Serialize runtime SQLite writes

[mjc]

Summary

• add a single in-process DbWriter to serialize runtime SQLite mutations
• route runtime write-heavy paths through the writer and simplify webhook processing
• reduce pool size and busy timeout defaults, and add focused DbWriter tests

Validation

• nix develop -c mix compile
• nix develop -c mix test test/reencodarr/db_writer_test.exs test/reencodarr/media/video_upsert_test.exs test/reencodarr/core/retry_test.exs

[Copilot]

Pull request overview

This PR introduces a global in-process database writer to serialize SQLite write operations at runtime, reducing lock contention and simplifying previously write-serialization logic in webhook processing.

Changes:

• Add Reencodarr.DbWriter (GenServer) to serialize runtime SQLite mutations and apply retry behavior centrally.
• Route multiple write-heavy code paths (Media, VideoUpsert, VideoStateMachine, Services, VideoFailure, webhooks) through DbWriter.
• Adjust SQLite/pool tuning (smaller repo pool, reduced busy timeout) and add focused DbWriter tests.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
lib/reencodarr/db_writer.ex	New GenServer-based writer that serializes write execution and wraps writes with retry logic.
test/reencodarr/db_writer_test.exs	Adds tests covering inline behavior in test env, nesting, enqueue, and transaction semantics.
lib/reencodarr_web/webhook_processor.ex	Simplifies webhook processing to enqueue work onto the global DbWriter.
lib/reencodarr/media.ex	Routes many write paths through DbWriter and adds labeled wrappers for writes/transactions.
lib/reencodarr/media/video_upsert.ex	Wraps upsert/batch_upsert in DbWriter and removes per-transaction retry helper usage.
lib/reencodarr/media/video_state_machine.ex	Routes Repo updates through DbWriter instead of direct retry helper.
lib/reencodarr/media/video_queries.ex	Wraps “claim for analysis” state transition in DbWriter transaction.
lib/reencodarr/media/video_failure.ex	Routes failure insert/update through DbWriter.
lib/reencodarr/services.ex	Routes config create/update/delete through DbWriter.
lib/reencodarr/sync.ex	Removes explicit Repo transactions around some upsert flows (now handled elsewhere).
lib/reencodarr/core/retry.ex	Tweaks transient SQLite retry log message format.
lib/reencodarr/application.ex	Adds Reencodarr.DbWriter to supervision tree.
config/config.exs	Reduces SQLite busy timeout default now that writes are serialized.
config/dev.exs / config/runtime.exs	Reduces repo pool size defaults to reflect serialized writes.

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[Copilot]

Pull request overview

Copilot reviewed 15 out of 15 changed files in this pull request and generated 6 comments.

Comments suppressed due to low confidence (1)

lib/reencodarr_web/webhook_processor.ex:12

• WebhookProcessor is still started under supervision as a GenServer, but it no longer queues/handles any messages (it just delegates to DbWriter.enqueue/2). This leaves an extra always-idle process and can be misleading to future maintainers. Consider converting this to a plain module (or keep the process only if you need it for compatibility/health checks) and update the supervision tree accordingly.

  use GenServer

  alias Reencodarr.DbWriter

  def start_link(opts) do
    GenServer.start_link(__MODULE__, opts, name: __MODULE__)
  end

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[Copilot]

Pull request overview

Copilot reviewed 14 out of 14 changed files in this pull request and generated 2 comments.

Comments suppressed due to low confidence (1)

lib/reencodarr_web/webhook_processor.ex:12

• WebhookProcessor no longer handles any GenServer messages (it just forwards to DbWriter.enqueue/2), but it is still started as a supervised GenServer and keeps an idle process around. Consider converting this to a plain module (remove use GenServer, start_link/1, and the child entry) or, if it must remain a child for compatibility, document why it needs to run and/or add minimal callbacks to justify the process.

  use GenServer

  alias Reencodarr.DbWriter

  def start_link(opts) do
    GenServer.start_link(__MODULE__, opts, name: __MODULE__)
  end

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[Copilot]

Application.put_env(:reencodarr, :env, :dev) mutates global application config and can leak into other tests running concurrently (this repo has many async: true tests). This can make the suite flaky. Prefer a test-only switch that forces non-inline mode without touching global env (e.g., an option like DbWriter.run(..., inline?: false) / mode: :writer) or isolate the behavior behind dedicated test config instead of runtime mutation.

[Copilot]

This test also changes :reencodarr, :env globally. Even with async: false for this module, ExUnit can still run other async tests in parallel, so this can cause cross-test interference. Consider avoiding Application.put_env/3 here by injecting a writer/inline mode override into DbWriter.enqueue/2 for tests.