fix: Skip cxx_builtin_include_directories filtering when remote (#415)

It is not possible to determine directory existence remotely, so for
remote execution the best option is not to filter. This PR addresses
https://github.com/bazel-contrib/toolchains_llvm/pull/280/files/1a11e9f0f58f89789b5cd0a389efc7e321e33b3b#r1830121103

toolchain/internal/common.bzl

@@ -115,7 +115,9 @@ def os(rctx):
             return name
         else:
             fail("Unsupported value for exec_os: %s" % name)
+    return os_from_rctx(rctx)
 
+def os_from_rctx(rctx):
     name = rctx.os.name
     if name == "linux":
         return "linux"
@@ -138,7 +140,9 @@ def arch(rctx):
             return "x86_64"
         else:
             fail("Unsupported value for exec_arch: %s" % arch)
+    return arch_from_rctx(rctx)
 
+def arch_from_rctx(rctx):
     arch = rctx.os.arch
     if arch == "arm64":
         return "aarch64"

toolchain/internal/configure.bzl

@@ -21,6 +21,7 @@ load(
 load(
     "//toolchain/internal:common.bzl",
     _arch = "arch",
+    _arch_from_rctx = "arch_from_rctx",
     _canonical_dir_path = "canonical_dir_path",
     _check_os_arch_keys = "check_os_arch_keys",
     _exec_os_arch_dict_value = "exec_os_arch_dict_value",
@@ -30,6 +31,7 @@ load(
     _os = "os",
     _os_arch_pair = "os_arch_pair",
     _os_bzl = "os_bzl",
+    _os_from_rctx = "os_from_rctx",
     _pkg_path_from_label = "pkg_path_from_label",
     _supported_targets = "SUPPORTED_TARGETS",
     _toolchain_tools = "toolchain_tools",
@@ -518,6 +520,16 @@ cc_toolchain(
 )
 """
 
+    # Filter out non-existing directories with absolute paths as they
+    # result in a -Wincomplete-umbrella warning when mentioned in the
+    # system module map. Note that this filtering is skipped for remote
+    # execution because it is not possible to check directory existence.
+    filtered_cxx_builtin_include_directories = cxx_builtin_include_directories if _is_remote(rctx, exec_os, exec_arch) else [
+        dir
+        for dir in cxx_builtin_include_directories
+        if _is_hermetic_or_exists(rctx, dir, sysroot_path)
+    ]
+
     return template.format(
         suffix = suffix,
         target_os = target_os,
@@ -548,14 +560,7 @@ cc_toolchain(
         coverage_link_flags = _list_to_string(_dict_value(toolchain_info.coverage_link_flags_dict, target_pair)),
         unfiltered_compile_flags = _list_to_string(_dict_value(toolchain_info.unfiltered_compile_flags_dict, target_pair)),
         extra_files_str = extra_files_str,
-        cxx_builtin_include_directories = _list_to_string([
-            # Filter out non-existing directories with absolute paths as they
-            # result in a -Wincomplete-umbrella warning when mentioned in the
-            # system module map.
-            dir
-            for dir in cxx_builtin_include_directories
-            if _is_hermetic_or_exists(rctx, dir, sysroot_path)
-        ]),
+        cxx_builtin_include_directories = _list_to_string(filtered_cxx_builtin_include_directories),
         extra_compiler_files = ("\"%s\"," % str(toolchain_info.extra_compiler_files)) if toolchain_info.extra_compiler_files else "",
         major_llvm_version = major_llvm_version,
         extra_exec_compatible_with_specific = toolchain_info.extra_exec_compatible_with.get(target_pair, []),
@@ -564,6 +569,9 @@ cc_toolchain(
         extra_target_compatible_with_all_targets = toolchain_info.extra_target_compatible_with.get("", []),
     )
 
+def _is_remote(rctx, exec_os, exec_arch):
+    return not (_os_from_rctx(rctx) == exec_os and _arch_from_rctx(rctx) == exec_arch)
+
 def _convenience_targets_str(rctx, use_absolute_paths, llvm_dist_rel_path, llvm_dist_label_prefix, exec_dl_ext):
     if use_absolute_paths:
         llvm_dist_label_prefix = ":"