修復 NetworkManager-wait-online 導致的 network-online.target active 過早

systemd 有個 network-online.target，許多程式會依賴它以在網路連線成功後纔啓動。但如果使用 NetworkManager 提供的 NetworkManager-wait-online.service，會導致某些程式啓動過早，如 shadowsocks-libev 使用域名作爲伺服器地址時報錯 Temporary failure in name resolution。

在換用百合仙子寫的 wait-online 後，問題消失。日誌中可以看到 shadowsocks-libev 確實在 network-online.target active 後纔啓動。於是查看 NetworkManager-wait-online.service：

# /usr/lib/systemd/system/NetworkManager-wait-online.service
[Unit]
Description=Network Manager Wait Online
Documentation=man:nm-online(1)
Requires=NetworkManager.service
After=NetworkManager.service
Before=network-online.target

[Service]
# `nm-online -s` waits until the point when NetworkManager logs
# "startup complete". That is when startup actions are settled and
# devices and profiles reached a conclusive activated or deactivated
# state. It depends on which profiles are configured to autoconnect and
# also depends on profile settings like ipv4.may-fail/ipv6.may-fail,
# which affect when a profile is considered fully activated.
# Check NetworkManager logs to find out why wait-online takes a certain
# time.

Type=oneshot
ExecStart=/usr/bin/nm-online -s -q
RemainAfterExit=yes

# Set $NM_ONLINE_TIMEOUT variable for timeout in seconds.
# Edit with `systemctl edit NetworkManager-wait-online`.
#
# Note, this timeout should commonly not be reached. If your boot
# gets delayed too long, then the solution is usually not to decrease
# the timeout, but to fix your setup so that the connected state
# gets reached earlier.
Environment=NM_ONLINE_TIMEOUT=60

[Install]
WantedBy=network-online.target

閱讀 nm-online 的 man page 後發現，-s 是在 NetworkManager 啓動連線時就退出，而非網路連線成功。我選擇使用 drop-in file 來去掉這個選項：

# /etc/systemd/system/NetworkManager-wait-online.service.d/exit-after-connected.conf
[Service]
ExecStart=
ExecStart=/usr/bin/nm-online -q

問題解決。