work-around res_search on older glibcs

`res_search` became a proper symbol only in glibc 2.34. Until that it
was re-defined in headers, hitting the (in-)famous
https://github.com/ziglang/zig/issues/9485

glibc 2.34+:

    resolv/resolv.h:int             res_search (const char *, int, int, unsigned char *, int)

Old glibc:

    resolv/resolv.h:#define res_search              __res_search

Also, remove the toplevel includes, as they should never be included in
the first place: the headers are included explicitly when needed.

test/cgo/cgo.go

@@ -4,8 +4,9 @@ package main
 // #include <unistd.h>
 // #include <fcntl.h>
 // #include <stdio.h>
+// #include <resolv.h>
 // char* hello() { return "hello, world"; }
-// void phello() { printf("%s, your lucky number is %p\n", hello(), fcntl); }
+// void phello() { printf("%s, your lucky numbers are %p and %p\n", hello(), fcntl, res_search); }
 import "C"
 
 func main() {

toolchain/BUILD.sdk.bazel

@@ -11,7 +11,9 @@ declare_files(
 
 exports_files([
     "glibc-hacks/fcntl.map",
-    "glibc-hacks/glibchack-fcntl.h",
+    "glibc-hacks/fcntl.h",
+    "glibc-hacks/res_search.map",
+    "glibc-hacks/res_search.h",
 ])
 
 declare_cc_toolchains(

toolchain/defs.bzl

@@ -10,19 +10,6 @@ _DEFAULT_INCLUDE_DIRECTORIES = [
     "libunwind/include",
 ]
 
-_fcntl_map = """
-GLIBC_2.2.5 {
-   fcntl;
-};
-"""
-_fcntl_h = """
-#ifdef __ASSEMBLER__
-.symver fcntl64, fcntl@GLIBC_2.2.5
-#else
-__asm__(".symver fcntl64, fcntl@GLIBC_2.2.5");
-#endif
-"""
-
 # Official recommended version. Should use this when we have a usable release.
 URL_FORMAT_RELEASE = "https://ziglang.org/download/{version}/zig-{host_platform}-{version}.{_ext}"
 
@@ -82,6 +69,36 @@ _ZIG_TOOLS = [
     "ar",
 ]
 
+_template_mapfile = """
+%s {
+    %s;
+};
+"""
+
+_template_linker = """
+#ifdef __ASSEMBLER__
+.symver {from_function}, {to_function_abi}
+#else
+__asm__(".symver {from_function}, {to_function_abi}");
+#endif
+"""
+
+def _glibc_hack(from_function, to_function_abi):
+    # Cannot use .format(...) here, because starlark thinks
+    # that the byte 3 (the opening brace on the first line)
+    # is a nested { ... }, returning an error:
+    # Error in format: Nested replacement fields are not supported
+    to_function, to_abi = to_function_abi.split("@")
+    mapfile = _template_mapfile % (to_abi, to_function)
+    header = _template_linker.format(
+        from_function = from_function,
+        to_function_abi = to_function_abi,
+    )
+    return struct(
+        mapfile = mapfile,
+        header = header,
+    )
+
 def _quote(s):
     return "'" + s.replace("'", "'\\''") + "'"
 
@@ -188,14 +205,12 @@ def _zig_repository_impl(repository_ctx):
             )
             repository_ctx.symlink("tools/launcher{}".format(exe), tool_path)
 
-    repository_ctx.file(
+    fcntl_hack = _glibc_hack("fcntl64", "fcntl@GLIBC_2.2.5")
-        "glibc-hacks/fcntl.map",
+    repository_ctx.file("glibc-hacks/fcntl.map", content = fcntl_hack.mapfile)
-        content = _fcntl_map,
+    repository_ctx.file("glibc-hacks/fcntl.h", content = fcntl_hack.header)
-    )
+    res_search_hack = _glibc_hack("res_search", "__res_search@GLIBC_2.2.5")
-    repository_ctx.file(
+    repository_ctx.file("glibc-hacks/res_search.map", content = res_search_hack.mapfile)
-        "glibc-hacks/glibchack-fcntl.h",
+    repository_ctx.file("glibc-hacks/res_search.h", content = res_search_hack.header)
-        content = _fcntl_h,
-    )
 
 zig_repository = repository_rule(
     attrs = {

toolchain/private/cc_toolchains.bzl

@@ -17,9 +17,6 @@ def declare_cc_toolchains(os, zig_sdk_path):
         zigtarget = target_config.zigtarget
 
         cxx_builtin_include_directories = []
-        for d in getattr(target_config, "toplevel_include", []):
-            cxx_builtin_include_directories.append(zig_sdk_path + "/" + d)
-
         absolute_tool_paths = {}
         for name, path in target_config.tool_paths.items() + DEFAULT_TOOL_PATHS:
             if path[0] == "/":

toolchain/private/defs.bzl

@@ -121,9 +121,15 @@ def _target_windows(gocpu, zigcpu):
 def _target_linux_gnu(gocpu, zigcpu, glibc_version):
     glibc_suffix = "gnu.{}".format(glibc_version)
 
+    compiler_extra_includes = []
+    linker_version_scripts = []
+    if glibc_version < "2.28":
         # https://github.com/ziglang/zig/issues/5882#issuecomment-888250676
-    # fcntl_hack is only required for glibc 2.27 or less.
+        compiler_extra_includes.append("glibc-hacks/fcntl.h")
-    fcntl_hack = glibc_version < "2.28"
+        linker_version_scripts.append("glibc-hacks/fcntl.map")
+    if glibc_version < "2.34":
+        compiler_extra_includes.append("glibc-hacks/res_search.h")
+        linker_version_scripts.append("glibc-hacks/res_search.map")
 
     return struct(
         gotarget = "linux_{}_{}".format(gocpu, glibc_suffix),
@@ -137,9 +143,8 @@ def _target_linux_gnu(gocpu, zigcpu, glibc_version):
                    (["libc/include/{}-linux-any".format(zigcpu)] if zigcpu != "x86_64" else []) + [
             "libc/include/any-linux-any",
         ] + _INCLUDE_TAIL,
-        toplevel_include = ["glibc-hacks"] if fcntl_hack else [],
+        compiler_extra_includes = compiler_extra_includes,
-        compiler_extra_includes = ["glibc-hacks/glibchack-fcntl.h"] if fcntl_hack else [],
+        linker_version_scripts = linker_version_scripts,
-        linker_version_scripts = ["glibc-hacks/fcntl.map"] if fcntl_hack else [],
         dynamic_library_linkopts = [],
         copts = [],
         libc = "glibc",