fix(agent): Allow signal propagation when running as PID 1 (#6141)

agent/reaper/reaper.go

@@ -1,6 +1,10 @@
 package reaper
 
-import "github.com/hashicorp/go-reap"
+import (
+	"os"
+
+	"github.com/hashicorp/go-reap"
+)
 
 type Option func(o *options)
 
@@ -22,7 +26,16 @@ func WithPIDCallback(ch reap.PidCh) Option {
 	}
 }
 
-type options struct {
-	ExecArgs []string
-	PIDs     reap.PidCh
+// WithCatchSignals sets the signals that are caught and forwarded to the
+// child process. By default no signals are forwarded.
+func WithCatchSignals(sigs ...os.Signal) Option {
+	return func(o *options) {
+		o.CatchSignals = sigs
+	}
+}
+
+type options struct {
+	ExecArgs     []string
+	PIDs         reap.PidCh
+	CatchSignals []os.Signal
 }

agent/reaper/reaper_test.go

@@ -3,8 +3,11 @@
 package reaper_test
 
 import (
+	"fmt"
 	"os"
 	"os/exec"
+	"os/signal"
+	"syscall"
 	"testing"
 	"time"
 
@@ -15,9 +18,8 @@ import (
 	"github.com/coder/coder/testutil"
 )
 
+//nolint:paralleltest // Non-parallel subtest.
 func TestReap(t *testing.T) {
-	t.Parallel()
-
 	// Don't run the reaper test in CI. It does weird
 	// things like forkexecing which may have unintended
 	// consequences in CI.
@@ -28,8 +30,9 @@ func TestReap(t *testing.T) {
 	// OK checks that's the reaper is successfully reaping
 	// exited processes and passing the PIDs through the shared
 	// channel.
+
+	//nolint:paralleltest // Signal handling.
 	t.Run("OK", func(t *testing.T) {
-		t.Parallel()
 		pids := make(reap.PidCh, 1)
 		err := reaper.ForkReap(
 			reaper.WithPIDCallback(pids),
@@ -64,3 +67,39 @@ func TestReap(t *testing.T) {
 		}
 	})
 }
+
+//nolint:paralleltest // Signal handling.
+func TestReapInterrupt(t *testing.T) {
+	// Don't run the reaper test in CI. It does weird
+	// things like forkexecing which may have unintended
+	// consequences in CI.
+	if _, ok := os.LookupEnv("CI"); ok {
+		t.Skip("Detected CI, skipping reaper tests")
+	}
+
+	errC := make(chan error, 1)
+	pids := make(reap.PidCh, 1)
+
+	// Use signals to notify when the child process is ready for the
+	//  next step of our test.
+	usrSig := make(chan os.Signal, 1)
+	signal.Notify(usrSig, syscall.SIGUSR1, syscall.SIGUSR2)
+	defer signal.Stop(usrSig)
+
+	go func() {
+		errC <- reaper.ForkReap(
+			reaper.WithPIDCallback(pids),
+			reaper.WithCatchSignals(os.Interrupt),
+			// Signal propagation does not extend to children of children, so
+			// we create a little bash script to ensure sleep is interrupted.
+			reaper.WithExecArgs("/bin/sh", "-c", fmt.Sprintf("pid=0; trap 'kill -USR2 %d; kill -TERM $pid' INT; sleep 10 &\npid=$!; kill -USR1 %d; wait", os.Getpid(), os.Getpid())),
+		)
+	}()
+
+	require.Equal(t, <-usrSig, syscall.SIGUSR1)
+	err := syscall.Kill(os.Getpid(), syscall.SIGINT)
+	require.NoError(t, err)
+	require.Equal(t, <-usrSig, syscall.SIGUSR2)
+
+	require.NoError(t, <-errC)
+}

agent/reaper/reaper_unix.go

@@ -4,6 +4,7 @@ package reaper
 
 import (
 	"os"
+	"os/signal"
 	"syscall"
 
 	"github.com/hashicorp/go-reap"
@@ -15,6 +16,24 @@ func IsInitProcess() bool {
 	return os.Getpid() == 1
 }
 
+func catchSignals(pid int, sigs []os.Signal) {
+	if len(sigs) == 0 {
+		return
+	}
+
+	sc := make(chan os.Signal, 1)
+	signal.Notify(sc, sigs...)
+	defer signal.Stop(sc)
+
+	for {
+		s := <-sc
+		sig, ok := s.(syscall.Signal)
+		if ok {
+			_ = syscall.Kill(pid, sig)
+		}
+	}
+}
+
 // ForkReap spawns a goroutine that reaps children. In order to avoid
 // complications with spawning `exec.Commands` in the same process that
 // is reaping, we forkexec a child process. This prevents a race between
@@ -51,13 +70,17 @@ func ForkReap(opt ...Option) error {
 	}
 
 	//#nosec G204
-	pid, _ := syscall.ForkExec(opts.ExecArgs[0], opts.ExecArgs, pattrs)
+	pid, err := syscall.ForkExec(opts.ExecArgs[0], opts.ExecArgs, pattrs)
+	if err != nil {
+		return xerrors.Errorf("fork exec: %w", err)
+	}
+
+	go catchSignals(pid, opts.CatchSignals)
 
 	var wstatus syscall.WaitStatus
 	_, err = syscall.Wait4(pid, &wstatus, 0, nil)
 	for xerrors.Is(err, syscall.EINTR) {
 		_, err = syscall.Wait4(pid, &wstatus, 0, nil)
 	}
-
-	return nil
+	return err
 }

cli/agent.go

@@ -68,7 +68,10 @@ func workspaceAgent() *cobra.Command {
 				// Do not start a reaper on the child process. It's important
 				// to do this else we fork bomb ourselves.
 				args := append(os.Args, "--no-reap")
-				err := reaper.ForkReap(reaper.WithExecArgs(args...))
+				err := reaper.ForkReap(
+					reaper.WithExecArgs(args...),
+					reaper.WithCatchSignals(InterruptSignals...),
+				)
 				if err != nil {
 					logger.Error(ctx, "failed to reap", slog.Error(err))
 					return xerrors.Errorf("fork reap: %w", err)