${target}；当前格式的最高保真结果本身更小，不会添加无效填充。`
+  }
+  return `体积上限 ${target}；结果已控制在上限内。`
+}
+
 function setCompressionMode(mode: CompressionMode) {
   options.mode = mode
   if (
@@ -159,7 +182,7 @@ async function runOne(item: UploadItem) {
     }
 
     const outputFormat: OutputFormat | undefined = options.outputFormat === 'auto'
-      ? (options.mode === 'size' ? 'webp' : undefined)
+      ? (options.mode === 'size' ? targetOutputFormat(item.file) : undefined)
       : options.outputFormat
 
     if (options.mode === 'size' && outputFormat && !targetSizeFormats.has(outputFormat)) {
@@ -188,6 +211,7 @@ async function runOne(item: UploadItem) {
       auth.token,
     )
 
+    item.targetSizeBytes = targetSizeBytes
     item.result = result
     item.status = 'done'
   } catch (err) {
@@ -452,6 +476,9 @@ async function resendVerification() {
                     {{ item.result.saved_percent.toFixed(2) }}%
                   </template>
                 </div>
+                <div v-if="targetResultHint(item)" class="mt-1 text-xs text-slate-500">
+                  {{ targetResultHint(item) }}
+                </div>
                 <div v-if="item.error" class="mt-1 text-xs text-rose-700">{{ item.error }}</div>
               </div>
 
@@ -547,7 +574,7 @@ async function resendVerification() {
                   :class="options.mode === 'size' ? 'bg-indigo-600 text-white' : 'text-slate-600 hover:bg-slate-100'"
                   @click="setCompressionMode('size')"
                 >
-                  按目标大小
+                  按体积上限
                 </button>
               </div>
             </div>
@@ -573,7 +600,7 @@ async function resendVerification() {
 
             <!-- 目标大小模式 -->
             <div v-else class="space-y-1">
-              <div class="text-xs font-medium text-slate-600">目标大小</div>
+              <div class="text-xs font-medium text-slate-600">最大输出大小</div>
               <div class="flex gap-2">
                 <input
                   v-model="options.targetSize"
@@ -591,7 +618,7 @@ async function resendVerification() {
                 </select>
               </div>
               <div class="text-xs text-slate-500">
-                仅支持 JPEG/WebP/AVIF；保持原格式时会自动输出 WebP，过小且无法保证清晰度的目标会被拒绝。
+                这是体积上限，不是固定输出大小。系统会优先使用原格式和最高可用画质；最高画质结果更小时不会填充无效数据。
               </div>
             </div>
 
@@ -601,7 +628,7 @@ async function resendVerification() {
                 v-model="options.outputFormat"
                 class="w-full rounded-md border border-slate-200 bg-white px-3 py-2 text-sm text-slate-800"
               >
-                <option value="auto">{{ options.mode === 'size' ? '自动选择 WebP（推荐）' : '保持原格式（推荐）' }}</option>
+                <option value="auto">{{ options.mode === 'size' ? '智能选择（优先原格式）' : '保持原格式（推荐）' }}</option>
                 <option value="jpeg">JPEG</option>
                 <option value="png" :disabled="options.mode === 'size'">PNG</option>
                 <option value="webp">WebP</option>
@@ -611,7 +638,7 @@ async function resendVerification() {
                 <option value="tiff" :disabled="options.mode === 'size'">TIFF</option>
                 <option value="ico" :disabled="options.mode === 'size'">ICO</option>
               </select>
-              <div class="text-xs text-slate-500">支持按需转码。目标大小模式建议配合 JPEG/WebP/AVIF。</div>
+              <div class="text-xs text-slate-500">支持按需转码。体积上限模式仅使用 JPEG/WebP/AVIF；其他输入会自动转为 WebP。</div>
             </label>
 
