Move `sighandling` package out of `sentry`.

PiperOrigin-RevId: 399295737

1 files changed, 102 insertions, 0 deletions

diff --git a/pkg/sighandling/sighandling.go b/pkg/sighandling/sighandling.go
new file mode 100644
index 000000000..bdaf8af29
--- /dev/null
+++ b/pkg/sighandling/sighandling.go
@@ -0,0 +1,102 @@
+// Copyright 2018 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package sighandling contains helpers for handling signals to applications.
+package sighandling
+
+import (
+	"os"
+	"os/signal"
+	"reflect"
+
+	"golang.org/x/sys/unix"
+	"gvisor.dev/gvisor/pkg/abi/linux"
+)
+
+// numSignals is the number of normal (non-realtime) signals on Linux.
+const numSignals = 32
+
+// handleSignals listens for incoming signals and calls the given handler
+// function.
+//
+// It stops when the stop channel is closed. The done channel is closed once it
+// will no longer deliver signals to k.
+func handleSignals(sigchans []chan os.Signal, handler func(linux.Signal), stop, done chan struct{}) {
+	// Build a select case.
+	sc := []reflect.SelectCase{{Dir: reflect.SelectRecv, Chan: reflect.ValueOf(stop)}}
+	for _, sigchan := range sigchans {
+		sc = append(sc, reflect.SelectCase{Dir: reflect.SelectRecv, Chan: reflect.ValueOf(sigchan)})
+	}
+
+	for {
+		// Wait for a notification.
+		index, _, ok := reflect.Select(sc)
+
+		// Was it the stop channel?
+		if index == 0 {
+			if !ok {
+				// Stop forwarding and notify that it's done.
+				close(done)
+				return
+			}
+			continue
+		}
+
+		// How about a different close?
+		if !ok {
+			panic("signal channel closed unexpectedly")
+		}
+
+		// Otherwise, it was a signal on channel N. Index 0 represents the stop
+		// channel, so index N represents the channel for signal N.
+		handler(linux.Signal(index))
+	}
+}
+
+// StartSignalForwarding ensures that synchronous signals are passed to the
+// given handler function and returns a callback that stops signal delivery.
+//
+// Note that this function permanently takes over signal handling. After the
+// stop callback, signals revert to the default Go runtime behavior, which
+// cannot be overridden with external calls to signal.Notify.
+func StartSignalForwarding(handler func(linux.Signal)) func() {
+	stop := make(chan struct{})
+	done := make(chan struct{})
+
+	// Register individual channels. One channel per standard signal is
+	// required as os.Notify() is non-blocking and may drop signals. To avoid
+	// this, standard signals have to be queued separately. Channel size 1 is
+	// enough for standard signals as their semantics allow de-duplication.
+	//
+	// External real-time signals are not supported. We rely on the go-runtime
+	// for their handling.
+	var sigchans []chan os.Signal
+	for sig := 1; sig <= numSignals+1; sig++ {
+		sigchan := make(chan os.Signal, 1)
+		sigchans = append(sigchans, sigchan)
+
+		// SIGURG is used by Go's runtime scheduler.
+		if sig == int(linux.SIGURG) {
+			continue
+		}
+		signal.Notify(sigchan, unix.Signal(sig))
+	}
+	// Start up our listener.
+	go handleSignals(sigchans, handler, stop, done) // S/R-SAFE: synchronized by Kernel.extMu.
+
+	return func() {
+		close(stop)
+		<-done
+	}
+}