502 error after the AGH update

[ameshkov]

2019/11/20 05:07:09 [info] AdGuard Home, version v0.99.2, channel release

2019/11/20 05:07:09 [info] AdGuard Home is running as a service
2019/11/20 05:07:09 [info] AdGuard Home is available on the following addresses:
2019/11/20 05:07:09 [info] Go to http://127.0.0.1:80
2019/11/20 05:07:09 [info] Go to http://192.168.1.131:80
2019/11/20 05:07:12 [info] Upstream 0: https://cloudflare-dns.com:443/dns-query
2019/11/20 05:07:12 [info] Starting the DNS proxy server
2019/11/20 05:07:12 [info] Ratelimit is enabled and set to 20 rps
2019/11/20 05:07:12 [info] The server is configured to refuse ANY requests
2019/11/20 05:07:12 [info] DNS cache is enabled
2019/11/20 05:07:12 [info] Creating the UDP server socket
2019/11/20 05:07:12 [info] Listening to udp://[::]:53
2019/11/20 05:07:12 [info] Creating the TCP server socket
2019/11/20 05:07:12 [info] Listening to tcp://[::]:53
2019/11/20 05:07:12 [info] Entering the tcp listener loop on [::]:53
2019/11/20 05:07:12 [info] Entering the UDP listener loop on [::]:53
2019/11/20 05:07:22 [info] Updating from v0.99.2 to v0.99.3.  URL:https://static.adguard.com/adguardhome/release/AdGuardHome_linux_arm.tar.gz  Package:/home/pi/AdGuardHome/agh-update-v0.99.3/AdGuardHome_linux_arm.tar.gz
2019/11/20 05:07:24 [info] AdGuard Home, version v0.99.3, channel release

2019/11/20 05:07:24 [info] Configuration file is OK
2019/11/20 05:07:25 [info] Stopping all tasks
2019/11/20 05:07:25 [info] Stopping AdGuard Home
2019/11/20 05:07:25 [info] Stopping the DNS proxy server
2019/11/20 05:07:25 [info] tcpListen.Accept() returned because we're reading from a closed connection, exiting loop
2019/11/20 05:07:25 [info] udpListen.ReadFrom() returned because we're reading from a closed connection, exiting loop
2019/11/20 05:07:25 [info] Stopped the DNS proxy server
2019/11/20 05:07:31 [info] Stopped
2019/11/20 05:07:31 [info] Restarting: [/home/pi/AdGuardHome/AdGuardHome -s run]
2019/11/20 05:07:32 [info] AdGuard Home, version v0.99.3, channel release

2019/11/20 05:07:32 [info] AdGuard Home is running as a service
2019/11/20 05:07:32 [info] AdGuard Home is available on the following addresses:
2019/11/20 05:07:32 [info] Go to http://127.0.0.1:80
2019/11/20 05:07:32 [info] Go to http://192.168.1.131:80
2019/11/20 05:07:33 [info] Couldn't get version check json from https://static.adguard.com/adguardhome/release/version.json: *url.Error Get https://static.adguard.com/adguardhome/release/version.json: read udp 127.0.0.1:46880->127.0.0.1:53: read: connection refused

2019/11/20 05:07:34 [info] Upstream 0: https://cloudflare-dns.com:443/dns-query
2019/11/20 05:07:34 [info] Starting the DNS proxy server
2019/11/20 05:07:34 [info] Ratelimit is enabled and set to 20 rps
2019/11/20 05:07:34 [info] The server is configured to refuse ANY requests
2019/11/20 05:07:34 [info] DNS cache is enabled

Also, here's another relevant discussion:
#886 (comment)

[szolin]

1. When there is running: false in /control/status response, UI shows a stub page.
2. We should replace it with an informative message saying that DNS server is starting up and it may take time (even more time if there are large filter lists enabled).
3. After that we should re-request the server every 1 second until we receive running: true. This means that DNS server is up and we can load Dashboard as usual.

[ameshkov]

One more thing. When the AGH update is in progress, we should not reload page unless running is true