DEV(cache_critical_dns): add SRV priority tunables

An SRV RR contains a priority value for each of the SRV targets that
are present, ranging from 0 - 65535. When caching SRV records we may want to
filter out any targets above or below a particular threshold.

This change adds support for specifying a lower and/or upper bound on
target priorities for any SRV RRs. Any targets returned when resolving
the SRV RR whose priority does not fall between the lower and upper
thresholds are ignored.

For example: Let's say we are running two Redis servers, a primary and
cold server as a backup (but not a replica). Both servers would pass health
checks, but clearly the primary should be preferred over the backup
server. In this case, we could configure our SRV RR with the primary
target as priority 1 and backup target as priority 10. The
`DISCOURSE_REDIS_HOST_SRV_LE` could then be set to 1 and the target with
priority 10 would be ignored.

See /t/66045.

script/cache_critical_dns

@@ -23,6 +23,22 @@ CRITICAL_HOST_ENV_VARS = %w{
 HOST_RESOLVER_CACHE = {}
 HOST_HEALTHY_CACHE = {}
 HOSTS_PATH = ENV['DISCOURSE_DNS_CACHE_HOSTS_FILE'] || "/etc/hosts"
+
+PrioFilter = Struct.new(:min, :max) do
+  # min and max must be integers and relate to the minimum or maximum accepted
+  # priority of an SRV RR target.
+  # The return value from within_threshold? indicates if the priority is less
+  # than or equal to the upper threshold, or greater than or equal to the
+  # lower threshold.
+  def within_threshold?(p)
+    p >= min && p <= max
+  end
+end
+SRV_PRIORITY_THRESHOLD_MIN = 0
+SRV_PRIORITY_THRESHOLD_MAX = 65535
+SRV_PRIORITY_FILTERS = Hash.new(
+  PrioFilter.new(SRV_PRIORITY_THRESHOLD_MIN, SRV_PRIORITY_THRESHOLD_MAX))
+
 REFRESH_SECONDS = 30
 
 module DNSClient
@@ -54,14 +70,16 @@ end
 class SRVName
   include DNSClient
 
-  def initialize(srv_hostname)
+  def initialize(srv_hostname, prio_filter)
     @name = srv_hostname
+    @prio_filter = prio_filter
   end
 
   def resolve
     dns_client_with_timeout do |dns_client|
       [].tap do |addresses|
         targets = dns_client.getresources(@name, Resolv::DNS::Resource::IN::SRV)
+        targets.delete_if { |t| !@prio_filter.within_threshold?(t.priority) }
         addresses.concat(targets.map { |t| Name.new(t.target.to_s).resolve }.flatten)
       end
     end
@@ -264,8 +282,12 @@ def nilempty(v)
   end
 end
 
+def env_srv_var(env_name)
+  "#{env_name}_SRV"
+end
+
 def env_srv_name(env_name)
-  nilempty(ENV["#{env_name}_SRV"])
+  nilempty(ENV[env_srv_var(env_name)])
 end
 
 def run(hostname_vars)
@@ -278,7 +300,7 @@ def run(hostname_vars)
     name = ENV[var]
     HOST_RESOLVER_CACHE[var] ||= ResolverCache.new(
       if (srv_name = env_srv_name(var))
-        SRVName.new(srv_name)
+        SRVName.new(srv_name, SRV_PRIORITY_FILTERS[env_srv_var(var)])
       else
         Name.new(name)
       end
@@ -339,6 +361,23 @@ all_hostname_vars = CRITICAL_HOST_ENV_VARS.select do |name|
   end
 end
 
+# Populate the SRV_PRIORITY_FILTERS for any name that has a priority present in
+# the environment. If no priority thresholds are found for the name, the default
+# is that no filtering based on priority will be performed.
+CRITICAL_HOST_ENV_VARS.each do |v|
+  if (name = env_srv_name(v))
+    max = ENV.fetch("#{env_srv_var(v)}_PRIORITY_LE", SRV_PRIORITY_THRESHOLD_MAX).to_i
+    min = ENV.fetch("#{env_srv_var(v)}_PRIORITY_GE", SRV_PRIORITY_THRESHOLD_MIN).to_i
+    if max > SRV_PRIORITY_THRESHOLD_MAX ||
+        min < SRV_PRIORITY_THRESHOLD_MIN ||
+        min > max
+      raise "invalid priority threshold set for #{v}"
+    end
+
+    SRV_PRIORITY_FILTERS[env_srv_var(v)] = PrioFilter.new(min, max)
+  end
+end
+
 while true
   run(all_hostname_vars)
   sleep REFRESH_SECONDS