Fix race condition in spawn error handling and extract FIFO functionality

[Copilot]

Command spawn errors resolved instead of rejecting in separate output mode due to event handler registration order. The setupSignalHandlers error handler fired first and resolved with exitCode: 0 before the main error handler could reject.

Changes

Event handler ordering (src/main.ts)

• Register main error and close handlers before setupSignalHandlers in separate output mode
• Prevents setupSignalHandlers' cleanup from settling the promise before error propagation

// Register error handler BEFORE setupSignalHandlers to avoid race condition
child.on('error', (error: Error) => {
  if (!settled) {
    settled = true
    reject(error)
  }
})

child.on('close', (code: number | null) => { /* ... */ })

// setupSignalHandlers now runs AFTER main handlers
setupSignalHandlers(child, () => { /* cleanup */ })

FIFO functionality extraction (src/fifo-stream.ts)

• Extracted FIFO pipe management to new FifoPipe class in separate module
• Clean API: open(), getOutput(), waitForCompletion(), close()
• Separates concerns: FIFO logic vs command execution
• Main.ts now uses FifoPipe to create pipe, then spawns child with the file descriptor
• Comprehensive unit tests added in __tests__/fifo-stream.test.ts (15 tests)
• FIFO module coverage: 84% statements, 73% branches, 93% functions

Test refactoring (tests/main.test.ts, tests/fifo-stream.test.ts)

• Split matrix tests into separate combined/separate mode suites to eliminate conditional expects
• Removes jest/no-conditional-expect lint violations
• Added 15 comprehensive FIFO-specific tests covering edge cases and error handling

Documentation (.github/copilot-instructions.md, eslint.config.mjs)

• Document that lint errors block PR merge
• Added super-linter Docker command instructions for local markdown/shell linting
• Fixed markdown lint errors (capitalization: Jest, TypeScript)
• Increase ESLint file match limit to handle test files

Test Results

76 tests now pass consistently (was 61). Previously skipped test "rejects when command not found" passes in both modes.

Overall Coverage: 89.29% (improved from 83.67%)

• Statements: 89.29%
• Branches: 83.48%
• Functions: 90.24%
• Lines: 89.09%

Original prompt

Support combining the executed command's stdout and stderr together.

When this is enabled:

• The same fd should be passed to both the child process's stdout and stderr file handles.
• The functionality for printing output in realtime should be run only once on that fd, and it should output to stdout.
• The combined output should go to an output named combined_output and the stdout and stderr outputs should be absent.

Enable this behavior by default, adding a separate_outputs boolean input that is false by default.

Ensure all examples reflect the new defaults, and add an example demonstrating the opt-in separate_outputs behavior.

---

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[eriksw]

@copilot The lint fails with findings in main.ts. This should have been run by the agent. Identify why lint was not run and fix the copilot instructions to ensure it is run before making the pull request.

All jobs in the signal handling test workflow failed because they were not updated. Update copilot instructions to ensure that the workflow is updated when changes that might affect the tests in the signal handling test workflow.

This PR reduces test coverage substantially. Ensure coverage is as good as possible.

The It is okay to swallow errors unlinking the fifo while handling unrelated, but errors unlinking it when it is supposed to be unlinked should still be fatal.

The executeCommand function has far too many responsibilities and is far too long. Consider extracting the pipe-creating functionality to another module.

Review the e2e tests and simplify away any redundant tests that do not improve code coverage.

[Copilot]

@copilot The lint fails with findings in main.ts. This should have been run by the agent. Identify why lint was not run and fix the copilot instructions to ensure it is run before making the pull requ...

Fixed in commit 9924cba:

Lint errors: Fixed all 4 unused variable errors in main.ts by removing unused error parameters in catch blocks.

Why lint wasn't run: The copilot instructions mentioned lint should pass (line 109) but didn't explicitly require running npm run lint after code changes. Updated instructions to:

• Add dedicated "Linting and Formatting" section requiring lint checks before bundling
• Add explicit reminder to run lint after code changes (line 85)
• Updated PR guidelines to emphasize running lint commands

Signal-test workflow: Updated all 4 test jobs to add separate_outputs: true since they expect separate stdout/stderr outputs.

Still addressing: extracting pipe functionality, simplifying e2e tests, improving coverage, and handling FIFO unlink errors appropriately.

[eriksw]

@copilot There are NATURAL_LANGUAGE lint errors in markdown files that must be fixed.

Incorporate the following into the instructions:

- **Before updating any pull request**, if there are changes to Markdown or
  shell files between the base branch and current state, run the super-linter
  Docker image exactly as the CI workflow does:

  ```bash
  docker run --rm \
    -e RUN_LOCAL=true \
    -e USE_FIND_ALGORITHM=true \
    -e CHECKOV_FILE_NAME=.checkov.yml \
    -e FILTER_REGEX_EXCLUDE="dist/**/*" \
    -e LINTER_RULES_PATH=. \
    -e VALIDATE_ALL_CODEBASE=true \
    -e VALIDATE_BIOME_FORMAT=false \
    -e VALIDATE_BIOME_LINT=false \
    -e VALIDATE_GITHUB_ACTIONS_ZIZMOR=false \
    -e VALIDATE_JAVASCRIPT_ES=false \
    -e VALIDATE_JSCPD=false \
    -e VALIDATE_TYPESCRIPT_ES=false \
    -e VALIDATE_JSON=false \
    -v "$(pwd)":/tmp/lint:ro \
    --workdir=/tmp/lint \
    ghcr.io/super-linter/super-linter:slim-v8
  ```

  This command uses all environment variables from the linter workflow
  (`.github/workflows/linter.yml`) except `GITHUB_TOKEN` (not needed for local
  runs) and `DEFAULT_BRANCH` (not compatible with `USE_FIND_ALGORITHM=true`).
  The `RUN_LOCAL=true` flag enables local execution mode. This ensures linting
  is performed using the same versions and configurations as CI.

  **Important:** Always check this command against
  `.github/workflows/linter.yml` and update it whenever there are changes to the
  linter workflow to ensure it stays synchronized with the CI configuration.

Next, and importantly: the main.ts is too large to review and maintain effectively. In particular, executeCommand is too long. Extract the code relating fifo-based pipe functionality to a different module. If practical, consider abstracting the fifo-related code into a class that can be treated like the child.stdout / child.stderr of a normal spawn() with pipe stdout/stderr.

[Copilot]

@copilot There are NATURAL_LANGUAGE lint errors in markdown files that must be fixed.

Incorporate the following into the instructions:
...

Fixed in commit b5766da:

Markdown lint errors: Fixed terminology (jest → Jest, typescript → TypeScript).

Super-linter instructions: Added to .github/copilot-instructions.md with complete Docker command synchronized with CI workflow.

FIFO code extracted: Created src/fifo-stream.ts module (213 lines) with FifoPipe class. Main.ts now uses FifoPipe to create pipe and get FD, then spawns child. No command execution in FIFO module.

FIFO tests: Added __tests__/fifo-stream.test.ts with 15 comprehensive tests. FIFO module coverage: 84% statements, 74% branches, 93% functions.

Overall: 76 tests passing (was 61), 89.29% coverage (was 83.67%), 0 lint errors, all TypeScript modules.

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[eriksw]

Investigate using this approach to create a pipe the same way spawn does, instead of using the fifo module.:

const { Pipe, constants: PipeConstants } = internalBinding('pipe_wrap');