             <div class="grid grid-cols-2 gap-3">

migrations/027_task_target_size.sql

@@ -0,0 +1,6 @@
+ALTER TABLE tasks
+  ADD COLUMN target_size_bytes BIGINT;
+
+ALTER TABLE tasks
+  ADD CONSTRAINT tasks_target_size_bytes_check
+  CHECK (target_size_bytes IS NULL OR target_size_bytes >= 1024);

src/api/compress.rs

@@ -397,6 +397,7 @@ async fn compress_json(
             req.max_height,
             effective_level,
             req.compression_rate,
+            req.target_size_bytes,
             format_in,
             format_out,
             original_size,
@@ -728,6 +729,7 @@ async fn compress_direct(
             req.max_height,
             effective_level,
             req.compression_rate,
+            req.target_size_bytes,
             format_in,
             format_out,
             original_size,
@@ -1022,7 +1024,6 @@ async fn parse_single_file_request(
                             "target_size_bytes 格式错误，需为正整数（字节）",
                         )
                     })?);
-                    // 最小目标大小限制：1KB
                     if let Some(size) = target_size_bytes {
                         if size < 1024 {
                             return Err(AppError::new(
@@ -1030,6 +1031,12 @@ async fn parse_single_file_request(
                                 "target_size_bytes 最小为 1024（1KB）",
                             ));
                         }
+                        if i64::try_from(size).is_err() {
+                            return Err(AppError::new(
+                                ErrorCode::InvalidRequest,
+                                "target_size_bytes 超出支持范围",
+                            ));
+                        }
                     }
                 }
             }
@@ -1229,6 +1236,7 @@ async fn record_task_and_metering(
     max_height: Option<u32>,
     level: CompressionLevel,
     compression_rate: Option<u8>,
+    target_size_bytes: Option<u64>,
     format_in: ImageFmt,
     format_out: ImageFmt,
     original_size: u64,
@@ -1264,16 +1272,16 @@ async fn record_task_and_metering(
         INSERT INTO tasks (
           id, user_id, session_id, api_key_id, client_ip, source, status,
           compression_level, output_format, max_width, max_height, preserve_metadata,
-          compression_rate,
+          compression_rate, target_size_bytes,
           total_files, completed_files, failed_files,
           total_original_size, total_compressed_size,
           started_at, completed_at, expires_at, retention_hours
         ) VALUES (
           $1, $2, $3, $4, $5::inet, $6::task_source, 'completed',
           $7::compression_level, $8, $9, $10, $11, $12,
-          1, 1, 0,
+          $13, 1, 1, 0,
-          $13, $14,
+          $14, $15,
-          NOW(), NOW(), $15, $16
+          NOW(), NOW(), $16, $17
         )
         "#,
     )
@@ -1289,6 +1297,7 @@ async fn record_task_and_metering(
     .bind(max_height.map(|v| v as i32))
     .bind(false)
     .bind(compression_rate.map(|v| v as i16))
+    .bind(target_size_bytes.map(|v| v as i64))
     .bind(original_size as i64)
     .bind(compressed_size as i64)
     .bind(expires_at)

