Why Rails Parallel Tests Hang Forever on a Postgres Lock

Thu, 22 Oct 2026 00:00:00 +0000

bin/rails test stopped printing and never finished.

Not a failure. Not a stack trace. The log just froze mid-run, and the parent process sat at 0% CPU:

Running 2014 tests in parallel using 10 processes
# Running:

CalculatorsControllerTest#test_each_tool_page_has_a_markdown_twin = 0.04 s = .
CompanyIssue::AccountResultSyncTest#test_names_an_application_it_can_already_identify_without_asking_twice = 0.01 s = .
ConfirmModalFormsTest#test_each_destructive_action_renders_a_hidden_form_the_controller_can_submit = 0.25 s = .
SocialImageGeneratorTest#test_all_templates_produce_images_of_similar_size = 13.75 s = .
SocialImageGeneratorTest#test_renders_valid_PNG_for_each_template = 15.49 s = .

Wait. Nothing else.

Those last tests render social cards in a headless browser, so they take 7–15 seconds — the slowest in the suite. Of course I went looking at that commit.

Testing on Saroj Maharjan

Why Rails Parallel Tests Hang Forever on a Postgres Lock