Require libfyaml 0.9 and build it from source in CI

The libfyaml-dev package on Ubuntu is 0.8, which segfaults psych's
emitter, while 0.9.6 (used on macOS via Homebrew) passes the whole suite.
Build the same 0.9.6 release from source in the Linux CI job, and reject
libfyaml older than 0.9 in extconf so users get a clear error instead of
a runtime crash. Also drop an unused variable in set_canonical.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: hsbt

.github/workflows/libfyaml.yml

@@ -26,14 +26,28 @@ jobs:
         ruby: ${{ fromJson(needs.ruby-versions.outputs.versions) }}
         os: [ ubuntu-latest, macos-latest ]
 
+    env:
+      LIBFYAML_VERSION: "0.9.6"
+
     steps:
       - uses: actions/checkout@v7.0.0
       - name: Set up Ruby and libfyaml
         uses: ruby/setup-ruby-pkgs@v1
         with:
           ruby-version: ${{ matrix.ruby }}
-          apt-get: "libfyaml-dev pkg-config"
+          apt-get: "pkg-config"
           brew: "libfyaml pkg-config"
+      - name: Build libfyaml from source
+        # The libfyaml-dev package on Ubuntu is 0.8, which crashes the emitter,
+        # so build a known-good release. macOS uses the Homebrew build above.
+        if: runner.os == 'Linux'
+        run: |
+          curl -fsSL "https://github.com/pantoniou/libfyaml/releases/download/v${LIBFYAML_VERSION}/libfyaml-${LIBFYAML_VERSION}.tar.gz" | tar xz
+          cd "libfyaml-${LIBFYAML_VERSION}"
+          ./configure --prefix=/usr/local
+          make -j"$(nproc)"
+          sudo make install
+          sudo ldconfig
       - name: Install dependencies
         run: bundle install --jobs 3
       - name: Compile with the libfyaml backend

ext/psych/extconf.rb

@@ -12,6 +12,12 @@
   unless pkg_config('libfyaml')
     abort "libfyaml was requested with --enable-libfyaml but was not found via pkg-config"
   end
+  # libfyaml 0.8 and earlier crash psych's emitter, so require a known-good
+  # version rather than building something that segfaults at runtime.
+  pkgconfig = ENV["PKG_CONFIG"] || "pkg-config"
+  unless system(pkgconfig, "--atleast-version=0.9", "libfyaml")
+    abort "The libfyaml backend requires libfyaml 0.9 or newer"
+  end
   $defs << "-DPSYCH_USE_LIBFYAML"
 
   create_makefile 'psych'

ext/psych/psych_emitter_fy.c

@@ -408,8 +408,6 @@ static VALUE alias(VALUE self, VALUE anchor)
 
 static VALUE set_canonical(VALUE self, VALUE style)
 {
-    psych_fy_emitter_t *e;
-    TypedData_Get_Struct(self, psych_fy_emitter_t, &psych_emitter_type, e);
     /* libfyaml has no canonical emit mode, so reject enabling it rather than
      * pretending to honor the request. */
     if (RTEST(style)) {