feat: build with LuaJIT 2.1 and test against latest C++ server-side SDK

[kinyoklion]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Related issues

Supersedes #123 (which only updated Redis Source to 2.2.2).

Describe the solution you've provided

Builds and tests against current toolchain and SDK versions, without changing what this SDK requires:

• launchdarkly-server-sdk-redis.c: removed the static luaL_setfuncs Lua 5.1 shim. It was never called, and LuaJIT 2.1's lauxlib.h declares luaL_setfuncs non-static, so the shim broke compilation there (static declaration of 'luaL_setfuncs' follows non-static declaration). launchdarkly-server-sdk.c has its own correctly-named ld_luaL_setfuncs and is unaffected.
• CI LuaJIT matrix luajit-2.0.5 → luajit-2.1. luarocks/gh-actions-lua clones --branch v2.0.5 from luajit/luajit and that branch/tag no longer exists upstream (only v2.0, v2.1, master remain), so those jobs failed at the Install Lua step. luajit-2.1 is LuaJIT's current rolling release. README's LuaJIT compatibility note updated to match.
• CI / example build versions: C++ Server-side SDK 3.10.1 → 3.13.1, Redis Source 2.1.19 → 2.4.1 (both current latest).
• The README minimum-version table stays at 3.9.0 / 2.1.19 — the Lua SDK still works with those; working with the latest is not the same as requiring it.

Files changed:

• launchdarkly-server-sdk-redis.c
• .github/variables/cpp-sdk-versions.env — central version config used by CI
• examples/hello-haproxy/Dockerfile, examples/hello-nginx/Dockerfile, examples/hello-debian/Dockerfile — default CPP_SDK_VERSION ARG for local builds
• .github/workflows/ci.yml, .github/workflows/install-lua-sdk.yml — LuaJIT version built and tested
• README.md — LuaJIT compatibility note

How to test it

CI builds and tests both rockspecs on Lua 5.1/5.2/5.3 and LuaJIT 2.1 against the C++ SDK 3.13.1 / Redis Source 2.4.1 artifacts, and builds the hello-haproxy, hello-nginx, and hello-debian example images. Also verified locally on Lua 5.1: luarocks make of both rockspecs against the prebuilt redis-source v2.4.1 linux-gcc-x64-dynamic artifact, test.lua 20/20 passing, and examples/hello-lua-server/hello.lua run against a real server-side SDK key (client init, data sync, flag evaluation).

Additional context

No deprecated APIs are used by the examples (clientInit, makeContext, boolVariation), and the C++ 3.13.1 bindings compile without warnings.

Link to Devin session: https://app.devin.ai/sessions/85ca4fcc915e4e38bdee004d3d493824
Requested by: @kinyoklion

Link to Devin session: https://app.devin.ai/sessions/1a367c31818a4fa6a1d33eb65a7b2331
Requested by: @jsonbailey

---

Note

Overview
CI and toolchain alignment for current LuaJIT and C++ artifacts: the matrix and install workflow now use LuaJIT 2.1 instead of luajit-2.0.5 (upstream tag/branch no longer available), and .github/variables/cpp-sdk-versions.env plus example Dockerfiles default to C++ server SDK 3.13.1 and Redis Source 2.4.1. The README’s supported LuaJIT line is updated to 2.1.

LuaJIT compile fix in launchdarkly-server-sdk-redis.c: drops an unused static luaL_setfuncs Lua 5.1 shim that conflicted with LuaJIT 2.1’s non-static declaration in lauxlib.h. The main SDK module keeps its separate ld_luaL_setfuncs helper unchanged.

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[devin-ai-integration]

Refreshed this PR to the current latest releases: C++ SDK 3.13.0 and Redis Source 2.4.0 (both published 2026-07-24), replacing the now-stale 3.12.0/2.3.1.

Verified locally on Ubuntu 22.04 / Lua 5.3 against the prebuilt launchdarkly-cpp-server-redis-source-v2.4.0 linux-gcc-x64-dynamic artifact: both rockspecs compile warning-free, test.lua passes 20/20, and examples/hello-lua-server/hello.lua initializes and evaluates against a live SDK key.

Heads up on CI: the two linux-build (luajit-2.0.5, ...) jobs fail before reaching this PR's changes, in the luarocks/gh-actions-lua install step:

git clone --branch v2.0.5 --single-branch https://github.com/luajit/luajit.git
fatal: Remote branch v2.0.5 not found in upstream origin

The luajit/luajit mirror no longer publishes a v2.0.5 branch (or any tags) — it currently only has master, v2.0, and v2.1 branches. This is an upstream change in the LuaJIT mirror, not related to the C++ SDK bump, and it will affect every run of this workflow until the luajit-2.0.5 matrix entry (or the action) is adjusted. All other checks, including the Lua 5.1/5.2/5.3 builds and the hello-haproxy/hello-nginx/hello-debian example builds, pass with 3.13.0/2.4.0. I've left the matrix alone since changing the tested LuaJIT version is a maintainer decision.

@cursor review