TSI-3418: fix(worker): 裸 -c copy 被静默改写为重新编码（HEVC→H264 有损转换）

[multica-tsic404]

Closes TSI-3418

Summary by Sourcery

Prevent rewrite decisions from replacing FFmpeg stream-copy commands with unintended video re-encoding.

Bug Fixes:

• Preserve bare general codec copy directives without silently converting them into lossy video re-encodes.
• Correctly recognize supported explicit video codec flag forms while allowing video-specific settings to override general copy directives.

Tests:

• Add regression coverage for bare and inline copy syntax, explicit video codec precedence, non-video codec flags, and verbatim argument passthrough.

[sourcery-ai]

Reviewer's Guide

Fixes silent lossy re-encoding of commands using bare -c/-codec copy by recognizing general stream-copy directives while preserving video-specific overrides, and adds parser plus end-to-end regression tests.

Flow diagram for FFmpeg encoder argument classification

flowchart TD
    A[FFmpeg arguments] --> B{Video-specific codec flag present?}
    B -->|Yes| C[parseEncoderFromArgs]
    B -->|No| D{General codec flag sets copy?}
    D -->|Yes| E[Return encoder.EncoderFamily copy]
    D -->|No| F[Return empty encoder family]
    C --> G[Use video-specific encoder override]
    E --> H[Preserve stream copy]
    G --> I[Rewrite engine continues with classified encoder]
    H --> I

Loading

File-Level Changes

Change	Details	Files
Preserve bare general stream-copy directives instead of treating them as missing encoder selection and rewriting them.	Detect -c copy, -codec copy, and inline -c=copy/-codec=copy. Prioritize video-specific codec flags over a general copy directive. Leave non-copy general codec values to the existing auto-selection path.	pkg/worker/rewrite_adapter.go
Add focused unit and end-to-end regression coverage for stream-copy handling.	Cover separated and inline general copy flag forms, precedence, non-copy codecs, and audio-only codec flags. Verify bare copy arguments pass through verbatim with no rewrite or resolved target encoder, including mixed audio re-encoding.	pkg/worker/rewrite_adapter_test.go

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[sourcery-ai]

Hey - I've found 1 issue

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="pkg/worker/rewrite_adapter.go" line_range="388-398" />
<code_context>
 		}
 	}
+
+	// General codec flag: bare "-c copy" sets every stream (video included)
+	// to stream-copy. Treat it as the "copy" passthrough encoder so the
+	// rewrite engine never injects a lossy re-encode.
+	for i, arg := range args {
+		if arg == "-c" || arg == "-codec" {
+			if i+1 < len(args) && args[i+1] == "copy" {
+				return encoder.EncoderFamily("copy")
+			}
+		}
+		if arg == "-c=copy" || arg == "-codec=copy" {
+			return encoder.EncoderFamily("copy")
+		}
+	}
</code_context>
<issue_to_address>
**issue (bug_risk):** When arguments contain an inline stream-specific option such as `-codec:v=h264_qsv` or `-vcodec=h264_qsv` together with `-c copy`, the first scan does not recognize that stream-specific form and the new general-option scan returns `copy`. The adapter therefore skips rewriting and leaves the explicitly requested video encoder unchanged, even when it is unavailable and should be handled by the rewrite or fallback path.

**Triggers:** When FFmpeg arguments combine general `-c copy` with inline `-codec:v=...` or `-vcodec=...`.

**Suggested fix:** Recognize `-codec:v=<encoder>` and `-vcodec=<encoder>` in the stream-specific scan before considering general codec flags.
</issue_to_address>

Sourcery assessment

Needs a human reviewer. 1 finding to address first, and if the parser misclassifies a command, the worker could leave an output video in an unsupported codec instead of re-encoding it, or fail to apply a needed conversion. That incorrect media artifact outlives a revert, but it is bounded and can be regenerated after reverting the change.

Blocking findings: pkg/worker/rewrite_adapter.go:398

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[multica-tsic404]

审查规模：L1（93 行 / 2 文件），Bug 修复。修复方向正确：裸 -c copy 短路为 passthrough，端到端回归测试覆盖透传与 ResolveTargetEncoder 置空，点赞。

通过维度：代码质量 ✓ 安全 ✓ 架构 ✓ 规范 ✓ 性能 ✓ 并发 ✓ API 兼容 ✓。安全：纯参数解析，无新增依赖、无 shell 拼接、无敏感信息（显式检查）。

已纳入 sourcery-ai 审查反馈（COMMENTED，非 CHANGES_REQUESTED）：其指出的内联 -codec:v=/-vcodec= 与 -c copy 组合误判已核实为真实缺陷，见内联评论 1；另见评论 2（注释语义与位置优先级）。

[sourcery-ai]

Sourcery assessment

Approved.

[multica-tsic404]

复审提交 5eb51c3：2 个阻塞问题均已解决。

1. 🔴 pkg/worker/rewrite_adapter.go（原 :392，现 :384-390）— 修复正确：首个扫描现通过 strings.HasPrefix 识别 -c:v= / -codec:v= / -vcodec= 三种内联形式（切片偏移 5/9/8 逐一核对正确），显式视频编码器优先于通用 -c copy，不再误判为纯 copy 透传。
2. 🟡 pkg/worker/rewrite_adapter.go:366-372 注释 — 现准确表述：显式视频标志优先、且注明 ffmpeg 位置语义（later wins）与本解析器的取舍，不再绝对化。

测试补齐：rewrite_adapter_test.go 新增内联形式与「内联 -codec:v=/-vcodec= 覆盖通用 -c copy」用例，恰好覆盖 Sourcery 原始 bug_risk 场景；端到端回归测试（透传 + ResolveTargetEncoder 置空）保留。

安全维度已显式检查：仍为纯参数解析，无新增依赖（go.mod 未动）、无 shell 拼接、无敏感信息输出。并发/性能/API 兼容同前轮：parseEncoderFromArgs 无共享状态、O(n) 单趟、公共接口未变。

sourcery-ai 已同步转 APPROVED。零发现。