making v up mirror-aware for better experience in regions with limited GitHub access

[CryingN]

Currently, V's updates — including v up and VPM package downloads — are only distributed via GitHub. However, in regions where GitHub is difficult to access (e.g., China), this often leads to frequent failures or extremely slow speeds when installing, updating, or fetching dependencies. Has the team ever considered implementing mirror support — such as allowing users to configure custom download sources, auto-selecting the fastest mirror, or specifying a mirror via a command-line flag?

---

1. Make the update URL configurable

Similar to Linux package manager mirror lists. For example, Arch Linux's /etc/pacman.d/mirrorlist:

## China
Server = https://mirrors.ustc.edu.cn/archlinux/$repo/os/$arch
Server = https://mirrors.tuna.tsinghua.edu.cn/archlinux/$repo/os/$arch
Server = https://mirrors.cqu.edu.cn/archlinux/$repo/os/$arch
Server = https://mirrors.aliyun.com/archlinux/$repo/os/$arch

V could have a similar config file or environment variable to let users specify a custom base URL.

---

2. Auto-select the fastest available mirror

I recently implemented something similar in v_llama_cpp to automatically pick a faster download source. The core logic looks like this:

module v_llama_cpp

import net.http
import time

fn check_speed(url string, timeout_ms time.Duration, ch chan string) {
        http.fetch(http.FetchConfig{
                url: url
                method: .head
                read_timeout: timeout_ms * time.second
        }) or {
                return
        }
        ch <- url
}

fn select_fastest_url(urls []string, timeout_ms int) !string {
        if urls.len == 0 {
                return error('[Error] ./v_llama_cpp/ez_connect.v select_fastest_url(): not found url.')
        }

        ch := chan string{cap: urls.len}

        for url in urls {
                go check_speed(url, timeout_ms * time.second, ch)
        }

        select {
                winner := <-ch {
                        return winner
                }
                timeout_ms * time.second {
                        return error('[Error] ./v_llama_cpp/ez_connect.v select_fastest_url(): timeout, not found url.')
                }
        }
        return error('[Error] ./v_llama_cpp/ez_connect.v select_fastest_url(): unexpected end of select.')
}

If v up could do something similar — probe a list of known mirrors (GitHub, Gitee, AtomGit, etc.) and pick the fastest one — that would be really helpful.

---

3. Specify a mirror via command-line flag

Like pip's -i flag in Python:

pip install some-package -i https://pypi.tuna.tsinghua.edu.cn/simple

V could support something like:

v up --mirror https://gitee.com/mirrors/v

This gives users a quick way to switch mirrors when the default one is slow or unreachable.

---

Platforms like GitHub, Gitee, AtomGit, etc., could all be used for real-time syncing if the official repo sets up mirrors there. Technically, none of these ideas seem too hard to implement, and they wouldn't affect the default GitHub workflow.

To be honest, we do care about the team's attitude on this. If there is a clear intention in this direction, we might be able to help with some related work. If not, we can just stop focusing on the mirroring issue for now.