Disable parallelization when running a single test via line filter #57172

Problem

When running a single test using a line filter:

bin/rails test test/controllers/foo_test.rb:24

Rails still enables parallelization if the test file contains more than the configured threshold (default: 50 tests).

This happens because the decision to enable parallelization is based on the total number of test methods in the test class, rather than the number of tests that will actually be executed after filtering (I think, not sure).

As a result:

• A single test run still boots multiple workers
• Multiple test databases are created (e.g. test-0, test-1, ...)
• There is unnecessary overhead for setup and teardown
• Debugging becomes slower and more complex

Expected behavior

When the test run is effectively targeting a single test (e.g. via file:line), parallelization should be disabled automatically.

Current behavior

Parallelization is enabled if the total number of test methods in the test class exceeds the configured threshold, regardless of runtime filtering.

Proposal

Detect when the test run is scoped to a single test (e.g. via line filter) and force:

parallelize(workers: 1)

This could be implemented by:

• Inspecting the test filter (line-based or name-based)
• Determining the number of runnable test methods after filtering
• Disabling parallelization when only one test is selected

Example

Given a test file with 100 test methods:

bin/rails test test/models/user_test.rb:42

Current behavior:

• Parallelization enabled
• Multiple workers spawned
• Multiple test databases created

Proposed behavior:

• Parallelization disabled
• Single worker
• Faster execution and simpler debugging

Notes

• This change would improve developer experience without affecting full test suite runs
• It aligns better with user expectations when targeting a single test
• Similar behavior is commonly implemented in custom test runners or wrappers

Alternatives considered

• Using environment variables (PARALLEL_WORKERS=1)
• Wrapper scripts to detect :line usage

These approaches work but require manual intervention and are not discoverable.