nixos/syncthing: Fix race condition when updating from 1.x to 2.x (#472606)

  ) (filterAttrs (_: folder: folder.enable) cfg.settings.folders);

  jq = "${pkgs.jq}/bin/jq";

  grep = lib.getExe pkgs.gnugrep;

  updateConfig = pkgs.writers.writeBash "merge-syncthing-config" (

    ''

      set -efu

              --retry 1000 --retry-delay 1 --retry-all-errors \

              "$@"

      }

      # Before version 2.0.0, Syncthing used LevelDB. In version 2.0.0,

      # Syncthing started using SQLite. If you upgrade from an older version of

      # Syncthing that uses LevelDB to a newer version of Syncthing that uses

      # SQLite, then Syncthing will automatically do a one time database

      # migration [1]. While the migration is happening, the regular Syncthing

      # REST API is not available. Instead, a temporary API is made available

      # in its place.

      #

      # The rest of this script depends on Syncthing’s regular REST API. This

      # next part checks to see if Syncthing is currently providing the

      # temporary API. If it is, this next part will wait until the regular API

      # is available.

      #

      # [1]: <https://github.com/syncthing/syncthing/releases/tag/v2.0.0>

      while true

      do

        # We can use pretty much any API endpoint here. I chose to use

        # /rest/noauth/health because it doesn’t return a lot of data and

        # because doing a “health check” seems like an appropriate way to check

        # to see if the regular API is “alive” or not.

        content_type="$(curl \

          -o /dev/null \

          -w '%header{Content-Type}' \

          ${curlAddressArgs "/rest/noauth/health"}

        )"

        # The “($|([ \t]*;.*))” part at the end allows us to not worry about

        # whether or not the Content-Type contains any parameters. “$” matches

        # the end of the string which indicates that no parameters were used

        # [1][2]. The “[ \t]*;” part matches OWS [3] followed by a semicolon

        # which indicates that at least one parameter was used [4].

        #

        # We use “grep -i” here because media types are case-insensitive [2].

        #

        # [1]: <https://httpwg.org/specs/rfc9110.html#field.content-type>

        # [2]: <https://httpwg.org/specs/rfc9110.html#media.type>

        # [3]: <https://httpwg.org/specs/rfc9110.html#whitespace>

        # [4]: <https://httpwg.org/specs/rfc9110.html#parameter>

        if printf %s "$content_type" | ${lib.escapeShellArg grep} -qiP '^text/plain($|([ \t]*;.*))'

        then

          echo Waiting for Syncthing to finish its database migration…

          sleep 30

        # TODO: This next regex pattern can be simplified if this Syncthing bug gets fixed [1].

        #

        # [1]: <https://github.com/syncthing/syncthing/issues/10500>

        elif printf %s "$content_type" | ${lib.escapeShellArg grep} -qiP '^application/json($|([ \t]*;.*))'

        then

          echo 'Syncthing is not doing a database migration (anymore).'

          break

        else

          printf 'ERROR: Syncthing responded with an unexpected Content-Type: %s\n' "$content_type"

          # This is the EX_PROTOCOL exit status from <man:sysexits.h(3head)>.

          exit 76

        fi

      done

    ''

    +