[POC][APR] Support for cross-building using configuration variables

recipes/apr/all/conanfile.py

@@ -1,3 +1,6 @@
+import os
+import re
+
 from conan import ConanFile
 from conan.errors import ConanException, ConanInvalidConfiguration
 from conan.tools.apple import fix_apple_shared_install_name
@@ -9,8 +12,6 @@
 from conan.tools.layout import basic_layout
 from conan.tools.microsoft import is_msvc
 from conan.tools.scm import Version
-import os
-import re
 
 required_conan_version = ">=1.54.0"
 
@@ -69,7 +70,20 @@ def layout(self):
 
     def validate_build(self):
         if cross_building(self) and not is_msvc(self):
-            raise ConanInvalidConfiguration("apr recipe doesn't support cross-build yet due to runtime checks in autoconf")
+            msg = ("apr recipe doesn't support cross-build for all the platforms"
+                   " due to runtime checks in autoconf. You can provide"
+                   " a pre-built cached file as an user Conan conf variable to try it.\n\n"
+                   "Via host profile:\n"
+                   "[conf]\nuser.apr:cache_file=/path/to/cache_file\n\n"
+                   "Via CLI: \n"
+                   "-c \"user.apr:cache_file='/path/to/cache_file'\"")
+            # Cross-building for apr < 1.7.4 is not supported without a pre-built cached file
+            if Version(self.version) < "1.7.4" and self.conf.get("user.apr:cache_file") is None:
+                raise ConanInvalidConfiguration(msg)
+            # Conan provides for apr >= 1.7.4 and Linux some configuration flags to avoid
+            # entering a pre-built cached file
+            if self.settings.os != "Linux" and self.conf.get("user.apr:cache_file") is None:
+                raise ConanInvalidConfiguration(msg)
 
     def build_requirements(self):
         if not is_msvc(self):
@@ -83,6 +97,47 @@ def build_requirements(self):
     def source(self):
         get(self, **self.conan_data["sources"][self.version], strip_root=True)
 
+    def _get_cross_building_configure_args(self):
+        """
+        The vast majority of projects that use autotools and make use of the AC_TRY_RUN macro,
+        do provide a default fallback when cross-compiling, as per the documentation here:
+
+            * https://ftp.gnu.org/old-gnu/Manuals/autoconf-2.53/html_node/Test-Programs.html
+
+        In that regard, APR cannot be cross-compiled by traditional means, and the only fallback
+        is to use a cache file. Indeed, the only way to cross-compile that is documented by upstream
+        is by pre-empting the configuration checks with a cache file that needs to be generated on
+        the target system:
+
+            ./configure --cache-file={gnu_host_triplet}.cache
+
+        The generated cache file can be repeatedly used to cross-compile to the targeted host system
+        by including it with the recipe data.
+
+        This recipe is reading this custom user conf variable:
+
+            [conf]
+            user.apr:cache_file=/path/to/{gnu_host_triplet}.cache
+
+        So you can use it to cross-compile on your system.
+        """
+        configure_args = []
+        user_cache_file = self.conf.get("user.apr:cache_file", check_type=str)
+        if user_cache_file:
+            configure_args.append(f"--cache-file={user_cache_file}")
+            return configure_args
+
+        self.output.warning("Trying to set some configuration arguments, but it"
+                            " could fail. The best approach is to provide a"
+                            " pre-built cached file.")
+        if self.settings.os == "Linux":
+            # Mandatory cross-building configuration flags (tested on Linux ARM and Intel)
+            configure_args.extend(["apr_cv_mutex_robust_shared=yes",
+                                   "ac_cv_file__dev_zero=yes",
+                                   "apr_cv_process_shared_works=yes",
+                                   "apr_cv_tcp_nodelay_with_cork=yes"])
+        return configure_args
+
     def generate(self):
         if is_msvc(self):
             tc = CMakeToolchain(self)
@@ -95,7 +150,7 @@ def generate(self):
             tc = AutotoolsToolchain(self)
             tc.configure_args.append("--with-installbuilddir=${prefix}/res/build-1")
             if cross_building(self):
-                tc.configure_args.append("apr_cv_mutex_robust_shared=yes")
+                tc.configure_args.extend(self._get_cross_building_configure_args())
             tc.generate()
 
     def _patch_sources(self):