From 3c44bed54575947b38b9aa96a50e05327af196f4 Mon Sep 17 00:00:00 2001
From: Jarek Radosz <jradosz@gmail.com>
Date: Tue, 5 Apr 2022 19:29:58 +0200
Subject: [PATCH] DEV: Make DistributedMemoizer use DistributedMutex (#16229)

Its implementation was already distributed-mutex-like, with slight differences that did not seem necessary.
---
 lib/distributed_memoizer.rb             | 65 +++++--------------------
 spec/lib/distributed_memoizer_spec.rb   | 18 ++-----
 spec/requests/static_controller_spec.rb |  8 ++-
 3 files changed, 18 insertions(+), 73 deletions(-)

diff --git a/lib/distributed_memoizer.rb b/lib/distributed_memoizer.rb
index 559833f2b6d..31b53434427 100644
--- a/lib/distributed_memoizer.rb
+++ b/lib/distributed_memoizer.rb
@@ -1,72 +1,31 @@
 # frozen_string_literal: true
 
 class DistributedMemoizer
-
   # never wait for longer that 1 second for a cross process lock
-  MAX_WAIT = 2
-  LOCK = Mutex.new
+  MAX_WAIT = 1
 
   # memoize a key across processes and machines
-  def self.memoize(key, duration = 60 * 60 * 24, redis = nil)
-    redis ||= Discourse.redis
-
+  def self.memoize(key, duration = 60 * 60 * 24, redis = Discourse.redis)
+    redis_lock_key = self.redis_lock_key(key)
     redis_key = self.redis_key(key)
 
-    unless result = redis.get(redis_key)
-      redis_lock_key = self.redis_lock_key(key)
+    DistributedMutex.synchronize(redis_lock_key, redis: redis, validity: MAX_WAIT) do
+      result = redis.get(redis_key)
 
-      start = Time.now
-      got_lock = false
-
-      begin
-        while Time.now < start + MAX_WAIT && !got_lock
-          LOCK.synchronize do
-            got_lock = get_lock(redis, redis_lock_key)
-          end
-          sleep 0.001
-        end
-
-        unless result = redis.get(redis_key)
-          result = yield
-          redis.setex(redis_key, duration, result)
-        end
-
-      ensure
-        # NOTE: delete regardless so next one in does not need to wait MAX_WAIT again
-        redis.del(redis_lock_key)
+      unless result
+        result = yield
+        redis.setex(redis_key, duration, result)
       end
-    end
 
-    result
+      result
+    end
   end
 
   def self.redis_lock_key(key)
-    +"memoize_lock_" << key
+    "memoize_lock_#{key}"
   end
 
   def self.redis_key(key)
-    +"memoize_" << key
-  end
-
-  # Used for testing
-  def self.flush!
-    Discourse.redis.scan_each(match: "memoize_*").each { |key| Discourse.redis.del(key) }
-  end
-
-  protected
-
-  def self.get_lock(redis, redis_lock_key)
-    redis.watch(redis_lock_key)
-    current = redis.get(redis_lock_key)
-    return false if current
-
-    unique = SecureRandom.hex
-
-    result = redis.multi do
-      redis.setex(redis_lock_key, MAX_WAIT, unique)
-    end
-
-    redis.unwatch
-    result == ["OK"]
+    "memoize_#{key}"
   end
 end
diff --git a/spec/lib/distributed_memoizer_spec.rb b/spec/lib/distributed_memoizer_spec.rb
index e875422f65e..256716aeccb 100644
--- a/spec/lib/distributed_memoizer_spec.rb
+++ b/spec/lib/distributed_memoizer_spec.rb
@@ -1,35 +1,25 @@
 # frozen_string_literal: true
 
 describe DistributedMemoizer do
-
-  before do
+  after do
     Discourse.redis.del(DistributedMemoizer.redis_key("hello"))
     Discourse.redis.del(DistributedMemoizer.redis_lock_key("hello"))
-    Discourse.redis.unwatch
   end
 
-  # NOTE we could use a mock redis here, but I think it makes sense to test the real thing
-  # let(:mock_redis) { MockRedis.new }
-
   def memoize(&block)
     DistributedMemoizer.memoize("hello", duration = 120, &block)
   end
 
   it "returns the value of a block" do
-    expect(memoize do
-      "abc"
-    end).to eq("abc")
+    expect(memoize { "abc" }).to eq("abc")
   end
 
   it "return the old value once memoized" do
-
     memoize do
       "abc"
     end
 
-    expect(memoize do
-      "world"
-    end).to eq("abc")
+    expect(memoize { "world" }).to eq("abc")
   end
 
   it "memoizes correctly when used concurrently" do
@@ -48,7 +38,5 @@ describe DistributedMemoizer do
     threads.each(&:join)
     expect(results.uniq.length).to eq(1)
     expect(results.count).to eq(5)
-
   end
-
 end
diff --git a/spec/requests/static_controller_spec.rb b/spec/requests/static_controller_spec.rb
index 21987f2a79a..4f0a926626a 100644
--- a/spec/requests/static_controller_spec.rb
+++ b/spec/requests/static_controller_spec.rb
@@ -11,12 +11,10 @@ describe StaticController do
       UploadCreator.new(file, filename).create_for(Discourse.system_user.id)
     end
 
-    before_all do
-      DistributedMemoizer.flush!
-    end
-
     after do
-      DistributedMemoizer.flush!
+      Discourse.redis.scan_each(match: "memoize_*").each do |key|
+        Discourse.redis.del(key)
+      end
     end
 
     describe 'local store' do