curl_get_line: error out on read errors

[bagder]

Missing ferror handling in Curl_get_line causes infinite loops on I/O errors, leading to denial-of-service hangs for config/cache file loads.

Follow-up to 769ccb4

Pointed out by Codex Security

[Copilot]

Pull request overview

Fixes a read-error hang in Curl_get_line() by returning an error when fgets() fails due to an underlying I/O error, preventing infinite loops during config/cache file loading.

Changes:

• Detect fgets() returning NULL with ferror() set and return CURLE_READ_ERROR.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

You can also share your feedback on Copilot code review. Take the survey.

[vszakats]

Seems to be a reboot of #20826 and may make #20873 unnecessary,
if this approach works as expected.

Would be interesting to revert #20873 and see if tests keep passing.

[testclutch]

Analysis of PR #20958 at e9f3f889:

Test ../../tests/http/test_05_errors.py::TestErrors::test_05_01_partial_1[h3] failed, but it has been 1.8% flaky lately, so it's probably NOT a fault of the PR. Note that this test has failed in 2 different CI jobs (the link just goes to one of them).

Generated by Testclutch

[bagder]

Would be interesting to revert #20873 and see if tests keep passing.

The previous problem was related to opening and reading a directory instead of a file, which probably is caught by this on most platforms but as noted before not on (Net)BSD - so we still need something like that.