runtime: linux iscgo support for not blocking nptl signals

Under linux+cgo, OS threads are launched via pthread_create().
This abstraction, under linux, requires we avoid blocking
signals 32,33 and 34 indefinitely because they are needed to
reliably execute POSIX-semantics threading in glibc and/or musl.

When blocking signals the go runtime generally re-enables them
quickly. However, when a thread exits (under cgo, this is
via a return from mstart()), we avoid a deadlock in C-code by
not blocking these three signals.

Fixes #42494

Change-Id: I02dfb2480a1f97d11679e0c4b132b51bddbe4c14
Reviewed-on: https://go-review.googlesource.com/c/go/+/269799
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Trust: Tobias Klauser <tobias.klauser@gmail.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/src/syscall/syscall_linux_test.go b/src/syscall/syscall_linux_test.go
index 153d0efef1..adeb7c9ebb 100644
--- a/src/syscall/syscall_linux_test.go
+++ b/src/syscall/syscall_linux_test.go
@@ -597,6 +597,14 @@ func compareStatus(filter, expect string) error {
 	return nil
 }
 
+// killAThread locks the goroutine to an OS thread and exits; this
+// causes an OS thread to terminate.
+func killAThread(c <-chan struct{}) {
+	runtime.LockOSThread()
+	<-c
+	return
+}
+
 // TestSetuidEtc performs tests on all of the wrapped system calls
 // that mirror to the 9 glibc syscalls with POSIX semantics. The test
 // here is considered authoritative and should compile and run
@@ -647,6 +655,11 @@ func TestSetuidEtc(t *testing.T) {
 	}
 
 	for i, v := range vs {
+		// Generate some thread churn as we execute the tests.
+		c := make(chan struct{})
+		go killAThread(c)
+		close(c)
+
 		if err := v.fn(); err != nil {
 			t.Errorf("[%d] %q failed: %v", i, v.call, err)
 			continue