Fix race condition between job enqueue and concurrency unblock

This addresses #456.

There is a race condition in the concurrency control mechanism where a
job that finishes and tries to unblock the next blocked execution can
miss a `BlockedExecution` that is being created concurrently. This
causes the blocked job to remain stuck until the `ConcurrencyMaintenance`
periodic task runs (potentially minutes later).

It happens as follows:

 1. Job A is running (semaphore value=0)

 2. Job B enqueue starts: reads semaphore (value=0, no row lock) →
    decides to block

 3. Job A finishes: `Semaphore.signal` → `UPDATE` value to 1 (succeeds
    immediately since no lock held)

 4. Job A: `BlockedExecution.release_one` → `SELECT` finds nothing (Job B's
    `BlockedExecution` not committed yet)

 5. Job B enqueue commits: `BlockedExecution` now exists but nobody will
    unblock it

The root cause is that `Semaphore::Proxy#wait` doesn't lock the
semaphore row when checking the semaphore. This allows the concurrent
`signal` to complete before the enqueue transaction commits, creating a
window where the `BlockedExecution` is invisible.

To fix, we lock the semaphore row with `FOR UPDATE` during the wait check so that
the enqueue transaction holds the lock from the check through
`BlockedExecution` creation and commit. This forces a concurrent signal
`UPDATE` to wait, guaranteeing the `BlockedExecution` is visible when
release_one runs.

This shouldn't introduce any dead locks, as there's no new circular
dependencies introduced by these two:

- Enqueue path: locks `Semaphore` row → `INSERT`s `BlockedExecution` (no lock
  on existing rows)
- `release_one` path: locks `BlockedExecution` row (`SKIP LOCKED`) → locks
  `Semaphore` row (via wait in release)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: rosa

app/models/solid_queue/semaphore.rb

@@ -40,7 +40,7 @@ def initialize(job)
       end
 
       def wait
-        if semaphore = Semaphore.find_by(key: key)
+        if semaphore = Semaphore.lock.find_by(key: key)
           semaphore.value > 0 && attempt_decrement
         else
           attempt_creation

test/models/solid_queue/semaphore_test.rb

@@ -0,0 +1,119 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+class SolidQueue::SemaphoreTest < ActiveSupport::TestCase
+  self.use_transactional_tests = false
+
+  setup do
+    @result = JobResult.create!(queue_name: "default")
+  end
+
+  test "wait acquires a row lock that blocks concurrent signal" do
+    skip_on_sqlite
+
+    # Enqueue first job to create semaphore with value=0
+    NonOverlappingUpdateResultJob.perform_later(@result)
+    concurrency_key = SolidQueue::Job.last.concurrency_key
+    assert_equal 0, SolidQueue::Semaphore.find_by(key: concurrency_key).value
+
+    lock_held = Concurrent::Event.new
+
+    # Background thread: holds a FOR UPDATE lock on the semaphore row
+    locker = Thread.new do
+      SolidQueue::Record.connection_pool.with_connection do
+        SolidQueue::Record.transaction do
+          SolidQueue::Semaphore.where(key: concurrency_key).lock.first
+          lock_held.set
+          sleep 1
+        end
+      end
+    end
+
+    lock_held.wait(5)
+    sleep 0.1
+
+    # Main thread: this UPDATE should block until the locker's transaction commits
+    start = monotonic_now
+    SolidQueue::Semaphore.where(key: concurrency_key).update_all("value = value + 1")
+    elapsed = monotonic_now - start
+
+    locker.join(5)
+
+    assert elapsed >= 0.5, "UPDATE should have been blocked by FOR UPDATE lock (took #{elapsed.round(3)}s)"
+    assert_equal 1, SolidQueue::Semaphore.find_by(key: concurrency_key).value
+  end
+
+  test "blocked execution created during enqueue is visible to release_one after signal" do
+    skip_on_sqlite
+
+    # Enqueue first job to create semaphore with value=0
+    NonOverlappingUpdateResultJob.perform_later(@result)
+    job_a = SolidQueue::Job.last
+    concurrency_key = job_a.concurrency_key
+    assert_equal 0, SolidQueue::Semaphore.find_by(key: concurrency_key).value
+
+    lock_held = Concurrent::Event.new
+
+    # Background thread: simulates the enqueue path for a second job.
+    # Locks the semaphore row (as the code does), creates a BlockedExecution,
+    # then holds the transaction open to simulate the window where the race occurs.
+    enqueue_thread = Thread.new do
+      SolidQueue::Record.connection_pool.with_connection do
+        SolidQueue::Record.transaction do
+          # Lock the semaphore (same as Semaphore::Proxy#wait)
+          SolidQueue::Semaphore.where(key: concurrency_key).lock.first
+
+          # Create job and blocked execution bypassing after_create callbacks
+          # to avoid re-entering Semaphore.wait
+          uuid = SecureRandom.uuid
+          SolidQueue::Job.insert({
+            queue_name: "default",
+            class_name: "NonOverlappingUpdateResultJob",
+            concurrency_key: concurrency_key,
+            active_job_id: uuid,
+            arguments: "{}",
+            scheduled_at: Time.current
+          })
+          job_b_id = SolidQueue::Job.where(active_job_id: uuid).pick(:id)
+
+          SolidQueue::BlockedExecution.insert({
+            job_id: job_b_id,
+            queue_name: "default",
+            concurrency_key: concurrency_key,
+            expires_at: SolidQueue.default_concurrency_control_period.from_now,
+            priority: 0
+          })
+
+          lock_held.set
+
+          # Hold the transaction open so the signal path must wait
+          sleep 1
+        end
+      end
+    end
+
+    lock_held.wait(5)
+    sleep 0.1
+
+    # Main thread: simulates job_a finishing — signal + release_one.
+    # The signal UPDATE will block until the enqueue transaction commits,
+    # guaranteeing the BlockedExecution is visible to release_one.
+    assert SolidQueue::Semaphore.signal(job_a)
+    assert SolidQueue::BlockedExecution.release_one(concurrency_key),
+      "release_one should find the BlockedExecution created by the concurrent enqueue"
+
+    enqueue_thread.join(5)
+
+    assert_equal 0, SolidQueue::BlockedExecution.where(concurrency_key: concurrency_key).count
+  end
+
+  private
+    def skip_on_sqlite
+      skip "Row-level locking not supported on SQLite" if SolidQueue::Record.connection.adapter_name.downcase.include?("sqlite")
+    end
+
+    def monotonic_now
+      Process.clock_gettime(Process::CLOCK_MONOTONIC)
+    end
+end