meson: add cross file support

Author: matt-sm

foreign_cc/meson.bzl

@@ -13,6 +13,7 @@ load(
     "//foreign_cc/private:detect_root.bzl",
     "detect_root",
 )
+load("//foreign_cc/private:detect_xcompile.bzl", "detect_xcompile")
 load(
     "//foreign_cc/private:framework.bzl",
     "CC_EXTERNAL_RULE_ATTRIBUTES",
@@ -23,6 +24,11 @@ load(
 )
 load("//foreign_cc/private:make_script.bzl", "pkgconfig_script")
 load("//foreign_cc/private:transitions.bzl", "foreign_cc_rule_variant")
+load(
+    "//foreign_cc/private/framework:platform.bzl",
+    "target_arch_name",
+    "target_os_name",
+)
 load("//toolchains/native_tools:native_tools_toolchain.bzl", "native_tool_toolchain")
 load("//toolchains/native_tools:tool_access.bzl", "get_cmake_data", "get_meson_data", "get_ninja_data", "get_pkgconfig_data")
 
@@ -141,6 +147,15 @@ def _create_meson_script(configureParameters):
 
         target_args[target_name] = args
 
+    # Generate cross file if needed
+    if ctx.attr.generate_crosstool_file:
+        cross_args = _write_cross_file(ctx, script)
+        if cross_args:
+            if "setup" not in target_args:
+                target_args["setup"] = cross_args
+            else:
+                target_args["setup"] += cross_args
+
     script.append("{meson} setup --prefix={install_dir} {setup_args} {options} {source_dir}".format(
         meson = meson_path,
         install_dir = "$$INSTALLDIR$$",
@@ -208,6 +223,15 @@ def _attrs():
             doc = "__deprecated__: please use `target_args` with `'build'` target key.",
             mandatory = False,
         ),
+        "generate_crosstool_file": attr.bool(
+            doc = (
+                "When True, a Meson cross file will be generated from the platform values. " +
+                "A cross compile must be detected and no existing --cross-file passed as a " +
+                "setup option."
+            ),
+            mandatory = False,
+            default = False,
+        ),
         "install": attr.bool(
             doc = "__deprecated__: please use `targets` if you want to skip install.",
             default = True,
@@ -300,3 +324,28 @@ def _absolutize(workspace_name, text, force = False):
 
 def _join_flags_list(workspace_name, flags):
     return " ".join([_absolutize(workspace_name, flag) for flag in flags])
+
+def _write_cross_file(ctx, script):
+    # generate a cross file from platform values
+    if not detect_xcompile(ctx, autoconf = False):
+        return []
+
+    # adjust the system name Meson expects
+    os_name = "darwin" if target_os_name(ctx) == "macos" else target_os_name(ctx)
+
+    # write cross file
+    script.append("cat > crosstool_bazel.txt << EOF")
+    script.append("[host_machine]")
+    script.append("system = '{}'".format(os_name))
+    script.append("cpu_family = '{}'".format(target_arch_name(ctx)))
+    script.append("cpu = '{}'".format(target_arch_name(ctx)))
+    script.append("endian = 'little'")
+    script.append("")
+    script.append("[properties]")
+    script.append("skip_sanity_check = true")
+    script.append("EOF")
+    script.append("")
+
+    # return setup arg
+    return ["--cross-file crosstool_bazel.txt"]
+

foreign_cc/private/detect_xcompile.bzl

@@ -8,17 +8,18 @@ load(
     "triplet_name",
 )
 
-def detect_xcompile(ctx):
+def detect_xcompile(ctx, autoconf = True):
     """A helper function for detecting and setting autoconf-style xcompile flags
 
     Args:
         ctx (ctx): The current rule's context object
+        autoconf: True if being called from make/pkgconfig
 
     Returns:
         list(str): The flags to set, or None if xcompiling is disabled
     """
 
-    if not ctx.attr.configure_xcompile:
+    if autoconf and not ctx.attr.configure_xcompile:
         return None
 
     host_triplet = triplet_name(