Stage uploads on the cache filesystem - #13814

Description

Multipart uploads were staged in the operating-system temp directory. When GRADIO_TEMP_DIR was on another filesystem, the final rename failed and /upload returned the destination before a background copy published it.

Stage upload temp files in Gradio's configured upload directory so the final content-addressed rename stays on one filesystem and completes before the response. This avoids both the publication race and an extra synchronous copy of large uploads.

Closes: #13722

AI Disclosure

• I used AI to reproduce the race, draft the fix and regression test, and prepare this PR description. I self-reviewed every changed line and verified the behavior locally.
• I did not use AI

🎯 PRs Should Target Issues

This PR targets #13722. I checked for overlapping open PRs; none were found.

Testing and Formatting Your Code

• python3 -m pytest test/test_routes.py::TestRoutes::test_upload_is_staged_in_custom_upload_path -q
• ruff check --no-cache gradio/route_utils.py test/test_routes.py
• ruff format --check gradio/route_utils.py test/test_routes.py
• ty check --python /Library/Frameworks/Python.framework/Versions/3.14/bin/python3 gradio/route_utils.py
• Local 32 MiB HTTP probe: before, the returned path was missing and an immediate Range GET returned 403; after, the path existed and the Range GET returned 206 with 1,024 bytes.

Minimal reproduction:

import errno
import os
import shutil
import tempfile
import time
from pathlib import Path

CACHE_DIR = Path(tempfile.gettempdir()) / "gradio-upload-race"
os.environ["GRADIO_TEMP_DIR"] = str(CACHE_DIR)

import gradio as gr
from gradio import route_utils, routes

original_rename = os.rename

def cross_filesystem_rename(source, destination):
    source = Path(source).resolve()
    destination = Path(destination).resolve()
    if destination.is_relative_to(CACHE_DIR.resolve()) and not source.is_relative_to(CACHE_DIR.resolve()):
        raise OSError(errno.EXDEV, "simulated cross-filesystem upload")
    return original_rename(source, destination)

def delayed_move(sources, destinations):
    time.sleep(5)
    for source, destination in zip(sources, destinations, strict=False):
        Path(destination).parent.mkdir(parents=True, exist_ok=True)
        shutil.move(source, destination)

route_utils.os.rename = cross_filesystem_rename
routes.move_uploaded_files_to_cache = delayed_move

with gr.Blocks() as demo:
    gr.File()

demo.launch()

The reproduction files were not committed.