src/api/tasks.rs

@@ -56,6 +56,7 @@ struct BatchFileInput {
 struct BatchOptions {
     level: CompressionLevel,
     compression_rate: Option<u8>,
+    target_size_bytes: Option<u64>,
     output_format: Option<ImageFmt>,
     max_width: Option<u32>,
     max_height: Option<u32>,
@@ -235,16 +236,16 @@ async fn create_batch_task(
             INSERT INTO tasks (
               id, user_id, session_id, api_key_id, client_ip, source, status,
               compression_level, output_format, max_width, max_height, preserve_metadata,
-              compression_rate,
+              compression_rate, target_size_bytes,
               total_files, completed_files, failed_files,
               total_original_size, total_compressed_size,
               expires_at, retention_hours, anonymous_units_reserved, anonymous_quota_date
             ) VALUES (
               $1, $2, $3, $4, $5::inet, $6::task_source, 'pending',
               $7::compression_level, $8, $9, $10, $11, $12,
-              $13, 0, 0,
+              $13, $14, 0, 0,
-              $14, 0,
+              $15, 0,
-              $15, $16, $17, $18
+              $16, $17, $18, $19
             )
             "#,
         )
@@ -258,8 +259,9 @@ async fn create_batch_task(
         .bind(opts.output_format.map(|f| f.as_str()))
         .bind(opts.max_width.map(|v| v as i32))
         .bind(opts.max_height.map(|v| v as i32))
-        .bind(false)
+        .bind(opts.preserve_metadata)
         .bind(opts.compression_rate.map(|v| v as i16))
+        .bind(opts.target_size_bytes.map(|v| v as i64))
         .bind(files.len() as i32)
         .bind(total_original_size)
         .bind(expires_at)
@@ -491,6 +493,7 @@ async fn parse_batch_request(
     let mut opts = BatchOptions {
         level: CompressionLevel::Medium,
         compression_rate: None,
+        target_size_bytes: None,
         output_format: None,
         max_width: None,
         max_height: None,
@@ -681,6 +684,36 @@ async fn parse_batch_request(
                     });
                 }
             }
+            "target_size_bytes" | "target_size" => {
+                let v = text.trim();
+                if !v.is_empty() {
+                    let parsed = match v.parse::<u64>() {
+                        Ok(parsed) if parsed >= 1024 && i64::try_from(parsed).is_ok() => parsed,
+                        Ok(parsed) if parsed >= 1024 => {
+                            cleanup_file_paths(&files).await;
+                            return Err(AppError::new(
+                                ErrorCode::InvalidRequest,
+                                "target_size_bytes 超出支持范围",
+                            ));
+                        }
+                        Ok(_) => {
+                            cleanup_file_paths(&files).await;
+                            return Err(AppError::new(
+                                ErrorCode::InvalidRequest,
+                                "target_size_bytes 最小为 1024（1KB）",
+                            ));
+                        }
+                        Err(_) => {
+                            cleanup_file_paths(&files).await;
+                            return Err(AppError::new(
+                                ErrorCode::InvalidRequest,
+                                "target_size_bytes 格式错误，需为正整数（字节）",
+                            ));
+                        }
+                    };
+                    opts.target_size_bytes = Some(parsed);
+                }
+            }
             "max_width" => {
                 let v = text.trim();
                 if !v.is_empty() {
@@ -721,6 +754,14 @@ async fn parse_batch_request(
         }
     }
 
+    if opts.compression_rate.is_some() && opts.target_size_bytes.is_some() {
+        cleanup_file_paths(&files).await;
+        return Err(AppError::new(
+            ErrorCode::InvalidRequest,
+            "compression_rate 与 target_size_bytes 不能同时指定",
+        ));
+    }
+
     if let Some(rate) = opts.compression_rate {
         opts.level = compress::rate_to_level(rate);
     }
@@ -731,6 +772,22 @@ async fn parse_batch_request(
         }
     }
 
+    if opts.target_size_bytes.is_some() {
+        if let Some(file) = files
+            .iter()
+            .find(|file| !compress::supports_target_size_format(file.output_format))
+        {
+            cleanup_file_paths(&files).await;
+            return Err(AppError::new(
+                ErrorCode::InvalidRequest,
+                format!(
+                    "target_size_bytes 仅支持输出 jpeg/webp/avif，当前为 {}",
+                    file.output_format.as_str()
+                ),
+            ));
+        }
+    }
+
     let mw = opts.max_width.map(|v| v.to_string()).unwrap_or_default();
     let mh = opts.max_height.map(|v| v.to_string()).unwrap_or_default();
     let out_fmt = opts.output_format.map(|f| f.as_str()).unwrap_or("");
@@ -738,6 +795,10 @@ async fn parse_batch_request(
         .compression_rate
         .map(|v| v.to_string())
         .unwrap_or_default();
+    let target_key = opts
+        .target_size_bytes
+        .map(|v| v.to_string())
+        .unwrap_or_default();
     let preserve = if opts.preserve_metadata { "1" } else { "0" };
 
     let mut h = Sha256::new();
@@ -745,6 +806,7 @@ async fn parse_batch_request(
     h.update(opts.level.as_str().as_bytes());
     h.update(out_fmt.as_bytes());
     h.update(rate_key.as_bytes());
+    h.update(target_key.as_bytes());
     h.update(mw.as_bytes());
     h.update(mh.as_bytes());
     h.update(preserve.as_bytes());

