[zig ld] --gc-sections workaround + tests

`zig cc` emits `--gc-sections` for the linker, which is incompatbile
with what CGo thinks about linking.

This commit adds a workaround: it will add `--no-gc-sections` to the
linking step if the command is not specified (falling back to the
default behavior of gcc/clang).

Related: https://github.com/golang/go/issues/52690

README.md

@@ -270,7 +270,12 @@ used, run:
 $ bazel query @zig_sdk//toolchain/...
 ```
 
-# Note: UBSAN and "SIGILL: Illegal Instruction"
+# Incompatibilities with clang and gcc
+
+`zig cc` is *almost* a drop-in replacement for clang/gcc. This section lists
+some of the discovered differences and ways to live with them.
+
+## UBSAN and "SIGILL: Illegal Instruction"
 
 `zig cc` differs from "mainstream" compilers by [enabling UBSAN by
 default][ubsan1]. Which means your program may compile successfully and crash
@@ -280,9 +285,14 @@ with:
 SIGILL: illegal instruction
 ```
 
-This is by design: it encourages program authors to fix the undefined behavior.
-There are [many ways][ubsan2] to find the undefined behavior.
+This flag encourages program authors to fix the undefined behavior. There are
+[many ways][ubsan2] to find the undefined behavior.
 
+## Use of `--gc-sections` by default
+
+`zig cc` passes `--gc-sections` to the ld.lld linker by default, this causes
+problems for CGo. See
+[below][#go-linker-does-not-put-libc-onto-the-linker-line].
 
 # Known Issues In bazel-zig-cc
 
@@ -313,6 +323,34 @@ currently targets the lowest version, without ability to change it.
 This section lists issues that I've stumbled into when using `zig cc`, and is
 outside of bazel-zig-cc's control.
 
+## Go linker does not put libc onto the linker line
+
+**Severity: Low**
+
+Task: [golang/go #52690 Go linker does not put libc onto the linker line, causing undefined symbol errors](https://github.com/golang/go/issues/52690)
+
+Background: when linking CGo programs that do not have C code by itself,
+the Golang linker does not link the C library, causing undefined symbols and
+a message similar to this:
+
+```
+runtime/race(.text): relocation target getuid not defined
+runtime/race(.text): relocation target pthread_self not defined
+```
+
+This is because `zig cc` emits `--gc-sections` for the linker, which is
+incompatbile with what CGo thinks about linking.
+
+A workaround until [#52690](https://github.com/golang/go/issues/52690) is
+resolved: add `--no-gc-sections` to the link step. So the resulting command to
+compile CGo code on Linux is:
+
+```
+CGO_ENABLED=1 CC="zig cc -Wl,--no-gc-sections" go build main.go
+```
+
+This is done automatically in bazel-zig-cc.
+
 ## using glibc 2.27 or older
 
 **Severity: Low**

test/cgo/BUILD

@@ -3,29 +3,29 @@ load("//rules:rules_go.bzl", "go_binary")
 load("@bazel-zig-cc//rules:platform.bzl", "platform_binary", "platform_test")
 
 go_library(
-    name = "go_lib",
-    srcs = ["hello.go"],
+    name = "cgo_lib",
+    srcs = ["cgo.go"],
     cgo = True,
-    importpath = "git.sr.ht/~motiejus/bazel-zig-cc/test/go",
+    importpath = "git.sr.ht/~motiejus/bazel-zig-cc/test/cgo",
     visibility = ["//visibility:private"],
 )
 
-go_binary(
-    name = "go",
-    embed = [":go_lib"],
-    visibility = ["//visibility:public"],
+go_test(
+    name = "cgo_test",
+    srcs = ["cgo_test.go"],
+    embed = [":cgo_lib"],
 )
 
-go_test(
-    name = "go_test",
-    srcs = ["hello_test.go"],
-    embed = [":go_lib"],
+go_binary(
+    name = "cgo",
+    embed = [":cgo_lib"],
+    visibility = ["//visibility:public"],
 )
 
 [
     platform_binary(
-        name = "go_{}".format(name),
-        src = "go",
+        name = "cgo_{}".format(name),
+        src = "cgo",
         platform = platform,
     )
     for name, platform in [
@@ -39,8 +39,8 @@ go_test(
 
 [
     platform_test(
-        name = "go_test_{}".format(name),
-        src = "go_test",
+        name = "cgo_test_{}".format(name),
+        src = "cgo_test",
         platform = platform,
     )
     for name, platform in [

test/gorace/BUILD

@@ -0,0 +1,24 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
+load("//rules:rules_go.bzl", "go_binary")
+
+go_library(
+    name = "gorace_lib",
+    srcs = ["main.go"],
+    # keep
+    cgo = True,
+    importpath = "git.sr.ht/~motiejus/bazel-zig-cc/test/gorace",
+    visibility = ["//visibility:private"],
+)
+
+go_binary(
+    name = "gorace",
+    embed = [":gorace_lib"],
+    visibility = ["//visibility:public"],
+)
+
+go_test(
+    name = "gorace_test",
+    srcs = ["main_test.go"],
+    embed = [":gorace_lib"],
+    race = "on",
+)

test/gorace/main.go

@@ -0,0 +1,13 @@
+// Package main tests that Zig can compile race-enabled tests.
+//
+// As of writing, this fails:
+//   CGO_ENABLED=1 CC="zig cc" go test -race .
+//
+// More context: https://github.com/ziglang/zig/issues/11398
+//
+// This fails, because `zig cc` adds `--gc-sections` to the linker
+// flag by default, which is incompatible with cgo. bazel-zig-cc
+// adds a workaround for it.
+package main
+
+func main() {}

test/gorace/main_test.go

@@ -0,0 +1,8 @@
+package main
+
+import "testing"
+
+func TestFoo(t *testing.T) {
+	_ = t
+	main()
+}

toolchain/defs.bzl

@@ -78,7 +78,33 @@ fi
 export ZIG_LOCAL_CACHE_DIR="$_cache_prefix/bazel-zig-cc"
 export ZIG_GLOBAL_CACHE_DIR=$ZIG_LOCAL_CACHE_DIR
 
-exec "{zig}" "{zig_tool}" "$@"
+lookup_args=()
+add_arg=
+case "{zig_tool}" in
+    cc|c++)
+        lookup_args=("-Wl,--gc-sections" "-Wl,--no-gc-sections")
+        add_arg="-Wl,--no-gc-sections"
+        ;;
+    ld.lld)
+        lookup_args=("--gc-sections" "--no-gc-sections")
+        add_arg="--no-gc-sections"
+        ;;
+esac
+
+has_special_arg=0
+for arg in "${{lookup_args[@]}}"; do
+    if [[ " ${{args[*]}} " == *" $arg "* ]]; then
+        has_special_arg=1
+        break
+    fi
+done
+
+args=( "$@" )
+if [[ "$has_special_arg" == 0 ]]; then
+    args+=( "$add_arg" )
+fi
+
+exec "{zig}" "{zig_tool}" "${{args[@]}}"
 """
 
 _ZIG_TOOLS = [