DEV: Make postgres_readonly cache work like other caches (#20879)

We didn't have an authoritative source for this data previously, so now
it's stored in redis.

lib/discourse.rb

@@ -594,6 +594,8 @@ module Discourse
     alias_method :base_url_no_path, :base_url_no_prefix
   end
 
+  LAST_POSTGRES_READONLY_KEY = "postgres:last_readonly"
+
   READONLY_MODE_KEY_TTL ||= 60
   READONLY_MODE_KEY ||= "readonly_mode"
   PG_READONLY_MODE_KEY ||= "readonly_mode:postgres"
@@ -704,7 +706,7 @@ module Discourse
 
   # Shared between processes
   def self.postgres_last_read_only
-    @postgres_last_read_only ||= DistributedCache.new("postgres_last_read_only", namespace: false)
+    @postgres_last_read_only ||= DistributedCache.new("postgres_last_read_only")
   end
 
   # Per-process
@@ -712,20 +714,33 @@ module Discourse
     @redis_last_read_only ||= {}
   end
 
+  def self.postgres_recently_readonly?
+    timestamp =
+      postgres_last_read_only.defer_get_set("timestamp") do
+        seconds = redis.get(LAST_POSTGRES_READONLY_KEY)
+        Time.zone.at(seconds.to_i) if seconds
+      end
+
+    timestamp.present? && timestamp > 15.seconds.ago
+  end
+
   def self.recently_readonly?
-    postgres_read_only = postgres_last_read_only[Discourse.redis.namespace]
     redis_read_only = redis_last_read_only[Discourse.redis.namespace]
 
-    (redis_read_only.present? && redis_read_only > 15.seconds.ago) ||
-      (postgres_read_only.present? && postgres_read_only > 15.seconds.ago)
+    (redis_read_only.present? && redis_read_only > 15.seconds.ago) || postgres_recently_readonly?
   end
 
   def self.received_postgres_readonly!
-    postgres_last_read_only[Discourse.redis.namespace] = Time.zone.now
+    time = Time.zone.now
+    redis.set(LAST_POSTGRES_READONLY_KEY, time.to_i.to_s)
+    postgres_last_read_only.clear
+
+    time
   end
 
   def self.clear_postgres_readonly!
-    postgres_last_read_only[Discourse.redis.namespace] = nil
+    redis.del(LAST_POSTGRES_READONLY_KEY)
+    postgres_last_read_only.clear
   end
 
   def self.received_redis_readonly!

spec/lib/discourse_spec.rb

@@ -311,7 +311,7 @@ RSpec.describe Discourse do
     describe ".received_postgres_readonly!" do
       it "sets the right time" do
         time = Discourse.received_postgres_readonly!
-        expect(Discourse.postgres_last_read_only["default"]).to eq(time)
+        expect(Discourse.redis.get(Discourse::LAST_POSTGRES_READONLY_KEY).to_i).to eq(time.to_i)
       end
     end
 
@@ -328,7 +328,7 @@ RSpec.describe Discourse do
         messages = []
 
         expect do messages = MessageBus.track_publish { Discourse.clear_readonly! } end.to change {
-          Discourse.postgres_last_read_only["default"]
+          Discourse.redis.get(Discourse::LAST_POSTGRES_READONLY_KEY)
         }.to(nil)
 
         expect(messages.any? { |m| m.channel == Site::SITE_JSON_CHANNEL }).to eq(true)