src/services/compress.rs

@@ -33,11 +33,12 @@ const AVIF_TARGET_MIN_QUALITY: u8 = 38;
 const JPEG_PERCEPTUAL_QUALITY: u8 = 72;
 const WEBP_PERCEPTUAL_QUALITY: u8 = 70;
 const AVIF_PERCEPTUAL_QUALITY: u8 = 55;
-const JPEG_TARGET_MAX_QUALITY: u8 = 90;
+const JPEG_TARGET_MAX_QUALITY: u8 = 100;
-const WEBP_TARGET_MAX_QUALITY: u8 = 92;
+const WEBP_TARGET_MAX_QUALITY: u8 = 100;
-const AVIF_TARGET_MAX_QUALITY: u8 = 90;
+const AVIF_TARGET_MAX_QUALITY: u8 = 100;
 const AVIF_ENCODER_SPEED: u8 = 5;
 const WEBP_TARGET_SAFETY_PERCENT: u64 = 97;
+const WEBP_HIGH_EFFORT_LOSSLESS_MAX_PIXELS: u64 = 2_100_000;
 const METADATA_TARGET_OVERHEAD: u64 = 1024;
 
 #[derive(Clone)]
@@ -927,6 +928,23 @@ fn encode_webp_target(
 ) -> Result<Vec<u8>, AppError> {
     deadline.check()?;
     let pixels = prepare_target_pixels(&image);
+    let lossless_candidate = encode_webp_lossless_pixels(&pixels);
+    deadline.check()?;
+    match lossless_candidate {
+        Ok(lossless) if lossless.len() as u64 <= target_size => return Ok(lossless),
+        Ok(_) => {}
+        Err(error) => {
+            tracing::debug!(error = %error, "WebP 无损候选编码失败，继续尝试有损编码");
+        }
+    }
+    deadline.check()?;
+
+    let max_lossy = encode_webp_pixels(&pixels, WEBP_TARGET_MAX_QUALITY)?;
+    deadline.check()?;
+    if max_lossy.len() as u64 <= target_size {
+        return Ok(max_lossy);
+    }
+
     let native_min_quality = if allow_resize {
         WEBP_PERCEPTUAL_QUALITY
     } else {
@@ -987,6 +1005,37 @@ fn encode_webp_native_target(
         })
 }
 
+fn encode_webp_lossless_pixels(pixels: &TargetPixels) -> Result<Vec<u8>, AppError> {
+    let mut config = webp::WebPConfig::new()
+        .map_err(|_| AppError::new(ErrorCode::CompressionFailed, "初始化 WebP 无损配置失败"))?;
+    config.lossless = 1;
+    config.quality = 100.0;
+    config.method = webp_lossless_method(pixels);
+    config.alpha_compression = 1;
+    config.near_lossless = 100;
+    config.exact = 1;
+    config.thread_level = 0;
+
+    webp_encoder(pixels)
+        .encode_advanced(&config)
+        .map(|bytes| bytes.to_vec())
+        .map_err(|err| {
+            AppError::new(
+                ErrorCode::CompressionFailed,
+                format!("WebP 无损编码失败: {err:?}"),
+            )
+        })
+}
+
+fn webp_lossless_method(pixels: &TargetPixels) -> i32 {
+    let pixel_count = u64::from(pixels.width).saturating_mul(u64::from(pixels.height));
+    if pixel_count <= WEBP_HIGH_EFFORT_LOSSLESS_MAX_PIXELS {
+        6
+    } else {
+        0
+    }
+}
+
 fn encode_avif_target(
     image: DynamicImage,
     target_size: u64,
@@ -1942,6 +1991,79 @@ mod tests {
         assert_eq!(detect_format(&output).unwrap(), ImageFmt::Webp);
     }
 
