unix_sigpipe: Add test for SIGPIPE disposition in child processes

For robustness, also test the disposition in our own process even if
other tests in `tests/ui/attributes/unix_sigpipe` already covers it.

tests/ui/attributes/unix_sigpipe/auxiliary/assert-sigpipe-disposition-bin.rs

@@ -0,0 +1,35 @@
+// It is UB to unwind out of `fn start()` according to
+// https://doc.rust-lang.org/beta/unstable-book/language-features/start.html so
+// panic with abort to avoid UB:
+//@ compile-flags:-C panic=abort
+//@ no-prefer-dynamic
+
+#![crate_type = "bin"]
+#![feature(start, rustc_private)]
+
+extern crate libc;
+
+// Use #[start] so we don't have a runtime that messes with SIGPIPE.
+#[start]
+fn start(argc: isize, argv: *const *const u8) -> isize {
+    assert_eq!(argc, 2, "Must pass SIG_IGN or SIG_DFL as first arg");
+    let arg1 = unsafe { std::ffi::CStr::from_ptr(*argv.offset(1) as *const libc::c_char) }
+        .to_str()
+        .unwrap();
+
+    let expected = match arg1 {
+        "SIG_IGN" => libc::SIG_IGN,
+        "SIG_DFL" => libc::SIG_DFL,
+        arg => panic!("Must pass SIG_IGN or SIG_DFL as first arg. Got: {}", arg),
+    };
+
+    let actual = unsafe {
+        let mut actual: libc::sigaction = std::mem::zeroed();
+        libc::sigaction(libc::SIGPIPE, std::ptr::null(), &mut actual);
+        actual.sa_sigaction
+    };
+
+    assert_eq!(actual, expected, "actual and expected SIGPIPE disposition in child differs");
+
+    0
+}

tests/ui/attributes/unix_sigpipe/unix_sigpipe-and-child-processes.rs

@@ -0,0 +1,55 @@
+//@ revisions: default sig_dfl sig_ign inherit
+//@ aux-build:assert-sigpipe-disposition-bin.rs
+//@ aux-build:sigpipe-utils.rs
+//@ ignore-cross-compile because we run the compiled code
+//@ only-unix because SIGPIPE is a unix thing
+//@ run-pass
+
+// Checks the signal disposition of `SIGPIPE` in child processes, and in our own
+// process for robustness. Without any `unix_sigpipe` attribute, `SIG_IGN` is
+// the default. But there is a difference in how `SIGPIPE` is treated in child
+// processes with and without the attribute. Search for
+// `unix_sigpipe_attr_specified()` in the code base to learn more.
+
+#![feature(rustc_private)]
+#![cfg_attr(any(sig_dfl, sig_ign, inherit), feature(unix_sigpipe))]
+
+extern crate libc;
+
+// We need `aux-build` for `assert-sigpipe-disposition-bin`, and `compiletest`
+// gets confused if we use both `aux-build` and `aux-crate` in the same test, so
+// we need to use the old style of bringing `sigpipe_utils` in scope.
+extern crate sigpipe_utils;
+
+use sigpipe_utils::*;
+
+#[cfg_attr(sig_dfl, unix_sigpipe = "sig_dfl")]
+#[cfg_attr(sig_ign, unix_sigpipe = "sig_ign")]
+#[cfg_attr(inherit, unix_sigpipe = "inherit")]
+fn main() {
+    // By default, we get SIG_IGN but the child gets SIG_DFL.
+    #[cfg(default)]
+    let (we_expect, child_expects) = (SignalHandler::Ignore, "SIG_DFL");
+
+    // With #[unix_sigpipe = "sig_dfl"] we get SIG_DFL and the child does too.
+    #[cfg(sig_dfl)]
+    let (we_expect, child_expects) = (SignalHandler::Default, "SIG_DFL");
+
+    // With #[unix_sigpipe = "sig_ign"] we get SIG_IGN and the child does too.
+    #[cfg(sig_ign)]
+    let (we_expect, child_expects) = (SignalHandler::Ignore, "SIG_IGN");
+
+    // With #[unix_sigpipe = "inherit"] we get SIG_DFL and the child does too.
+    #[cfg(inherit)]
+    let (we_expect, child_expects) = (SignalHandler::Default, "SIG_DFL");
+
+    assert_sigpipe_handler(we_expect);
+
+    assert!(
+        std::process::Command::new("./auxiliary/assert-sigpipe-disposition-bin")
+            .arg(child_expects)
+            .status()
+            .unwrap()
+            .success()
+    );
+}