Auto merge of #131155 - jieyouxu:always-kill, r=onur-ozkan

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

This PR fixes #130980 where building cargo invalidated the tool build caches of other tools (such as rustdoc) because `-Zon-broken-pipe=kill` was conditionally passed via `RUSTFLAGS` for other tools *except* for cargo. The differing `RUSTFLAGS` triggered tool build cache invalidation as `RUSTFLAGS` is a tracked env var -- any changes in `RUSTFLAGS` requires a rebuild.

`-Zon-broken-pipe=kill` is load-bearing for rustc and rustdoc to not ICE on broken pipes due to usages of raw std `println!` that panics without the flag being set, which manifests in ICEs.

I can't say I like the changes here, but it is what it is...

See detailed discussions and history of `-Zon-broken-pipe=kill` usage in https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Internal.20lint.20for.20raw.20.60print!.60.20and.20.60println!.60.3F/near/474593815.

## Approach

This PR fixes the tool build cache invalidation by informing the `rustc` binary shim when to apply `-Zon-broken-pipe=kill` (i.e. when the rustc binary shim is not used to build cargo). This information is not communicated by `RUSTFLAGS`, which is an env var tracked by cargo, and instead uses an untracked env var `UNTRACKED_BROKEN_PIPE_FLAG` so we won't trigger tool build cache invalidation. We preserve bootstrap's behavior of not setting that flag for cargo by conditionally omitting setting `UNTRACKED_BROKEN_PIPE_FLAG` when building cargo.

Notably, the `-Zon-broken-pipe=kill` instance in https://github.com/rust-lang/rust/blob/1e5719bdc40bb553089ce83525f07dfe0b2e71e9/src/bootstrap/src/core/build_steps/compile.rs#L1058 is not modified because that is used to build rustc only and not cargo itself.

Thanks to `@cuviper` for the idea!

## Testing

### Integration testing

This PR introduces a run-make test for rustc and rustdoc that checks that when they do not ICE/panic when they encounter a broken pipe of the stdout stream.

I checked this test will catch the broken pipe ICE regression for rustc on Linux (at least) by commenting out https://github.com/rust-lang/rust/blob/1e5719bdc40bb553089ce83525f07dfe0b2e71e9/src/bootstrap/src/core/build_steps/compile.rs#L1058, and the test failed because rustc ICE'd.

### Manual testing

I have manually tried:

1. `./x clean && `./x test build --stage 1` -> `rustc +stage1 --print=sysroot | false`: no ICE.
2. `./x clean` -> `./x test run-make` twice: no stage 1 cargo rebuilds.
3. `./x clean` -> `./x build rustdoc` -> `rustdoc +stage1 --version | false`: no panics.
4. `./x test src/tools/cargo`: tests pass, notably `build::close_output` and `cargo_command::closed_output_ok` do not fail which would fail if cargo was built with `-Zon-broken-pipe=kill`.

## Related discussions

Thanks to everyone who helped!
- https://rust-lang.zulipchat.com/#narrow/stream/246057-t-cargo/topic/Applying.20.60-Zon-broken-pipe.3Dkill.60.20flags.20in.20bootstrap.3F
- https://rust-lang.zulipchat.com/#narrow/stream/326414-t-infra.2Fbootstrap/topic/Modifying.20run-make.20tests.20unnecessarily.20rebuild.20stage.201.20.2E.2E.2E
- https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Internal.20lint.20for.20raw.20.60print!.60.20and.20.60println!.60.3F

Fixes https://github.com/rust-lang/rust/issues/130980
Closes https://github.com/rust-lang/rust/issues/131059

---

try-job: aarch64-apple
try-job: x86_64-msvc
try-job: x86_64-mingw

3 files changed, 40 insertions, 6 deletions

diff --git a/src/bootstrap/src/bin/rustc.rs b/src/bootstrap/src/bin/rustc.rs
index 780979ed981..18f5a1a58db 100644
--- a/src/bootstrap/src/bin/rustc.rs
+++ b/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,
diff --git a/src/bootstrap/src/core/build_steps/compile.rs b/src/bootstrap/src/core/build_steps/compile.rs
index b5be7d841dd..27bbc8bd8ff 100644
--- a/src/bootstrap/src/core/build_steps/compile.rs
+++ b/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 {
diff --git a/src/bootstrap/src/core/build_steps/tool.rs b/src/bootstrap/src/core/build_steps/tool.rs
index e2fcd13efe3..a01497c2bb9 100644
--- a/src/bootstrap/src/core/build_steps/tool.rs
+++ b/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