+    #[test]
+    fn webp_target_prefers_lossless_when_it_fits() {
+        let image = DynamicImage::ImageRgb8(RgbImage::from_fn(160, 120, |x, y| {
+            let block = ((x / 20) + (y / 20) * 3) as u8;
+            Rgb([
+                block.wrapping_mul(31),
+                block.wrapping_mul(17),
+                block.wrapping_mul(11),
+            ])
+        }));
+        let pixels = prepare_target_pixels(&image);
+        let lossless = encode_webp_lossless_pixels(&pixels).unwrap();
+        let output = encode_webp_target(
+            image.clone(),
+            lossless.len() as u64,
+            true,
+            &CompressionDeadline::unlimited(),
+        )
+        .unwrap();
+
+        assert_eq!(output, lossless);
+        assert_eq!(
+            image::load_from_memory(&output).unwrap().to_rgb8(),
+            image.to_rgb8()
+        );
+    }
+
+    #[test]
+    fn webp_target_uses_quality_100_when_lossless_exceeds_the_cap() {
+        let mut state = 0x7f4a_7c15_u32;
+        let image = DynamicImage::ImageRgb8(RgbImage::from_fn(160, 120, |_x, _y| {
+            let mut channel = || {
+                state ^= state << 13;
+                state ^= state >> 17;
+                state ^= state << 5;
+                state as u8
+            };
+            Rgb([channel(), channel(), channel()])
+        }));
+        let pixels = prepare_target_pixels(&image);
+        let max_lossy = encode_webp_pixels(&pixels, WEBP_TARGET_MAX_QUALITY).unwrap();
+        let lossless = encode_webp_lossless_pixels(&pixels).unwrap();
+        assert!(max_lossy.len() < lossless.len());
+
+        let output = encode_webp_target(
+            image,
+            max_lossy.len() as u64,
+            true,
+            &CompressionDeadline::unlimited(),
+        )
+        .unwrap();
+        assert_eq!(output, max_lossy);
+    }
+
+    #[test]
+    fn large_webp_targets_use_the_fast_lossless_probe() {
+        let small = TargetPixels {
+            bytes: Vec::new(),
+            width: 1920,
+            height: 1080,
+            layout: TargetPixelLayout::Rgb,
+        };
+        let large = TargetPixels {
+            bytes: Vec::new(),
+            width: 4096,
+            height: 3072,
+            layout: TargetPixelLayout::Rgb,
+        };
+
+        assert_eq!(webp_lossless_method(&small), 6);
+        assert_eq!(webp_lossless_method(&large), 0);
+    }
+
     #[test]
     fn jpeg_target_encoder_prefers_perceptual_downscaling() {
         let image = DynamicImage::ImageRgb8(RgbImage::from_fn(800, 600, |x, y| {

src/worker/mod.rs

@@ -659,6 +659,7 @@ async fn ack_message(
 struct TaskProcRow {
     compression_level: String,
     compression_rate: Option<i16>,
+    target_size_bytes: Option<i64>,
     max_width: Option<i32>,
     max_height: Option<i32>,
     preserve_metadata: bool,
@@ -735,6 +736,7 @@ pub(crate) async fn process_task(
         RETURNING
           compression_level::text AS compression_level,
           compression_rate,
+          target_size_bytes,
           max_width,
           max_height,
           preserve_metadata,
@@ -782,6 +784,7 @@ pub(crate) async fn process_task(
     };
 
     let compression_rate = task.compression_rate.and_then(|v| u8::try_from(v).ok());
+    let target_size_bytes = task.target_size_bytes.and_then(|v| u64::try_from(v).ok());
     let level = compression_rate
         .map(compress::rate_to_level)
         .unwrap_or(compress::parse_level(&task.compression_level)?);
@@ -849,6 +852,7 @@ pub(crate) async fn process_task(
                 file,
                 level,
                 compression_rate,
+                target_size_bytes,
                 max_width,
                 max_height,
                 ctx,
@@ -953,25 +957,18 @@ async fn file_attempt_is_current(state: &AppState, fence: &FileFence) -> Result<
     .map_err(|err| AppError::new(ErrorCode::Internal, "检查文件处理租约失败").with_source(err))
 }
 
