Change SIGPIPE ui from #[unix_sigpipe = "..."] to -Zon-broken-pipe=...

In the stabilization attempt of `#[unix_sigpipe = "sig_dfl"]`, a concern
was raised related to using a language attribute for the feature: Long
term, we want `fn lang_start()` to be definable by any crate, not just
libstd. Having a special language attribute in that case becomes
awkward.

So as a first step towards towards the next stabilization attempt, this
PR changes the `#[unix_sigpipe = "..."]` attribute to a compiler flag
`-Zon-broken-pipe=...` to remove that concern, since now the language
is not "contaminated" by this feature.

Another point was also raised, namely that the ui should not leak
**how** it does things, but rather what the **end effect** is. The new
flag uses the proposed naming. This is of course something that can be
iterated on further before stabilization.

Author: Enselic

std/src/rt.rs

@@ -74,7 +74,7 @@ macro_rules! rtunwrap {
 //
 // Since 2014, the Rust runtime on Unix has set the `SIGPIPE` handler to
 // `SIG_IGN`. Applications have good reasons to want a different behavior
-// though, so there is a `#[unix_sigpipe = "..."]` attribute on `fn main()` that
+// though, so there is a `-Zon-broken-pipe` compiler flag that
 // can be used to select how `SIGPIPE` shall be setup (if changed at all) before
 // `fn main()` is called. See <https://github.com/rust-lang/rust/issues/97889>
 // for more info.

std/src/sys/pal/unix/mod.rs

@@ -55,8 +55,8 @@ pub unsafe fn init(argc: isize, argv: *const *const u8, sigpipe: u8) {
     // want!
     //
     // Hence, we set SIGPIPE to ignore when the program starts up in order
-    // to prevent this problem. Add `#[unix_sigpipe = "..."]` above `fn main()` to
-    // alter this behavior.
+    // to prevent this problem. Use `-Zon-broken-pipe=...` to alter this
+    // behavior.
     reset_sigpipe(sigpipe);
 
     stack_overflow::init();
@@ -194,7 +194,7 @@ pub unsafe fn init(argc: isize, argv: *const *const u8, sigpipe: u8) {
                 _ => unreachable!(),
             };
             if sigpipe_attr_specified {
-                UNIX_SIGPIPE_ATTR_SPECIFIED.store(true, crate::sync::atomic::Ordering::Relaxed);
+                ON_BROKEN_PIPE_FLAG_USED.store(true, crate::sync::atomic::Ordering::Relaxed);
             }
             if let Some(handler) = handler {
                 rtassert!(signal(libc::SIGPIPE, handler) != libc::SIG_ERR);
@@ -214,7 +214,7 @@ pub unsafe fn init(argc: isize, argv: *const *const u8, sigpipe: u8) {
     target_os = "fuchsia",
     target_os = "horizon",
 )))]
-static UNIX_SIGPIPE_ATTR_SPECIFIED: crate::sync::atomic::AtomicBool =
+static ON_BROKEN_PIPE_FLAG_USED: crate::sync::atomic::AtomicBool =
     crate::sync::atomic::AtomicBool::new(false);
 
 #[cfg(not(any(
@@ -223,8 +223,8 @@ static UNIX_SIGPIPE_ATTR_SPECIFIED: crate::sync::atomic::AtomicBool =
     target_os = "fuchsia",
     target_os = "horizon",
 )))]
-pub(crate) fn unix_sigpipe_attr_specified() -> bool {
-    UNIX_SIGPIPE_ATTR_SPECIFIED.load(crate::sync::atomic::Ordering::Relaxed)
+pub(crate) fn on_broken_pipe_flag_used() -> bool {
+    ON_BROKEN_PIPE_FLAG_USED.load(crate::sync::atomic::Ordering::Relaxed)
 }
 
 // SAFETY: must be called only once during runtime cleanup.

std/src/sys/pal/unix/process/process_unix.rs

@@ -353,11 +353,11 @@ impl Command {
             // Inherit the signal mask from the parent rather than resetting it (i.e. do not call
             // pthread_sigmask).
 
-            // If #[unix_sigpipe] is specified, don't reset SIGPIPE to SIG_DFL.
-            // If #[unix_sigpipe] is not specified, reset SIGPIPE to SIG_DFL for backward compatibility.
+            // If -Zon-broken-pipe is used, don't reset SIGPIPE to SIG_DFL.
+            // If -Zon-broken-pipe is not used, reset SIGPIPE to SIG_DFL for backward compatibility.
             //
-            // #[unix_sigpipe] is an opportunity to change the default here.
-            if !crate::sys::pal::unix_sigpipe_attr_specified() {
+            // -Zon-broken-pipe is an opportunity to change the default here.
+            if !crate::sys::pal::on_broken_pipe_flag_used() {
                 #[cfg(target_os = "android")] // see issue #88585
                 {
                     let mut action: libc::sigaction = mem::zeroed();
@@ -455,7 +455,7 @@ impl Command {
     ) -> io::Result<Option<Process>> {
         use crate::mem::MaybeUninit;
         use crate::sys::weak::weak;
-        use crate::sys::{self, cvt_nz, unix_sigpipe_attr_specified};
+        use crate::sys::{self, cvt_nz, on_broken_pipe_flag_used};
 
         if self.get_gid().is_some()
             || self.get_uid().is_some()
@@ -617,11 +617,11 @@ impl Command {
             // Inherit the signal mask from this process rather than resetting it (i.e. do not call
             // posix_spawnattr_setsigmask).
 
-            // If #[unix_sigpipe] is specified, don't reset SIGPIPE to SIG_DFL.
-            // If #[unix_sigpipe] is not specified, reset SIGPIPE to SIG_DFL for backward compatibility.
+            // If -Zon-broken-pipe is used, don't reset SIGPIPE to SIG_DFL.
+            // If -Zon-broken-pipe is not used, reset SIGPIPE to SIG_DFL for backward compatibility.
             //
-            // #[unix_sigpipe] is an opportunity to change the default here.
-            if !unix_sigpipe_attr_specified() {
+            // -Zon-broken-pipe is an opportunity to change the default here.
+            if !on_broken_pipe_flag_used() {
                 let mut default_set = MaybeUninit::<libc::sigset_t>::uninit();
                 cvt(sigemptyset(default_set.as_mut_ptr()))?;
                 cvt(sigaddset(default_set.as_mut_ptr(), libc::SIGPIPE))?;