Support #[unix_sigpipe = "inherit|sig_dfl|sig_ign"] on fn main()

This makes it possible to instruct libstd to never touch the signal
handler for `SIGPIPE`, which makes programs pipeable by default (e.g.
with `./your-program | head -n 1`) without `ErrorKind::BrokenPipe`
errors.

src/test/ui/attributes/unix_sigpipe/auxiliary/sigpipe-utils.rs

@@ -0,0 +1,26 @@
+#![feature(rustc_private)]
+extern crate libc;
+
+/// So tests don't have to bring libc in scope themselves
+pub enum SignalHandler {
+    Ignore,
+    Default,
+}
+
+/// Helper to assert that [`libc::SIGPIPE`] has the expected signal handler.
+pub fn assert_sigpipe_handler(expected_handler: SignalHandler) {
+    #[cfg(unix)]
+    #[cfg(not(any(target_os = "emscripten", target_os = "fuchsia", target_os = "horizon")))]
+    {
+        let prev = unsafe { libc::signal(libc::SIGPIPE, libc::SIG_IGN) };
+
+        let expected = match expected_handler {
+            SignalHandler::Ignore => libc::SIG_IGN,
+            SignalHandler::Default => libc::SIG_DFL,
+        };
+        assert_eq!(prev, expected);
+
+        // Unlikely to matter, but restore the old value anyway
+        unsafe { libc::signal(libc::SIGPIPE, prev); };
+    }
+}

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-crate.rs

@@ -0,0 +1,4 @@
+#![feature(unix_sigpipe)]
+#![unix_sigpipe = "inherit"] //~ error: `unix_sigpipe` attribute cannot be used at crate level
+
+fn main() {}

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-crate.stderr

@@ -0,0 +1,13 @@
+error: `unix_sigpipe` attribute cannot be used at crate level
+  --> $DIR/unix_sigpipe-crate.rs:2:1
+   |
+LL | #![unix_sigpipe = "inherit"]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: perhaps you meant to use an outer attribute
+   |
+LL | #[unix_sigpipe = "inherit"]
+   | ~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+error: aborting due to previous error
+

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-duplicates.rs

@@ -0,0 +1,5 @@
+#![feature(unix_sigpipe)]
+
+#[unix_sigpipe = "sig_ign"]
+#[unix_sigpipe = "inherit"] //~ error: multiple `unix_sigpipe` attributes
+fn main() {}

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-duplicates.stderr

@@ -0,0 +1,14 @@
+error: multiple `unix_sigpipe` attributes
+  --> $DIR/unix_sigpipe-duplicates.rs:4:1
+   |
+LL | #[unix_sigpipe = "inherit"]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove this attribute
+   |
+note: attribute also specified here
+  --> $DIR/unix_sigpipe-duplicates.rs:3:1
+   |
+LL | #[unix_sigpipe = "sig_ign"]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-error.rs

@@ -0,0 +1,13 @@
+// run-pass
+// aux-build:sigpipe-utils.rs
+
+#![feature(unix_sigpipe)]
+
+#[unix_sigpipe = "sig_ign"]
+fn main() {
+    extern crate sigpipe_utils;
+
+    // #[unix_sigpipe = "sig_ign"] is active, so the legacy behavior of ignoring
+    // SIGPIPE shall be in effect
+    sigpipe_utils::assert_sigpipe_handler(sigpipe_utils::SignalHandler::Ignore);
+}

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-inherit.rs

@@ -0,0 +1,14 @@
+// run-pass
+// aux-build:sigpipe-utils.rs
+
+#![feature(unix_sigpipe)]
+
+#[unix_sigpipe = "inherit"]
+fn main() {
+    extern crate sigpipe_utils;
+
+    // #[unix_sigpipe = "inherit"] is active, so SIGPIPE shall NOT be ignored,
+    // instead the default handler shall be installed. (We assume that the
+    // process that runs these tests have the default handler.)
+    sigpipe_utils::assert_sigpipe_handler(sigpipe_utils::SignalHandler::Default);
+}

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-list.rs

@@ -0,0 +1,4 @@
+#![feature(unix_sigpipe)]
+
+#[unix_sigpipe(inherit)] //~ error: malformed `unix_sigpipe` attribute input
+fn main() {}

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-list.stderr

@@ -0,0 +1,15 @@
+error: malformed `unix_sigpipe` attribute input
+  --> $DIR/unix_sigpipe-list.rs:3:1
+   |
+LL | #[unix_sigpipe(inherit)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: the following are the possible correct uses
+   |
+LL | #[unix_sigpipe = "inherit|sig_ign|sig_dfl"]
+   | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+LL | #[unix_sigpipe]
+   | ~~~~~~~~~~~~~~~
+
+error: aborting due to previous error
+

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-non-main-fn.rs

@@ -0,0 +1,6 @@
+#![feature(unix_sigpipe)]
+
+#[unix_sigpipe = "inherit"] //~ error: `unix_sigpipe` attribute can only be used on `fn main()`
+fn f() {}
+
+fn main() {}

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-non-main-fn.stderr

@@ -0,0 +1,8 @@
+error: `unix_sigpipe` attribute can only be used on `fn main()`
+  --> $DIR/unix_sigpipe-non-main-fn.rs:3:1
+   |
+LL | #[unix_sigpipe = "inherit"]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-non-root-main.rs

