fix: do not absorb a signal while subscribing

A notification arriving after listen released the mutex but before wait snapshotted the counter became the new baseline, and its broadcast reached no registered waiter, so the role slept until its polling interval expired. Snapshot before subscribing and re-check before blocking, so a signal delivered while a waiter was getting ready is observed rather than swallowed.

Author: cardmagic

lib/solid_objects/wake_up_adapters/redis.rb

@@ -49,12 +49,17 @@ def signal
         false
       end
 
+      # The counter is snapshotted before subscribing, and re-checked before
+      # blocking, so a signal delivered while this caller was still getting
+      # ready is observed rather than absorbed into the new baseline.
       # @rbs (timeout: Numeric) -> bool
       def wait(timeout:)
+        signalled = mutex.synchronize { @signalled }
         return paced_failure(timeout) unless listen
 
         mutex.synchronize do
-          signalled = @signalled
+          return true unless @signalled == signalled
+
           condition.wait(mutex, timeout.to_f)
           @signalled != signalled
         end

sig/generated/lib/solid_objects/wake_up_adapters/redis.rbs

@@ -40,6 +40,9 @@ module SolidObjects
       # @rbs () -> bool
       def signal: () -> bool
 
+      # The counter is snapshotted before subscribing, and re-checked before
+      # blocking, so a signal delivered while this caller was still getting
+      # ready is observed rather than absorbed into the new baseline.
       # @rbs (timeout: Numeric) -> bool
       def wait: (timeout: Numeric) -> bool
 

test/integration/redis_wake_up_test.rb

@@ -78,6 +78,25 @@ class RedisWakeUpTest < ActiveSupport::TestCase
     assert subscribers.alive?, "one background subscription should serve the process"
   end
 
+  test "a signal arriving while a waiter is subscribing is not absorbed" do
+    @adapter.listen
+    real = @adapter.method(:listen)
+    # Publish inside listen, modelling a signal that lands after the counter
+    # would have been snapshotted but before the waiter blocks.
+    @adapter.define_singleton_method(:listen) do
+      result = real.call
+      SolidObjects::WakeUpAdapters::Redis.new(url: REDIS_URL).tap(&:signal).stop
+      sleep 0.1
+      result
+    end
+    started = monotonic_now
+
+    woken = @adapter.wait(timeout: 5)
+
+    assert woken, "the signal must not be absorbed by the subscribing waiter"
+    assert_operator monotonic_now - started, :<, 1.0
+  end
+
   test "signalling never raises into the caller" do
     broken = SolidObjects::WakeUpAdapters::Redis.new(url: "redis://127.0.0.1:1/0")