fix: return 502/504 instead of 204 on navigation failure

[kitsuyui]

Summary

When page.goto() throws a network error (DNS failure, connection refused, TLS error, etc.), the proxy was silently returning HTTP 204 No Content. This is misleading: 204 means the server explicitly sent "no content", but a thrown error means the upstream was unreachable.

This PR changes the behaviour:

• TimeoutError from Playwright → 504 Gateway Timeout
• Other network errors (DNS, connection refused, TLS) → 502 Bad Gateway

Distinguishing errors from valid 204 responses

page.goto() also throws when the server legitimately returns 204 No Content, because Playwright aborts the document load when there is no HTML body to parse. A naive error→502 mapping would break the existing 204 pass-through.

To solve this, a response event listener captures the main-frame HTTP response before page.goto() throws. If a response was captured, the server did reply — Playwright simply could not load a document from it — so the captured response is forwarded as-is. Only when no response was captured at all (genuine connection failure) does the proxy return 502/504.

Changes

• src/render/index.ts:

  • captureNavigationResponse() — attaches a response event listener and exposes the captured main-frame response.
  • classifyNavigationError() — maps a caught error to 502 or 504.
  • tryGoto() — wraps page.goto() in a try/catch using the helpers above.
  • navigatePage() — orchestrates the capture and classifies the outcome.
  • hasNoBody() / readRenderedBody() — skip response.body() for status codes that carry no body by spec (204, 304, 1xx).
  • errorRenderResult() — builds an empty error response for 502/504.

• src/render/index.spec.ts: Add a test that navigates to http://127.0.0.1:19999/ (nothing listening) and expects HTTP 502.

Verification

All 36 tests pass, including the pre-existing "can handle empty response" test (204 pass-through) and the new 502 test.

Trade-offs

The response-event approach is more robust than inspecting error message strings, but it does add a response listener per request. The listener is always detached before the function returns, so there is no leak across requests.

[github-actions]

gh-counter

PR gate

	Removed	Added	+/-
TODO/expect-error	0	0	0

Repo dashboard

	main (b5aa171)	#1598 (0bb7cdc)	+/-
TODO/expect-error	4	4	0

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Reported by gh-counter

[github-actions]

Code Metrics Report

	main (b5aa171)	#1598 (0bb7cdc)	+/-
Coverage	87.5%	87.9%	+0.3%
Code to Test Ratio	1:0.5	1:0.5	-0.1
Test Execution Time	6s	7s	+1s

Details

  |                     | main (b5aa171) | #1598 (0bb7cdc) |  +/-  |
  |---------------------|----------------|-----------------|-------|
+ | Coverage            |          87.5% |           87.9% | +0.3% |
  |   Files             |             11 |              11 |     0 |
  |   Lines             |            177 |             191 |   +14 |
+ |   Covered           |            155 |             168 |   +13 |
- | Code to Test Ratio  |          1:0.5 |           1:0.5 |  -0.1 |
  |   Code              |           1428 |            1491 |   +63 |
+ |   Test              |            819 |             828 |    +9 |
- | Test Execution Time |             6s |              7s |   +1s |

Code coverage of files in pull request scope (100.0% → 97.5%)

Files	Coverage	+/-	Status
src/render/index.ts	97.5%	-2.5%	modified

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Reported by octocov