ci: move macOS jobs to macos-15 and keep botan@2 installable

.github/homebrew/botan@2.rb

@@ -0,0 +1,80 @@
+# Copied from Homebrew/homebrew-core at commit c0c9ff1d10f9b975f2d3fc2385c042a579749815
+# (Formula/b/botan@2.rb) with its deprecate!/disable! lines removed, so that it
+# still installs after Homebrew removes botan@2. Used by .github/workflows/ci.yml.
+#
+# BSD 2-Clause License
+#
+# Copyright (c) 2009-present, Homebrew contributors
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are met:
+#
+# * Redistributions of source code must retain the above copyright notice, this
+#   list of conditions and the following disclaimer.
+#
+# * Redistributions in binary form must reproduce the above copyright notice,
+#   this list of conditions and the following disclaimer in the documentation
+#   and/or other materials provided with the distribution.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+# AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+# DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+# FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+# SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+# CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+# OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+class BotanAT2 < Formula
+  desc "Cryptographic algorithms and formats library in C++"
+  homepage "https://botan.randombit.net/"
+  url "https://botan.randombit.net/releases/Botan-2.19.5.tar.xz"
+  sha256 "dfeea0e0a6f26d6724c4af01da9a7b88487adb2d81ba7c72fcaf52db522c9ad4"
+  license "BSD-2-Clause"
+
+  bottle do
+    rebuild 2
+    sha256 arm64_tahoe:   "dd852d6fe3cc65e68fe497d5582c7f3dbaa4c2afced2acea3a9b0f412042bd82"
+    sha256 arm64_sequoia: "3b52c4abedf724426066b343384e553b586d7a3d69e437a85839ad1adde27868"
+    sha256 arm64_sonoma:  "dc8d1646e46dab1be1f4f53b46f2e111b92610998ec700579c702abe3bee9baf"
+    sha256 sonoma:        "534d9389e8a96ed9c171e1c457c931f70783c94515ff502c6fd876c369666f09"
+    sha256 arm64_linux:   "faf67b146845389405939b4803884a521fd4210e441b078fea28cdeca78e78ac"
+    sha256 x86_64_linux:  "a4d30c4277c84df5b6a0e51b82e049df3bbe90204151143bf68bf214f44cbd86"
+  end
+
+  keg_only :versioned_formula
+
+  # Botan2 is currently scheduled to reach end of life at the end of 2024
+  # Ref: https://botan.randombit.net/#releases
+
+  depends_on "pkgconf" => :build
+  depends_on "python@3.14"
+  depends_on "sqlite"
+
+  uses_from_macos "bzip2"
+  uses_from_macos "zlib"
+
+  def install
+    ENV.cxx11
+
+    args = %W[
+      --prefix=#{prefix}
+      --docdir=share/doc
+      --with-zlib
+      --with-bzip2
+      --with-sqlite3
+    ]
+
+    system python3, "configure.py", *args
+    system "make", "install"
+  end
+
+  test do
+    (testpath/"test.txt").write "Homebrew"
+    (testpath/"testout.txt").write shell_output("#{bin}/botan base64_enc test.txt")
+    assert_match "Homebrew", shell_output("#{bin}/botan base64_dec testout.txt")
+  end
+end

.github/workflows/ci.yml

@@ -129,7 +129,7 @@ jobs:
 
   macos:
     name: macOS (${{ matrix.backend }})
-    runs-on: macos-14
+    runs-on: macos-15
     strategy:
       fail-fast: false
       matrix:
@@ -142,7 +142,13 @@ jobs:
       - uses: actions/checkout@v6
       - name: Prepare
         run: |
-          brew install automake libtool cppunit botan@2
+          brew install automake libtool cppunit
+          # Homebrew disabled botan@2 on 2025-12-31 and removes disabled formulae one year
+          # after that date. Install it from a local tap holding .github/homebrew/botan@2.rb,
+          # the homebrew-core formula pinned to a commit and without its deprecate!/disable! lines.
+          brew tap-new --no-git local/pins
+          cp .github/homebrew/botan@2.rb "$(brew --repository local/pins)/Formula/"
+          brew install local/pins/botan@2
       - name: Build
         env:
           CXXFLAGS: -Werror -DBOTAN_NO_DEPRECATED_WARNINGS