vquic: drop support for OpenSSL-QUIC

[bagder]

• It is slower and uses more memory than the alternatives and is only experimental in curl.
• We disable a few tests for OpenSSL-QUIC because of flakiness
• It gets little attention from OpenSSL and we have no expectation of the major flaws getting corrected anytime soon.
• No one has spoken up for keeping it
• curl users building with vanilla OpenSSL can still use QUIC through the means of ngtcp2

[Copilot]

Pull request overview

This pull request removes support for OpenSSL-QUIC from curl. OpenSSL-QUIC was an experimental HTTP/3 backend that used OpenSSL's QUIC implementation. The removal is justified due to performance issues, high memory usage, lack of attention from OpenSSL maintainers, and the availability of better alternatives like ngtcp2.

Key Changes:

• Removes 2509 lines of OpenSSL-QUIC implementation code (curl_osslq.c and curl_osslq.h)
• Removes all build system support (CMake, autotools) for --with-openssl-quic option
• Removes OpenSSL-QUIC specific test skips (tests are now expected to run with alternative QUIC implementations)
• Updates documentation to remove OpenSSL-QUIC build instructions and marks it as removed in deprecation documentation
• Updates CI workflows to remove OpenSSL-QUIC specific builds

Reviewed changes

Copilot reviewed 21 out of 21 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
lib/vquic/curl_osslq.c	Entire OpenSSL-QUIC implementation file removed (2458 lines)
lib/vquic/curl_osslq.h	Header file for OpenSSL-QUIC removed (51 lines)
lib/vquic/vquic.c	Removed OpenSSL-QUIC include and version/factory function calls
lib/vtls/openssl.c	Removed USE_OPENSSL_QUIC conditional compilation block
lib/Makefile.inc	Removed curl_osslq.c and curl_osslq.h from build sources
lib/curl_config-cmake.h.in	Removed USE_OPENSSL_QUIC CMake define
m4/curl-openssl.m4	Removed OpenSSL QUIC capability detection code
configure.ac	Removed --with-openssl-quic option and related configuration logic
CMakeLists.txt	Removed USE_OPENSSL_QUIC option and related checks
tests/http/testenv/curl.py	Removed outdated comment about OpenSSL-QUIC connect time issue
tests/http/test_*.py	Removed OpenSSL-QUIC specific test skip conditions (5 files)
docs/HTTP3.md	Removed OpenSSL-QUIC build instructions section (54 lines)
docs/INSTALL-CMAKE.md	Removed USE_OPENSSL_QUIC documentation
docs/DEPRECATE.md	Moved OpenSSL-QUIC from planned removal to removed list
.github/workflows/windows.yml	Removed --with-openssl-quic from test configuration
.github/workflows/macos.yml	Removed -DUSE_OPENSSL_QUIC=ON from test configurations (3 instances)
.github/workflows/http3-linux.yml	Removed openssl-quic test job entries (2 variants)

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[bagder]

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🤖 Augment PR Summary

Summary: This PR removes curl’s experimental OpenSSL-QUIC (OpenSSL’s built-in QUIC stack) HTTP/3 backend due to higher resource usage, poorer performance, flakiness, and low upstream maintenance.

Changes:

• Removes the OpenSSL-QUIC implementation (lib/vquic/curl_osslq.c/.h) from the source tree/build.
• Drops CMake and autotools toggles/flags for it (e.g., `USE_OPENSSL_QUIC`, `--with-openssl-quic`).
• Updates CI workflows to stop building/testing OpenSSL-QUIC variants on Linux/macOS/Windows.
• Updates docs to remove OpenSSL-QUIC build instructions and record the backend as removed.
• Removes OpenSSL-QUIC-specific test skips that existed due to backend flakiness.

Technical Notes: Users building with “vanilla” OpenSSL can still use HTTP/3 via the ngtcp2+nghttp3 backend.

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