Change process spawning to inherit the parent's signal mask by default

Previously, the signal mask is always reset when a child process is
started. This breaks tools like `nohup` which expect `SIGHUP` to be
blocked.

With this change, the default behavior changes to inherit the signal mask.

This also changes the signal disposition for `SIGPIPE` to only be
changed if the `#[unix_sigpipe]` attribute isn't set.

2 files changed, 13 insertions, 3 deletions

diff --git a/src/doc/unstable-book/src/language-features/unix-sigpipe.md b/src/doc/unstable-book/src/language-features/unix-sigpipe.md
index aa39b6eb288..7ed6a7de895 100644
--- a/src/doc/unstable-book/src/language-features/unix-sigpipe.md
+++ b/src/doc/unstable-book/src/language-features/unix-sigpipe.md
@@ -36,7 +36,7 @@ hello world
 
 Set the `SIGPIPE` handler to `SIG_IGN` before invoking `fn main()`. This will result in `ErrorKind::BrokenPipe` errors if you program tries to write to a closed pipe. This is normally what you want if you for example write socket servers, socket clients, or pipe peers.
 
-This is what libstd has done by default since 2014. Omitting `#[unix_sigpipe = "..."]` is the same as having `#[unix_sigpipe = "sig_ign"]`.
+This is what libstd has done by default since 2014. (However, see the note on child processes below.)
 
 ### Example
 
@@ -52,3 +52,11 @@ hello world
 thread 'main' panicked at 'failed printing to stdout: Broken pipe (os error 32)', library/std/src/io/stdio.rs:1016:9
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
 ```
+
+### Note on child processes
+
+When spawning child processes, the legacy Rust behavior if `#[unix_sigpipe]` is not specified is to
+reset `SIGPIPE` to `SIG_DFL`.
+
+If `#[unix_sigpipe = "..."]` is specified, no matter what its value is, the signal disposition of
+`SIGPIPE` is no longer reset. This means that the child inherits the parent's `SIGPIPE` behavior.
diff --git a/src/test/ui/attributes/unix_sigpipe/auxiliary/sigpipe-utils.rs b/src/test/ui/attributes/unix_sigpipe/auxiliary/sigpipe-utils.rs
index e8b4fe7ae52..74fbae0350e 100644
--- a/src/test/ui/attributes/unix_sigpipe/auxiliary/sigpipe-utils.rs
+++ b/src/test/ui/attributes/unix_sigpipe/auxiliary/sigpipe-utils.rs
@@ -23,9 +23,11 @@ pub fn assert_sigpipe_handler(expected_handler: SignalHandler) {
             SignalHandler::Ignore => libc::SIG_IGN,
             SignalHandler::Default => libc::SIG_DFL,
         };
-        assert_eq!(prev, expected);
+        assert_eq!(prev, expected, "expected sigpipe value matches actual value");
 
         // Unlikely to matter, but restore the old value anyway
-        unsafe { libc::signal(libc::SIGPIPE, prev); };
+        unsafe {
+            libc::signal(libc::SIGPIPE, prev);
+        };
     }
 }