CPack/AppImage: Derive CPACK_APPIMAGE_ARCH from the target processor

cpack only knows the host CMAKE_SYSTEM_PROCESSOR, so cross-compiled
AppImages got the host architecture.  Set the default in the CPack
module instead, where the project's target processor is known.

Closes: #27556

Help/cpack_gen/appimage.rst

@@ -190,13 +190,19 @@ Variables Specific to CPack AppImage Generator
 
   .. versionadded:: 4.6
 
-  Needs to be set whenever :variable:`CMAKE_SYSTEM_PROCESSOR` is not transformed
-  to a proper architecture and ``appimagetool`` cannot automatically determine the
-  architecture of the binaries inside the AppDir to choose a suitable runtime
-  (e.g., when binaries for multiple architectures or just shell scripts are
-  contained in there).
+  Architecture of the AppImage, passed to ``appimagetool`` in the ``ARCH``
+  environment variable.  The :module:`CPack` module derives it from the
+  :variable:`CMAKE_SYSTEM_PROCESSOR` of the project, so it matches the target
+  when cross-compiling.  The supported values are ``x86_64``, ``i686``,
+  ``aarch64``, and ``armhf``.
 
-  :Default: CMAKE_SYSTEM_PROCESSOR
+  It needs to be set when the processor cannot be mapped to one of those
+  values and ``appimagetool`` cannot automatically determine the architecture
+  of the binaries inside the AppDir to choose a suitable runtime (e.g., when
+  binaries for multiple architectures or just shell scripts are contained in
+  there).
+
+  :Default: Derived from :variable:`CMAKE_SYSTEM_PROCESSOR` on Linux
 
 .. variable:: CPACK_APPIMAGE_VERSION
 

Modules/CPack.cmake

@@ -930,6 +930,19 @@ elseif(CMAKE_SIZEOF_VOID_P EQUAL 8)
   _cpack_set_default(CPACK_INNOSETUP_ARCHITECTURE "x64")
 endif()
 
+# AppImage specific variables
+if(CMAKE_SYSTEM_NAME STREQUAL "Linux")
+  if(CMAKE_SYSTEM_PROCESSOR MATCHES "^(x86_64|amd64)$")
+    _cpack_set_default(CPACK_APPIMAGE_ARCH "x86_64")
+  elseif(CMAKE_SYSTEM_PROCESSOR MATCHES "^(i386|i686|x86)$")
+    _cpack_set_default(CPACK_APPIMAGE_ARCH "i686")
+  elseif(CMAKE_SYSTEM_PROCESSOR MATCHES "^(aarch64|arm64)$")
+    _cpack_set_default(CPACK_APPIMAGE_ARCH "aarch64")
+  elseif(CMAKE_SYSTEM_PROCESSOR MATCHES "^arm")
+    _cpack_set_default(CPACK_APPIMAGE_ARCH "armhf")
+  endif()
+endif()
+
 # WiX specific variables
 _cpack_set_default(CPACK_WIX_SIZEOF_VOID_P "${CMAKE_SIZEOF_VOID_P}")
 if(NOT DEFINED CPACK_WIX_INSTALL_SCOPE)

Source/CPack/cmCPackAppImageGenerator.cxx

@@ -62,26 +62,6 @@ int cmCPackAppImageGenerator::InitializeInternal()
     this->SetOptionIfNotSet("CPACK_APPIMAGE_VERSION", packageVersion);
   }
 
-  if (!this->IsSet("CPACK_APPIMAGE_ARCH")) {
-    if (cmValue systemProcessor = this->GetOption("CMAKE_SYSTEM_PROCESSOR")) {
-      if (systemProcessor == "x86_64" || systemProcessor == "amd64") {
-        this->SetOption("CPACK_APPIMAGE_ARCH", "x86_64");
-      } else if (systemProcessor == "i386" || systemProcessor == "i686" ||
-                 systemProcessor == "x86") {
-        this->SetOption("CPACK_APPIMAGE_ARCH", "i686");
-      } else if (systemProcessor == "aarch64" || systemProcessor == "arm64") {
-        this->SetOption("CPACK_APPIMAGE_ARCH", "aarch64");
-      } else if (cmSystemTools::StringStartsWith(systemProcessor, "arm")) {
-        this->SetOption("CPACK_APPIMAGE_ARCH", "armhf");
-      } else {
-        cmCPackLogger(cmCPackLog::LOG_OUTPUT,
-                      "Could not transform CMAKE_SYSTEM_PROCESSOR, counting "
-                      "on appimagetool architecture detection."
-                        << std::endl);
-      }
-    }
-  }
-
   return Superclass::InitializeInternal();
 }
 

Tests/RunCMake/CPackConfig/AppImageArch-check.cmake

@@ -0,0 +1,3 @@
+include(${RunCMake_SOURCE_DIR}/check.cmake)
+
+test_variable(CPACK_APPIMAGE_ARCH "aarch64")

Tests/RunCMake/CPackConfig/AppImageArch.cmake

@@ -0,0 +1,3 @@
+# Simulate cross-compiling to a target processor that differs from the host
+set(CMAKE_SYSTEM_NAME Linux)
+set(CMAKE_SYSTEM_PROCESSOR arm64)

Tests/RunCMake/CPackConfig/AppImageArchUnknown-check.cmake

@@ -0,0 +1,3 @@
+include(${RunCMake_SOURCE_DIR}/check.cmake)
+
+test_variable(CPACK_APPIMAGE_ARCH "")

Tests/RunCMake/CPackConfig/AppImageArchUnknown.cmake

@@ -0,0 +1,2 @@
+set(CMAKE_SYSTEM_NAME Linux)
+set(CMAKE_SYSTEM_PROCESSOR riscv64)

Tests/RunCMake/CPackConfig/RunCMakeTest.cmake

@@ -11,6 +11,8 @@ run_cmake(CMP0172-WARN)
 run_cmake(CMP0206-NEW)
 run_cmake(CMP0206-OLD)
 run_cmake(CMP0206-WARN)
+run_cmake(AppImageArch)
+run_cmake(AppImageArchUnknown)
 run_cmake(Simple)
 run_cmake(Default)
 run_cmake(Special)