Rollup merge of rust-lang#75172 - tmandry:test-thread-capture, r=dtolnay

Capture output from threads spawned in tests

Fixes rust-lang#42474.

r? @dtolnay since you expressed interest in this, but feel free to redirect if you aren't the right person anymore.

Author: tmandry

library/std/src/io/impls.rs

@@ -210,13 +210,13 @@ impl<B: BufRead + ?Sized> BufRead for Box<B> {
 #[cfg(test)]
 /// This impl is only used by printing logic, so any error returned is always
 /// of kind `Other`, and should be ignored.
-impl Write for Box<dyn (::realstd::io::Write) + Send> {
+impl Write for dyn ::realstd::io::LocalOutput {
     fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
-        (**self).write(buf).map_err(|_| ErrorKind::Other.into())
+        (*self).write(buf).map_err(|_| ErrorKind::Other.into())
     }
 
     fn flush(&mut self) -> io::Result<()> {
-        (**self).flush().map_err(|_| ErrorKind::Other.into())
+        (*self).flush().map_err(|_| ErrorKind::Other.into())
     }
 }
 

library/std/src/io/mod.rs

@@ -274,10 +274,12 @@ pub use self::stdio::{StderrLock, StdinLock, StdoutLock};
 pub use self::stdio::{_eprint, _print};
 #[unstable(feature = "libstd_io_internals", issue = "42788")]
 #[doc(no_inline, hidden)]
-pub use self::stdio::{set_panic, set_print};
+pub use self::stdio::{set_panic, set_print, LocalOutput};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::util::{copy, empty, repeat, sink, Empty, Repeat, Sink};
 
+pub(crate) use self::stdio::clone_io;
+
 mod buffered;
 mod cursor;
 mod error;

library/std/src/io/stdio.rs

@@ -13,14 +13,14 @@ use crate::thread::LocalKey;
 
 thread_local! {
     /// Stdout used by print! and println! macros
-    static LOCAL_STDOUT: RefCell<Option<Box<dyn Write + Send>>> = {
+    static LOCAL_STDOUT: RefCell<Option<Box<dyn LocalOutput>>> = {
         RefCell::new(None)
     }
 }
 
 thread_local! {
     /// Stderr used by eprint! and eprintln! macros, and panics
-    static LOCAL_STDERR: RefCell<Option<Box<dyn Write + Send>>> = {
+    static LOCAL_STDERR: RefCell<Option<Box<dyn LocalOutput>>> = {
         RefCell::new(None)
     }
 }
@@ -903,6 +903,18 @@ impl fmt::Debug for StderrLock<'_> {
     }
 }
 
+/// A writer than can be cloned to new threads.
+#[unstable(
+    feature = "set_stdio",
+    reason = "this trait may disappear completely or be replaced \
+                     with a more general mechanism",
+    issue = "none"
+)]
+#[doc(hidden)]
+pub trait LocalOutput: Write + Send {
+    fn clone_box(&self) -> Box<dyn LocalOutput>;
+}
+
 /// Resets the thread-local stderr handle to the specified writer
 ///
 /// This will replace the current thread's stderr handle, returning the old
@@ -918,7 +930,7 @@ impl fmt::Debug for StderrLock<'_> {
     issue = "none"
 )]
 #[doc(hidden)]
-pub fn set_panic(sink: Option<Box<dyn Write + Send>>) -> Option<Box<dyn Write + Send>> {
+pub fn set_panic(sink: Option<Box<dyn LocalOutput>>) -> Option<Box<dyn LocalOutput>> {
     use crate::mem;
     LOCAL_STDERR.with(move |slot| mem::replace(&mut *slot.borrow_mut(), sink)).and_then(|mut s| {
         let _ = s.flush();
@@ -941,14 +953,25 @@ pub fn set_panic(sink: Option<Box<dyn Write + Send>>) -> Option<Box<dyn Write +
     issue = "none"
 )]
 #[doc(hidden)]
-pub fn set_print(sink: Option<Box<dyn Write + Send>>) -> Option<Box<dyn Write + Send>> {
+pub fn set_print(sink: Option<Box<dyn LocalOutput>>) -> Option<Box<dyn LocalOutput>> {
     use crate::mem;
     LOCAL_STDOUT.with(move |slot| mem::replace(&mut *slot.borrow_mut(), sink)).and_then(|mut s| {
         let _ = s.flush();
         Some(s)
     })
 }
 
