Prevent building cargo from invalidating build cache of other tools due to conditionally applied -Zon-broken-pipe=kill via tracked RUSTFLAGS

src/bootstrap/src/bin/rustc.rs

@@ -136,6 +136,12 @@ fn main() {
         cmd.args(lint_flags.split_whitespace());
     }
 
+    // Conditionally pass `-Zon-broken-pipe=kill` to underlying rustc. Not all binaries want
+    // `-Zon-broken-pipe=kill`, which includes cargo itself.
+    if env::var_os("FORCE_ON_BROKEN_PIPE_KILL").is_some() {
+        cmd.arg("-Z").arg("on-broken-pipe=kill");
+    }
+
     if target.is_some() {
         // The stage0 compiler has a special sysroot distinct from what we
         // actually downloaded, so we just always pass the `--sysroot` option,

src/bootstrap/src/core/build_steps/compile.rs

@@ -1053,8 +1053,19 @@ pub fn rustc_cargo(
 
     cargo.rustdocflag("-Zcrate-attr=warn(rust_2018_idioms)");
 
-    // If the rustc output is piped to e.g. `head -n1` we want the process to be
-    // killed, rather than having an error bubble up and cause a panic.
+    // If the rustc output is piped to e.g. `head -n1` we want the process to be killed, rather than
+    // having an error bubble up and cause a panic.
+    //
+    // FIXME(jieyouxu): this flag is load-bearing for rustc to not ICE on broken pipes, because
+    // rustc internally sometimes uses std `println!` -- but std `println!` by default will panic on
+    // broken pipes, and uncaught panics will manifest as an ICE. The compiler *should* handle this
+    // properly, but this flag is set in the meantime to paper over the I/O errors.
+    //
+    // See <https://github.com/rust-lang/rust/issues/131059> for details.
+    //
+    // Also see the discussion for properly handling I/O errors related to broken pipes, i.e. safe
+    // variants of `println!` in
+    // <https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Internal.20lint.20for.20raw.20.60print!.60.20and.20.60println!.60.3F>.
     cargo.rustflag("-Zon-broken-pipe=kill");
 
     if builder.config.llvm_enzyme {

src/bootstrap/src/core/build_steps/tool.rs

@@ -209,11 +209,28 @@ pub fn prepare_tool_cargo(
     // See https://github.com/rust-lang/rust/issues/116538
     cargo.rustflag("-Zunstable-options");
 
-    // `-Zon-broken-pipe=kill` breaks cargo tests
+    // NOTE: The root cause of needing `-Zon-broken-pipe=kill` in the first place is because `rustc`
+    // and `rustdoc` doesn't gracefully handle I/O errors due to usages of raw std `println!` macros
+    // which panics upon encountering broken pipes. `-Zon-broken-pipe=kill` just papers over that
+    // and stops rustc/rustdoc ICEing on e.g. `rustc --print=sysroot | false`.
+    //
+    // cargo explicitly does not want the `-Zon-broken-pipe=kill` paper because it does actually use
+    // variants of `println!` that handles I/O errors gracefully. It's also a breaking change for a
+    // spawn process not written in Rust, especially if the language default handler is not
+    // `SIG_IGN`. Thankfully cargo tests will break if we do set the flag.
+    //
+    // For the cargo discussion, see
+    // <https://rust-lang.zulipchat.com/#narrow/stream/246057-t-cargo/topic/Applying.20.60-Zon-broken-pipe.3Dkill.60.20flags.20in.20bootstrap.3F>.
+    //
+    // For the rustc discussion, see
+    // <https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Internal.20lint.20for.20raw.20.60print!.60.20and.20.60println!.60.3F>
+    // for proper solutions.
     if !path.ends_with("cargo") {
-        // If the output is piped to e.g. `head -n1` we want the process to be killed,
-        // rather than having an error bubble up and cause a panic.
-        cargo.rustflag("-Zon-broken-pipe=kill");
+        // Use an untracked env var `FORCE_ON_BROKEN_PIPE_KILL` here instead of `RUSTFLAGS`.
+        // `RUSTFLAGS` is tracked by cargo. Conditionally omitting `-Zon-broken-pipe=kill` from
+        // `RUSTFLAGS` causes unnecessary tool rebuilds due to cache invalidation from building e.g.
+        // cargo *without* `-Zon-broken-pipe=kill` but then rustdoc *with* `-Zon-broken-pipe=kill`.
+        cargo.env("FORCE_ON_BROKEN_PIPE_KILL", "-Zon-broken-pipe=kill");
     }
 
     cargo

tests/run-make/broken-pipe-no-ice/rmake.rs

@@ -0,0 +1,73 @@
+//! Check that `rustc` and `rustdoc` does not ICE upon encountering a broken pipe due to unhandled
+//! panics from raw std `println!` usages.
+//!
+//! Regression test for <https://github.com/rust-lang/rust/issues/34376>.
+
+//@ ignore-cross-compile (needs to run test binary)
+
+#![feature(anonymous_pipe)]
+
+use std::io::Read;
+use std::process::{Command, Stdio};
+
+use run_make_support::env_var;
+
+#[derive(Debug, PartialEq)]
+enum Binary {
+    Rustc,
+    Rustdoc,
+}
+
+fn check_broken_pipe_handled_gracefully(bin: Binary, mut cmd: Command) {
+    let (reader, writer) = std::pipe::pipe().unwrap();
+    drop(reader); // close read-end
+    cmd.stdout(writer).stderr(Stdio::piped());
+
+    let mut child = cmd.spawn().unwrap();
+
+    let mut stderr = String::new();
+    child.stderr.as_mut().unwrap().read_to_string(&mut stderr).unwrap();
+    let status = child.wait().unwrap();
+
+    assert!(!status.success(), "{bin:?} unexpectedly succeeded");
+
+    const PANIC_ICE_EXIT_CODE: i32 = 101;
+
+    #[cfg(not(windows))]
+    {
+        // On non-Windows, rustc/rustdoc built with `-Zon-broken-pipe=kill` shouldn't have an exit
+        // code of 101 because it should have an wait status that corresponds to SIGPIPE signal
+        // number.
+        assert_ne!(status.code(), Some(PANIC_ICE_EXIT_CODE), "{bin:?}");
+        // And the stderr should be empty because rustc/rustdoc should've gotten killed.
+        assert!(stderr.is_empty(), "{bin:?} stderr:\n{}", stderr);
+    }
+
+    #[cfg(windows)]
+    {
+        match bin {
+            // On Windows, rustc has a paper that propagates the panic exit code of 101 but converts
+            // broken pipe errors into fatal errors instead of ICEs.
+            Binary::Rustc => {
+                assert_eq!(status.code(), Some(PANIC_ICE_EXIT_CODE), "{bin:?}");
+                // But make sure it doesn't manifest as an ICE.
+                assert!(!stderr.contains("internal compiler error"), "{bin:?} ICE'd");
+            }
+            // On Windows, rustdoc seems to cleanly exit with exit code of 1.
+            Binary::Rustdoc => {
+                assert_eq!(status.code(), Some(1), "{bin:?}");
+                assert!(!stderr.contains("panic"), "{bin:?} stderr contains panic");
+            }
+        }
+    }
+}
+
+fn main() {
+    let mut rustc = Command::new(env_var("RUSTC"));
+    rustc.arg("--print=sysroot");
+    check_broken_pipe_handled_gracefully(Binary::Rustc, rustc);
+
+    let mut rustdoc = Command::new(env_var("RUSTDOC"));
+    rustdoc.arg("--version");
+    check_broken_pipe_handled_gracefully(Binary::Rustdoc, rustdoc);
+}