dotnet/aspnetcore · error · Exception

Failed to download {url} after {max_retries} attempts.

Error message

Failed to download {url} after {max_retries} attempts.

What it means

download_file retries up to max_retries (default 3) but only catches asyncio.CancelledError, asyncio.TimeoutError, and aiohttp.ClientError. If all attempts fail with one of those exception types, the loop exhausts and raises a final Exception naming the URL and attempt count. This is the terminal failure for transient network problems that did not self-heal within the retry budget.

Source

Thrown at eng/common/cross/install-debs.py:47

                        # verify checksum if provided
                        if checksum:
                            sha256 = hashlib.sha256(content).hexdigest()
                            if sha256 != checksum:
                                raise Exception(f"SHA256 mismatch for {url}: expected {checksum}, got {sha256}")

                        f.write(content)
                    print(f"Downloaded {url} at {dest_path}")
                    return
                else:
                    raise Exception(f"Failed to download {url}, Status Code: {response.status}")
        except (asyncio.CancelledError, asyncio.TimeoutError, aiohttp.ClientError) as e:
            print(f"Error downloading {url}: {type(e).__name__} - {e}. Retrying...")

        attempt += 1
        await asyncio.sleep(retry_delay)

    raise Exception(f"Failed to download {url} after {max_retries} attempts.")

async def download_deb_files_parallel(mirror, packages, tmp_dir):
    """Download .deb files in parallel."""
    os.makedirs(tmp_dir, exist_ok=True)

    tasks = []
    timeout = aiohttp.ClientTimeout(total=60)
    async with aiohttp.ClientSession(timeout=timeout) as session:
        for pkg, info in packages.items():
            filename = info.get("Filename")
            if filename:
                url = f"{mirror}/{filename}"
                dest_path = os.path.join(tmp_dir, os.path.basename(filename))
                tasks.append(asyncio.create_task(download_file(session, url, dest_path, checksum=info.get("SHA256"))))

        await asyncio.gather(*tasks)

async def download_package_index_parallel(mirror, arch, suites, check_sig, keyring):

View on GitHub (pinned to 294cab2f9b)

Solutions

1. Verify outbound connectivity from the host: curl -I <mirror-url> before re-running.
2. Set correct HTTP_PROXY/HTTPS_PROXY/NO_PROXY environment variables if behind a corporate proxy, or unset them if they are wrong.
3. Increase robustness by passing a larger max_retries/retry_delay to download_file (requires code change) for flaky links.
4. Switch to a mirror reachable from the build environment (e.g., a locally mirrored copy or snapshot.debian.org).
5. If building in CI, confirm the runner has network access and is not sandboxed to a package allowlist.

Example fix

# before — running with no proxy in a corporate network
HTTPS_PROXY=http://dead-proxy:8080 python3 install-debs.py ...
# Failed to download after 3 attempts

# after
unset HTTPS_PROXY HTTP_PROXY
python3 install-debs.py --mirror http://deb.debian.org/debian ...

# Verify connectivity before invoking install-debs
import socket
host = urllib.parse.urlparse(mirror).hostname
try:
    socket.create_connection((host, 443), timeout=5).close()
except OSError:
    raise SystemExit(f'No route to mirror {host}; fix network/proxy first')

# download_file only catches transient errors; wrap the whole gather for resilience
import asyncio
for attempt in range(3):
    try:
        await asyncio.gather(*tasks)
        break
    except Exception as e:
        if 'after' in str(e) and 'attempts' in str(e):
            print(f'All retries exhausted ({attempt+1}/3); switching mirror or backoff')
            await asyncio.sleep(2 ** attempt)
            continue
        raise

When it happens

Trigger: Sustained DNS failure, connection refused, TLS handshake error, or read timeout that persists across all 3 attempts spaced retry_delay (default 2s) apart. Each failure prints 'Error downloading ... Retrying...' and the loop increments attempt. After the final failure, this exception is raised and aborts asyncio.gather.

Common situations: No network egress from the build host to the mirror; DNS misconfigured in the build container; mirror is down or unreachable; firewall blocking outbound HTTPS; proxy environment variables (HTTP_PROXY/HTTPS_PROXY) point at a dead proxy.

Related errors

• SHA256 mismatch for {url}: expected {checksum}, got {sha256}
• Failed to download {url}, Status Code: {response.status}
• SHA256 mismatch for {path}: expected {packages_sha}, got {sh
• Signature verification failed: {result.stderr.decode('utf-8'
• Could not find checksum for {path} in Release file.

AI-assisted analysis of dotnet/aspnetcore@294cab2f9b (2026-08-06). Data as JSON: /api/errors/6b10c30ff60cbd35. Report an issue: GitHub.