+pub(crate) fn clone_io() -> (Option<Box<dyn LocalOutput>>, Option<Box<dyn LocalOutput>>) {
+    LOCAL_STDOUT.with(|stdout| {
+        LOCAL_STDERR.with(|stderr| {
+            (
+                stdout.borrow().as_ref().map(|o| o.clone_box()),
+                stderr.borrow().as_ref().map(|o| o.clone_box()),
+            )
+        })
+    })
+}
+
 /// Write `args` to output stream `local_s` if possible, `global_s`
 /// otherwise. `label` identifies the stream in a panic message.
 ///
@@ -961,7 +984,7 @@ pub fn set_print(sink: Option<Box<dyn Write + Send>>) -> Option<Box<dyn Write +
 /// However, if the actual I/O causes an error, this function does panic.
 fn print_to<T>(
     args: fmt::Arguments<'_>,
-    local_s: &'static LocalKey<RefCell<Option<Box<dyn Write + Send>>>>,
+    local_s: &'static LocalKey<RefCell<Option<Box<dyn LocalOutput>>>>,
     global_s: fn() -> T,
     label: &str,
 ) where

library/std/src/panicking.rs

@@ -212,7 +212,7 @@ fn default_hook(info: &PanicInfo<'_>) {
 
     if let Some(mut local) = set_panic(None) {
         // NB. In `cfg(test)` this uses the forwarding impl
-        // for `Box<dyn (::realstd::io::Write) + Send>`.
+        // for `dyn ::realstd::io::LocalOutput`.
         write(&mut local);
         set_panic(Some(local));
     } else if let Some(mut out) = panic_output() {

library/std/src/thread/mod.rs

@@ -465,11 +465,16 @@ impl Builder {
         let my_packet: Arc<UnsafeCell<Option<Result<T>>>> = Arc::new(UnsafeCell::new(None));
         let their_packet = my_packet.clone();
 
+        let (stdout, stderr) = crate::io::clone_io();
+
         let main = move || {
             if let Some(name) = their_thread.cname() {
                 imp::Thread::set_name(name);
             }
 
+            crate::io::set_print(stdout);
+            crate::io::set_panic(stderr);
+
             thread_info::set(imp::guard::current(), their_thread);
             let try_result = panic::catch_unwind(panic::AssertUnwindSafe(|| {
                 crate::sys_common::backtrace::__rust_begin_short_backtrace(f)

library/test/src/helpers/sink.rs

@@ -6,6 +6,7 @@ use std::{
     sync::{Arc, Mutex},
 };
 
+#[derive(Clone)]
 pub struct Sink(Arc<Mutex<Vec<u8>>>);
 
 impl Sink {
@@ -14,6 +15,12 @@ impl Sink {
     }
 }
 
+impl io::LocalOutput for Sink {
+    fn clone_box(&self) -> Box<dyn io::LocalOutput> {
+        Box::new(Self(self.0.clone()))
+    }
+}
+
 impl Write for Sink {
     fn write(&mut self, data: &[u8]) -> io::Result<usize> {
         Write::write(&mut *self.0.lock().unwrap(), data)

src/librustc_interface/util.rs

@@ -99,6 +99,11 @@ impl Write for Sink {
         Ok(())
     }
 }
+impl io::LocalOutput for Sink {
+    fn clone_box(&self) -> Box<dyn io::LocalOutput> {
+        Box::new(Self(self.0.clone()))
+    }
+}
 
 /// Like a `thread::Builder::spawn` followed by a `join()`, but avoids the need
 /// for `'static` bounds.

src/test/ui/panic-while-printing.rs

@@ -5,7 +5,7 @@
 
 use std::fmt;
 use std::fmt::{Display, Formatter};
-use std::io::set_panic;
+use std::io::{self, set_panic, LocalOutput, Write};
 
 pub struct A;
 
@@ -15,8 +15,23 @@ impl Display for A {
     }
 }
 
+struct Sink;
+impl Write for Sink {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        Ok(buf.len())
+    }
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
+}
+impl LocalOutput for Sink {
+    fn clone_box(&self) -> Box<dyn LocalOutput> {
+        Box::new(Sink)
+    }
+}
+
 fn main() {
-    set_panic(Some(Box::new(Vec::new())));
+    set_panic(Some(Box::new(Sink)));
     assert!(std::panic::catch_unwind(|| {
         eprintln!("{}", A);
     })

src/test/ui/test-thread-capture.rs

@@ -0,0 +1,30 @@
+// compile-flags: --test
+// run-fail
+// run-flags: --test-threads=1
+// check-run-results
+// exec-env:RUST_BACKTRACE=0
+
+#[test]
+fn thready_pass() {
+    println!("fee");
+    std::thread::spawn(|| {
+        println!("fie");
+        println!("foe");
+    })
+    .join()
+    .unwrap();
+    println!("fum");
+}
+
+#[test]
+fn thready_fail() {
+    println!("fee");
+    std::thread::spawn(|| {
+        println!("fie");
+        println!("foe");
+    })
+    .join()
+    .unwrap();
+    println!("fum");
+    panic!();
+}

src/test/ui/test-thread-capture.run.stdout

@@ -0,0 +1,21 @@
+
+running 2 tests
+test thready_fail ... FAILED
+test thready_pass ... ok
+
+failures:
+
+---- thready_fail stdout ----
+fee
+fie
+foe
+fum
+thread 'main' panicked at 'explicit panic', $DIR/test-thread-capture.rs:29:5
+note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
+
+
+failures:
+    thready_fail
+
+test result: FAILED. 1 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out
+