sphinx-quickstart: Enter should exit when existing conf.py is detected

[rowan-stein]

User Request

Bug: sphinx-quickstart with existing conf.py doesn't exit easily.

• Running sphinx-quickstart in a folder with a conf.py already existing.
• It says "Please enter a new root path name (or just Enter to exit)".
• Pressing Enter returns an error: "Please enter a valid path name" (the prompt does not exit).

To Reproduce:

$ sphinx-quickstart
$ sphinx-quickstart

Expected behavior:

• After pressing Enter, sphinx-quickstart exits.

Environment:

• OS: Ubuntu 20.04
• Python: 3.8.5
• Sphinx: 3.2.1

Researcher Specification

Current behavior and root cause:

• In sphinx/cmd/quickstart.py (interactive flow ask_user), when an existing conf.py is found in the selected root path (<root>/conf.py or <root>/source/conf.py), quickstart prints an error and prompts:
  • "Please enter a new root path (or just Enter to exit)"
• The prompt uses do_prompt(..., default='', validator=is_path). An empty input is rejected by is_path('') with the message "Please enter a valid path name.", causing a loop. Therefore, pressing Enter never exits.

Expected behavior:

• At the above prompt, pressing Enter should exit immediately with a non-zero status (exit code 1), without further prompting.
• Suggested exit message: "Found existing conf.py — aborting."
• Quiet mode (-q) behavior remains unchanged (return 1 if unsuitable path; no prompt).

Edge cases (left unchanged unless noted):

• Detection remains limited to <root>/conf.py and <root>/source/conf.py.
• Non-writable paths and non-Sphinx files are handled as today.
• Symlinked conf.py is treated as existing.

Test plan:

• Interactive: with <root>/conf.py present, run sphinx-quickstart, press Enter at the prompt → exit code 1; message includes error and abort text.
• Interactive: with <root>/source/conf.py present → same.
• Interactive: provide a valid new path → proceed to generate in the new path; no writes to original dir.
• Quiet mode: sphinx-quickstart -q ... <root> with conf.py → exit code 1 and existing messages.

Implementation outline (no code):

• In ask_user, change the prompt handling to accept empty input (Enter) and exit with code 1 when empty; otherwise, validate the provided path (directory) and continue.
• Add or adjust tests in tests/test_quickstart.py to cover the above scenarios.

Acceptance criteria:

• Interactive Enter exits with code 1 and prints abort message when conf.py exists.
• Quiet mode unchanged.
• Valid new path generation unaffected.