-#[allow(clippy::too_many_arguments)]
+async fn claim_task_file_attempt(
-async fn process_task_file(
+    state: &AppState,
-    state: AppState,
     task_id: Uuid,
     task_attempt: i64,
+    file_id: Uuid,
     worker_id: Uuid,
-    file: TaskFileProcRow,
+) -> Result<Option<i64>, AppError> {
-    level: compress::CompressionLevel,
+    sqlx::query_scalar(
-    compression_rate: Option<u8>,
-    max_width: Option<u32>,
-    max_height: Option<u32>,
-    ctx: TaskContext,
-    billing_ctx: Option<billing::BillingContext>,
-) -> Result<(), AppError> {
-    let file_attempt: Option<i64> = sqlx::query_scalar(
         r#"
         UPDATE task_files AS f
         SET status = 'processing',
-            processing_attempt = processing_attempt + 1,
+            processing_attempt = f.processing_attempt + 1,
             lease_owner = $4,
             lease_until = NOW() + $5 * INTERVAL '1 second',
             error_message = NULL
@@ -998,14 +995,33 @@ async fn process_task_file(
         RETURNING f.processing_attempt
         "#,
     )
-    .bind(file.id)
+    .bind(file_id)
     .bind(task_id)
     .bind(task_attempt)
     .bind(worker_id)
     .bind(PROCESSING_LEASE_SECONDS)
     .fetch_optional(&state.db)
     .await
-    .map_err(|err| AppError::new(ErrorCode::Internal, "更新文件处理状态失败").with_source(err))?;
+    .map_err(|err| AppError::new(ErrorCode::Internal, "更新文件处理状态失败").with_source(err))
+}
+
+#[allow(clippy::too_many_arguments)]
+async fn process_task_file(
+    state: AppState,
+    task_id: Uuid,
+    task_attempt: i64,
+    worker_id: Uuid,
+    file: TaskFileProcRow,
+    level: compress::CompressionLevel,
+    compression_rate: Option<u8>,
+    target_size_bytes: Option<u64>,
+    max_width: Option<u32>,
+    max_height: Option<u32>,
+    ctx: TaskContext,
+    billing_ctx: Option<billing::BillingContext>,
+) -> Result<(), AppError> {
+    let file_attempt =
+        claim_task_file_attempt(&state, task_id, task_attempt, file.id, worker_id).await?;
     let Some(file_attempt) = file_attempt else {
         return Ok(());
     };
@@ -1053,7 +1069,7 @@ async fn process_task_file(
         format_out,
         level,
         compression_rate,
-        None, // target_size_bytes: worker 批量任务不支持精确大小
+        target_size_bytes,
         max_width,
         max_height,
         ctx.preserve_metadata,
@@ -1082,7 +1098,7 @@ async fn process_task_file(
         compression_rate,
         format_in == format_out,
         max_width.is_some() || max_height.is_some(),
-        false,
+        target_size_bytes.is_some(),
         original_size,
         compressed_size,
     );
@@ -1990,7 +2006,10 @@ mod tests {
     use crate::services::mail::Mailer;
     use bytes::Bytes;
     use chrono::Utc;
+    use image::{DynamicImage, ImageFormat, Rgb, RgbImage};
     use sqlx::postgres::PgPoolOptions;
+    use std::io::Cursor;
+    use std::path::PathBuf;
     use tokio::sync::Barrier;
 
     #[test]
@@ -2140,16 +2159,35 @@ mod tests {
               original_size, status, processing_attempt, lease_owner, lease_until
             ) VALUES (
               $1, $2, 'fence.png', 'png', 'png',
-              100, 'processing', 2, $3, NOW() + INTERVAL '5 minutes'
+              100, 'pending', 0, NULL, NULL
             )
             "#,
         )
         .bind(file_id)
         .bind(task_id)
-        .bind(winning_owner)
         .execute(&pool)
         .await
         .expect("insert test task file");
+        assert_eq!(
+            claim_task_file_attempt(&state, task_id, 2, file_id, winning_owner)
+                .await
+                .expect("claim pending task file"),
+            Some(1)
+        );
+        sqlx::query(
+            r#"
+            UPDATE task_files
+            SET processing_attempt = 2,
+                lease_owner = $2,
+                lease_until = NOW() + INTERVAL '5 minutes'
+            WHERE id = $1
+            "#,
+        )
+        .bind(file_id)
+        .bind(winning_owner)
+        .execute(&pool)
+        .await
+        .expect("prepare winning file fence");
 
         let stale_key = storage::result_attempt_key(24, task_id, file_id, 1, 1, "png");
         let winning_key = storage::result_attempt_key(24, task_id, file_id, 2, 2, "png");
@@ -2327,6 +2365,123 @@ mod tests {
         .expect("query used units");
         assert_eq!(used_units, 1);
 
