refactor: #85 T6c extrait media-pdf-export vers backend/routers (comportement inchange)

backend/routers/helpers.py

@@ -10,9 +10,14 @@ de comportement. Regroupées ici car utilisées par plusieurs routers
 
 from __future__ import annotations
 
+import asyncio
 import os
+import re
 from pathlib import Path
 
+from fastapi import HTTPException, Request
+from fastapi.responses import FileResponse, StreamingResponse
+
 
 def content_disposition(disposition: str, filename: str) -> str:
     """Build a header-safe Content-Disposition value.
@@ -49,3 +54,76 @@ def media_max_inline_bytes() -> int:
     if mb <= 0:
         return default_mb * 1024 * 1024
     return int(mb * 1024 * 1024)
+
+
+def stream_file_with_range(file_path: Path, request: Request, media_type: str):
+    """Return a file response honouring the HTTP ``Range`` header (roadmap #109).
+
+    Extrait de :mod:`backend.main` (``_stream_file_with_range``) sans
+    changement de comportement. Shared by ``pdf/stream`` and ``/api/media``:
+    a plain :class:`FileResponse` with ``Accept-Ranges: bytes`` when no range
+    is requested, or a :class:`StreamingResponse` (206 Partial Content,
+    64 KiB chunks) for a valid single range. An unsatisfiable range yields
+    ``416`` with a ``Content-Range: bytes */size`` header.
+
+    Reads are offloaded to threads so the event loop is never blocked
+    (ASYNC230), matching the previous inline implementation.
+    """
+    file_size = file_path.stat().st_size
+    range_header = request.headers.get("range")
+    disposition = content_disposition("inline", file_path.name)
+
+    if range_header:
+        # Parse "bytes=start-end" (single range only; multi-range is not used by viewers)
+        m = re.match(r"bytes=(\d*)-(\d*)", range_header)
+        if not m:
+            raise HTTPException(status_code=416,
+                                headers={"Content-Range": f"bytes */{file_size}"})
+        start_s, end_s = m.group(1), m.group(2)
+        if start_s == "" and end_s == "":
+            raise HTTPException(status_code=416,
+                                headers={"Content-Range": f"bytes */{file_size}"})
+        if start_s == "":
+            # suffix range: last N bytes
+            length = min(int(end_s), file_size)
+            start = file_size - length
+            end = file_size - 1
+        else:
+            start = int(start_s)
+            end = int(end_s) if end_s else file_size - 1
+            end = min(end, file_size - 1)
+        if start > end or start >= file_size:
+            raise HTTPException(status_code=416,
+                                headers={"Content-Range": f"bytes */{file_size}"})
+
+        chunk_size = end - start + 1
+
+        async def _partial():
+            f = await asyncio.to_thread(open, str(file_path), "rb")
+            try:
+                await asyncio.to_thread(f.seek, start)
+                remaining = chunk_size
+                while remaining > 0:
+                    data = await asyncio.to_thread(f.read, min(64 * 1024, remaining))
+                    if not data:
+                        break
+                    remaining -= len(data)
+                    yield data
+            finally:
+                await asyncio.to_thread(f.close)
+
+        return StreamingResponse(
+            _partial(),
+            status_code=206,
+            media_type=media_type,
+            headers={
+                "Content-Range": f"bytes {start}-{end}/{file_size}",
+                "Accept-Ranges": "bytes",
+                "Content-Length": str(chunk_size),
+                "Content-Disposition": disposition,
+            },
+        )
+
+    return FileResponse(str(file_path), media_type=media_type, headers={
+        "Accept-Ranges": "bytes",
+        "Content-Disposition": disposition})