@@ -0,0 +1,8 @@
+#![feature(unix_sigpipe)]
+
+mod m {
+    #[unix_sigpipe = "inherit"] //~ error: `unix_sigpipe` attribute can only be used on root `fn main()`
+    fn main() {}
+}
+
+fn main() {}

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-non-root-main.stderr

@@ -0,0 +1,8 @@
+error: `unix_sigpipe` attribute can only be used on root `fn main()`
+  --> $DIR/unix_sigpipe-non-root-main.rs:4:5
+   |
+LL |     #[unix_sigpipe = "inherit"]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-not-used.rs

@@ -0,0 +1,9 @@
+// run-pass
+// aux-build:sigpipe-utils.rs
+
+fn main() {
+    extern crate sigpipe_utils;
+
+    // SIGPIPE shall be ignored since #[unix_sigpipe = "..."] is not used
+    sigpipe_utils::assert_sigpipe_handler(sigpipe_utils::SignalHandler::Ignore);
+}

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-only-feature.rs

@@ -0,0 +1,13 @@
+// run-pass
+// aux-build:sigpipe-utils.rs
+
+#![feature(unix_sigpipe)]
+
+fn main() {
+    extern crate sigpipe_utils;
+
+    // Only #![feature(unix_sigpipe)] is enabled, not #[unix_sigpipe = "..."].
+    // This shall not change any behavior, so we still expect SIGPIPE to be
+    // ignored
+    sigpipe_utils::assert_sigpipe_handler(sigpipe_utils::SignalHandler::Ignore);
+}

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-rustc_main.rs

@@ -0,0 +1,15 @@
+// run-pass
+// aux-build:sigpipe-utils.rs
+
+#![feature(unix_sigpipe)]
+#![feature(rustc_attrs)]
+
+#[unix_sigpipe = "sig_dfl"]
+#[rustc_main]
+fn rustc_main() {
+    extern crate sigpipe_utils;
+
+    // #[unix_sigpipe = "sig_dfl"] is active, so SIGPIPE handler shall be
+    // SIG_DFL. Note that we have a #[rustc_main], but it should still work.
+    sigpipe_utils::assert_sigpipe_handler(sigpipe_utils::SignalHandler::Default);
+}

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-sig_dfl.rs

@@ -0,0 +1,13 @@
+// run-pass
+// aux-build:sigpipe-utils.rs
+
+#![feature(unix_sigpipe)]
+
+#[unix_sigpipe = "sig_dfl"]
+fn main() {
+    extern crate sigpipe_utils;
+
+    // #[unix_sigpipe = "sig_dfl"] is active, so SIGPIPE shall NOT be ignored, instead
+    // the default handler shall be installed
+    sigpipe_utils::assert_sigpipe_handler(sigpipe_utils::SignalHandler::Default);
+}

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-start.rs

@@ -0,0 +1,6 @@
+#![feature(start)]
+#![feature(unix_sigpipe)]
+
+#[start]
+#[unix_sigpipe = "inherit"] //~ error: `unix_sigpipe` attribute can only be used on `fn main()`
+fn custom_start(argc: isize, argv: *const *const u8) -> isize { 0 }

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-start.stderr

@@ -0,0 +1,8 @@
+error: `unix_sigpipe` attribute can only be used on `fn main()`
+  --> $DIR/unix_sigpipe-start.rs:5:1
+   |
+LL | #[unix_sigpipe = "inherit"]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-struct.rs

@@ -0,0 +1,6 @@
+#![feature(unix_sigpipe)]
+
+#[unix_sigpipe = "inherit"] //~ error: `unix_sigpipe` attribute can only be used on `fn main()`
+struct S;
+
+fn main() {}

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-struct.stderr

@@ -0,0 +1,8 @@
+error: `unix_sigpipe` attribute can only be used on `fn main()`
+  --> $DIR/unix_sigpipe-struct.rs:3:1
+   |
+LL | #[unix_sigpipe = "inherit"]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-wrong.rs

@@ -0,0 +1,4 @@
+#![feature(unix_sigpipe)]
+
+#[unix_sigpipe = "wrong"] //~ error: valid values for `#[unix_sigpipe = "..."]` are `inherit`, `sig_ign`, or `sig_dfl`
+fn main() {}

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-wrong.stderr

@@ -0,0 +1,8 @@
+error: valid values for `#[unix_sigpipe = "..."]` are `inherit`, `sig_ign`, or `sig_dfl`
+  --> $DIR/unix_sigpipe-wrong.rs:3:1
+   |
+LL | #[unix_sigpipe = "wrong"]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/attributes/unix_sigpipe/unix_sigpipe.rs

@@ -0,0 +1,4 @@
+#![feature(unix_sigpipe)]
+
+#[unix_sigpipe] //~ error: valid values for `#[unix_sigpipe = "..."]` are `inherit`, `sig_ign`, or `sig_dfl`
+fn main() {}

src/test/ui/attributes/unix_sigpipe/unix_sigpipe.stderr

@@ -0,0 +1,8 @@
+error: valid values for `#[unix_sigpipe = "..."]` are `inherit`, `sig_ign`, or `sig_dfl`
+  --> $DIR/unix_sigpipe.rs:3:1
+   |
+LL | #[unix_sigpipe]
+   | ^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+