[ceritium]

When Solid Queue shares the app's database (no connects_to) and the enqueue isn't deferred to after commit, enqueueing a due job with limits_concurrency acquires its semaphore (SELECT ... FOR UPDATE or insert) inside the caller's transaction. The row lock lasts until that transaction ends, so:

1. Another enqueue with the same key waits for it.
2. A worker finishing a job with that key waits to signal the semaphore.
3. Two transactions enqueueing two keys in opposite order deadlock, raised as SolidQueue::Job::EnqueueError and aborting the caller's transaction.

Reproduction against 1.7.0 (Rails 8.1.4, PostgreSQL 17), single file: https://gist.github.com/ceritium/8d733493fef1c18f8f143fb8a2144c60
With transactions held 3 s, (1) and (2) wait ~2.5 s and (3) deadlocks. FORK=1 runs this branch, where none of them happens.

The change

A job enqueued with a wait: that isn't due yet already avoids this: it's inserted as scheduled and the dispatcher acquires the lock later, in its own short transaction. This makes that path available without a delay, through an option:

• :on_enqueue (default): current behaviour.
• :on_dispatch: insert the job as scheduled and already due; the dispatcher acquires the lock on its next poll.
• :outside_transaction: :on_dispatch when a transaction is open on Solid Queue's connection, :on_enqueue otherwise.

config.solid_queue.concurrency_lock_acquisition = :outside_transaction
limits_concurrency key: ->(record) { record }, acquire: :on_dispatch
MyJob.set(acquire: :on_enqueue).perform_later(record)

The job row is still written in the caller's transaction; only the semaphore moves out. Trade-offs, documented in the README: up to polling_interval of extra latency, a dispatcher is required, and on_conflict: :discard happens at dispatch (as for scheduled jobs).

Related: rails#812 fixes a deadlock between dispatchers on that same path.

Tests: model tests for each mode, precedence and bulk enqueue, and an integration test per effect above showing it with :on_enqueue and not with :outside_transaction (skipped on SQLite). Full suite green on PostgreSQL; MySQL not run.