Move `safecopy.ReplaceSignalHandler` into `sighandling` package.

PiperOrigin-RevId: 399560357

1 files changed, 34 insertions, 0 deletions

diff --git a/pkg/sighandling/sighandling_unsafe.go b/pkg/sighandling/sighandling_unsafe.go
index 3fe5c6770..7deeda042 100644
--- a/pkg/sighandling/sighandling_unsafe.go
+++ b/pkg/sighandling/sighandling_unsafe.go
@@ -15,6 +15,7 @@
 package sighandling
 
 import (
+	"fmt"
 	"unsafe"
 
 	"golang.org/x/sys/unix"
@@ -37,3 +38,36 @@ func IgnoreChildStop() error {
 
 	return nil
 }
+
+// ReplaceSignalHandler replaces the existing signal handler for the provided
+// signal with the function pointer at `handler`. This bypasses the Go runtime
+// signal handlers, and should only be used for low-level signal handlers where
+// use of signal.Notify is not appropriate.
+//
+// It stores the value of the previously set handler in previous.
+func ReplaceSignalHandler(sig unix.Signal, handler uintptr, previous *uintptr) error {
+	var sa linux.SigAction
+	const maskLen = 8
+
+	// Get the existing signal handler information, and save the current
+	// handler. Once we replace it, we will use this pointer to fall back to
+	// it when we receive other signals.
+	if _, _, e := unix.RawSyscall6(unix.SYS_RT_SIGACTION, uintptr(sig), 0, uintptr(unsafe.Pointer(&sa)), maskLen, 0, 0); e != 0 {
+		return e
+	}
+
+	// Fail if there isn't a previous handler.
+	if sa.Handler == 0 {
+		return fmt.Errorf("previous handler for signal %x isn't set", sig)
+	}
+
+	*previous = uintptr(sa.Handler)
+
+	// Install our own handler.
+	sa.Handler = uint64(handler)
+	if _, _, e := unix.RawSyscall6(unix.SYS_RT_SIGACTION, uintptr(sig), uintptr(unsafe.Pointer(&sa)), 0, maskLen, 0, 0); e != 0 {
+		return e
+	}
+
+	return nil
+}