Fix Matrix Synapse not allowing video seeking (nginx)

default.conf

# Fix video buffering
# https://github.com/matrix-org/synapse/issues/4780#issuecomment-1003101558
# luarocks install pgmoon

set $our_homeserver "matrix.example.com"; # Set this to the hostname of your server
set $media_store "/var/lib/matrix-synapse/media"; # no trailing slash
set $media_status_header true; # add a header to show the status of the media.

location ~* "^/_matrix/media/r0/download/(.*?)/([A-Za-z0-9]{2})([A-Za-z0-9]{2})(.*)$" {
    # Enable video seeking.
    add_header Accept-Ranges bytes;

    # Emulate the headers.
    add_header 'Access-Control-Allow-Origin' '*' always;
    add_header 'Access-Control-Allow-Methods' 'GET, HEAD, POST, PUT, DELETE, OPTIONS' always;
    add_header 'Access-Control-Allow-Headers' 'X-Requested-With, Content-Type, Authorization, Date, Range' always;
    add_header 'Access-Control-Expose-Headers' 'Content-Length,Content-Range' always;
    add_header 'Cross-Origin-Resource-Policy' 'cross-origin' always;
    add_header 'Content-Security-Policy' "sandbox; default-src 'none'; script-src 'none'; plugin-types application/pdf; style-src 'unsafe-inline'; media-src 'self'; object-src 'self';" always;
    add_header 'Content-Security-Policy' 'sandbox' always;
    add_header 'Referrer-Policy' 'no-referrer' always;
    add_header 'X-Frame-Options' 'DENY' always;
    add_header 'X-Content-Type-Options' 'nosniff' always;

    # Set variables to use in lua.
    set $media_host $1;
    set $part_a $2;
    set $part_b $3;
    set $part_c $4;

    # Need to figure out where the file will be located.
    set_by_lua_block $file_location {
      -- Determine if our homeserver is on a subdomain so that we can serve local files mapped to the root domain too.
      -- This should work if you have something wierd like matrix.sub.example.com
      _, _, our_root_domain = string.find(ngx.var.our_homeserver, ".*[.](.*[.].*)")

      if ngx.var.media_host == ngx.var.our_homeserver or our_root_domain == ngx.var.media_host then
        return ngx.var.media_store .. '/local_content'
      else
        return ngx.var.media_store .. '/remote_content/' .. ngx.var.media_host
      end
    }

    # Get the correct file type to use.
    # access_by_lua_block is a workaround for cosocket limitations.
    access_by_lua_block {
      local pgmoon = require("pgmoon")
      local http = require("resty.http")
      local media_status_header_set = false
      function media_status_header(status)
        if ngx.var.media_status_header == "true" and not media_status_header_set then
          ngx.header["Synapse-Remote-Media-Status"] = status
          media_status_header_set = true
        end
      end
      local pg = pgmoon.new({
        host = "127.0.0.1",
        port = "5432",
        database = "synapse",
        user = "synapse_user",
        password = "klmdkjlsadlkj"
      })
      assert(pg:connect())
      local esc = pg:escape_literal(ngx.var.part_a .. ngx.var.part_b .. ngx.var.part_c)
      success = false

      -- Add a custom header to let you know it was served through nginx instead of Synapse.
      if ngx.var.media_status_header == "true" then
        ngx.header["Synapse-Media-Server"] = "nginx"
      end

      -- Check both the remote and local file store for our file ID.
      -- The column filesystem_id in remote_media_cache should get mapped to media_id.
      local sql = "(SELECT media_type, upload_name, media_id FROM local_media_repository WHERE media_id = " .. esc .. ") UNION ALL (SELECT media_type, upload_name, filesystem_id FROM remote_media_cache WHERE media_id = " .. esc .. ")"

      res = assert(pg:query(sql))

      -- If the file doesnt exist, ask Synapse to fetch it for us and wait a max of 10s for it to finish.
      i = 0
      if res == nil or res[1] == nil then
        local http_res, http_err = http.new():request_uri("http://127.0.0.1:8008/_matrix/media/r0/download/" .. ngx.var.media_host .. "/" .. ngx.var.part_a .. ngx.var.part_b .. ngx.var.part_c, { method = "GET" })
        media_status_header(http_res.status)
        res = assert(pg:query(sql))
      end

      if res ~= nil and res[1] ~= nil then
        res = res[1]
        -- Check that the data is present actually.
        if res["media_type"] ~= nil and res["media_type"] ~= "" and res['media_id'] ~= nil and res['media_id'] ~= "" then
          -- Update the parts of the file ID since it may have changed if it was a remote file.
          _, _, ngx.var.part_a, ngx.var.part_b, ngx.var.part_c = string.find(res['media_id'], "(%w%w)(%w%w)(.*)$")

          -- Sometimes internal/backend services wont populate the fields.
          if res["upload_name"] ~= nil and res["upload_name"] ~= "" then
            ngx.header["Content-Disposition"] = 'inline; filename=' .. res["upload_name"]
          end

          ngx.header["Content-Type"] = res["media_type"]
          media_status_header("local")
          success = true
        else
          media_status_header("data-error")
        end
      else
        media_status_header("entry-not-found")
      end

      if not success then
        -- If it doesnt exist on our disk then we will tell the browsers and CDN not to cache the page.
        ngx.header["Cache-Control"] = "no-store, must-revalidate"
        ngx.header["Pragma"] = "no-cache"
        ngx.header["Expires"] = 0
      else
        ngx.header["Cache-Control"] = "public, max-age=86400" -- cache for 1 day
      end
      pg:keepalive()
      pg = nil
    }
    alias $file_location/;
    try_files $part_a/$part_b/$part_c =404;
}