+        let target_task_id = Uuid::new_v4();
+        let target_file_id = Uuid::new_v4();
+        let target_worker = Uuid::new_v4();
+        let target_image = DynamicImage::ImageRgb8(RgbImage::from_fn(160, 120, |x, y| {
+            let block = ((x / 20) + (y / 20) * 3) as u8;
+            Rgb([
+                block.wrapping_mul(31),
+                block.wrapping_mul(17),
+                block.wrapping_mul(11),
+            ])
+        }));
+        let mut input_cursor = Cursor::new(Vec::new());
+        target_image
+            .write_to(&mut input_cursor, ImageFormat::Png)
+            .expect("encode target-size input PNG");
+        let target_input = input_cursor.into_inner();
+        let target_input_dir = PathBuf::from(&state.config.storage_path)
+            .join("orig")
+            .join(target_task_id.to_string());
+        tokio::fs::create_dir_all(&target_input_dir)
+            .await
+            .expect("create target-size input directory");
+        let target_input_path = target_input_dir.join("source.png");
+        tokio::fs::write(&target_input_path, &target_input)
+            .await
+            .expect("write target-size input");
+
+        sqlx::query(
+            r#"
+            INSERT INTO tasks (
+              id, user_id, status, compression_level, output_format,
+              target_size_bytes, total_files, total_original_size,
+              expires_at, retention_hours
+            ) VALUES (
+              $1, $2, 'pending', 'medium', 'webp',
+              $3, 1, $4,
+              NOW() + INTERVAL '1 day', 24
+            )
+            "#,
+        )
+        .bind(target_task_id)
+        .bind(user_id)
+        .bind(1_048_576_i64)
+        .bind(target_input.len() as i64)
+        .execute(&pool)
+        .await
+        .expect("insert target-size task");
+        sqlx::query(
+            r#"
+            INSERT INTO task_files (
+              id, task_id, original_name, original_format, output_format,
+              original_size, input_path, status
+            ) VALUES ($1, $2, 'source.png', 'png', 'webp', $3, $4, 'pending')
+            "#,
+        )
+        .bind(target_file_id)
+        .bind(target_task_id)
+        .bind(target_input.len() as i64)
+        .bind(target_input_path.to_string_lossy().to_string())
+        .execute(&pool)
+        .await
+        .expect("insert target-size task file");
+
+        assert_eq!(
+            process_task(&state, target_task_id, target_worker)
+                .await
+                .expect("process target-size task"),
+            TaskProcessOutcome::Done
+        );
+        let target_task_status: String =
+            sqlx::query_scalar("SELECT status::text FROM tasks WHERE id = $1")
+                .bind(target_task_id)
+                .fetch_one(&pool)
+                .await
+                .expect("query target-size task status");
+        assert_eq!(target_task_status, "completed");
+        let target_result: (String, Option<Uuid>, String, i64) = sqlx::query_as(
+            r#"
+            SELECT storage_backend, storage_endpoint_id, storage_key, compressed_size
+            FROM task_files
+            WHERE id = $1
+            "#,
+        )
+        .bind(target_file_id)
+        .fetch_one(&pool)
+        .await
+        .expect("query target-size result");
+        assert!(target_result.3 <= 1_048_576);
+        let target_locator = storage::ObjectLocator {
+            backend: target_result.0,
+            endpoint_id: target_result.1,
+            key: target_result.2,
+        };
+        let target_output = storage::read_bytes(&state, &target_locator)
+            .await
+            .expect("read target-size result");
+        assert_eq!(
+            image::load_from_memory(&target_output)
+                .expect("decode target-size result")
+                .to_rgb8(),
+            target_image.to_rgb8(),
+            "worker must forward target_size_bytes and select the lossless candidate"
+        );
+        storage::delete_object(&state, &target_locator)
+            .await
+            .expect("delete target-size result object");
+        sqlx::query("DELETE FROM usage_events WHERE task_id = $1")
+            .bind(target_task_id)
+            .execute(&pool)
+            .await
+            .expect("delete target-size usage event");
+        sqlx::query("DELETE FROM tasks WHERE id = $1")
+            .bind(target_task_id)
+            .execute(&pool)
+            .await
+            .expect("delete target-size task");
+
         discard_tracked_result(&state, &winning_object, None).await;
         sqlx::query("DELETE FROM usage_events WHERE task_id = $1")
             .bind(task_id)