Rollup merge of #47334 - etaoins:only-call-res-init-on-gnu-unix, r=alexcrichton

Only link res_init() on GNU/*nix

To workaround a bug in glibc <= 2.26 lookup_host() calls res_init() based on the glibc version detected at runtime. While this avoids calling res_init() on platforms where it's not required we will still end up linking against the symbol.

This causes an issue on macOS where res_init() is implemented in a separate library (libresolv.9.dylib) from the main libc. While this is harmless for standalone programs it becomes a problem if Rust code is statically linked against another program. If the linked program doesn't already specify -lresolv it will cause the link to fail. This is captured in issue #46797

Fix this by hooking in to the glibc workaround in `cvt_gai` and only activating it for the "gnu" environment on Unix This should include all glibc platforms while excluding musl, windows-gnu, macOS, FreeBSD, etc.

This has the side benefit of removing the #[cfg] in sys_common; only unix.rs has code related to the workaround now.

Before this commit:
```shell
> cat main.rs
use std::net::ToSocketAddrs;

#[no_mangle]
pub extern "C" fn resolve_test() -> () {
    let addr_list = ("google.com.au", 0).to_socket_addrs().unwrap();
    println!("{:?}", addr_list);
}
> rustc --crate-type=staticlib main.rs
> clang libmain.a test.c -o combined
Undefined symbols for architecture x86_64:
  "_res_9_init", referenced from:
      std::net::lookup_host::h93c17fe9ad38464a in libmain.a(std-826c8d3b356e180c.std0.rcgu.o)
ld: symbol(s) not found for architecture x86_64
clang-5.0: error: linker command failed with exit code 1 (use -v to see invocation)
```

Afterwards:
```shell
> rustc --crate-type=staticlib main.rs
> clang libmain.a test.c -o combined
> ./combined
IntoIter([V4(172.217.25.131:0)])
```

Fixes  #46797

2 files changed, 13 insertions, 11 deletions

diff --git a/src/libstd/sys/unix/l4re.rs b/src/libstd/sys/unix/l4re.rs
index c3e8d0b7d95..21218489679 100644
--- a/src/libstd/sys/unix/l4re.rs
+++ b/src/libstd/sys/unix/l4re.rs
@@ -437,9 +437,5 @@ pub mod net {
     pub fn lookup_host(_: &str) -> io::Result<LookupHost> {
         unimpl!();
     }
-
-    pub fn res_init_if_glibc_before_2_26() -> io::Result<()> {
-        unimpl!();
-    }
 }
 
diff --git a/src/libstd/sys/unix/net.rs b/src/libstd/sys/unix/net.rs
index e775f857f2b..3f65975e608 100644
--- a/src/libstd/sys/unix/net.rs
+++ b/src/libstd/sys/unix/net.rs
@@ -51,6 +51,10 @@ pub fn cvt_gai(err: c_int) -> io::Result<()> {
     if err == 0 {
         return Ok(())
     }
+
+    // We may need to trigger a glibc workaround. See on_resolver_failure() for details.
+    on_resolver_failure();
+
     if err == EAI_SYSTEM {
         return Err(io::Error::last_os_error())
     }
@@ -377,21 +381,22 @@ impl IntoInner<c_int> for Socket {
 // res_init unconditionally, we call it only when we detect we're linking
 // against glibc version < 2.26. (That is, when we both know its needed and
 // believe it's thread-safe).
-pub fn res_init_if_glibc_before_2_26() -> io::Result<()> {
+#[cfg(target_env = "gnu")]
+fn on_resolver_failure() {
     // If the version fails to parse, we treat it the same as "not glibc".
     if let Some(Ok(version_str)) = glibc_version_cstr().map(CStr::to_str) {
         if let Some(version) = parse_glibc_version(version_str) {
             if version < (2, 26) {
-                let ret = unsafe { libc::res_init() };
-                if ret != 0 {
-                    return Err(io::Error::last_os_error());
-                }
+                unsafe { libc::res_init() };
             }
         }
     }
-    Ok(())
 }
 
+#[cfg(not(target_env = "gnu"))]
+fn on_resolver_failure() {}
+
+#[cfg(target_env = "gnu")]
 fn glibc_version_cstr() -> Option<&'static CStr> {
     weak! {
         fn gnu_get_libc_version() -> *const libc::c_char
@@ -405,6 +410,7 @@ fn glibc_version_cstr() -> Option<&'static CStr> {
 
 // Returns Some((major, minor)) if the string is a valid "x.y" version,
 // ignoring any extra dot-separated parts. Otherwise return None.
+#[cfg(target_env = "gnu")]
 fn parse_glibc_version(version: &str) -> Option<(usize, usize)> {
     let mut parsed_ints = version.split(".").map(str::parse::<usize>).fuse();
     match (parsed_ints.next(), parsed_ints.next()) {
@@ -413,7 +419,7 @@ fn parse_glibc_version(version: &str) -> Option<(usize, usize)> {
     }
 }
 
-#[cfg(test)]
+#[cfg(all(test, taget_env = "gnu"))]
 mod test {
     use super::*;