From bf2a584788b20fd5cc69f2143454cf45f3556402 Mon Sep 17 00:00:00 2001
From: Peter Bourgon <peter@bourgon.org>
Date: Wed, 20 Sep 2023 01:00:34 +0200
Subject: [PATCH] Signal errors, context handler

---
 actors.go      | 74 ++++++++++++++++++++++++++++++++++++++++++++------
 actors_test.go | 59 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 124 insertions(+), 9 deletions(-)
 create mode 100644 actors_test.go

diff --git a/actors.go b/actors.go
index d1bc754..10d50e2 100644
--- a/actors.go
+++ b/actors.go
@@ -2,23 +2,41 @@ package run
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"os"
 	"os/signal"
 )
 
+// ContextHandler returns an actor, i.e. an execute and interrupt func, that
+// terminates when the provided context is canceled.
+func ContextHandler(ctx context.Context) (execute func() error, interrupt func(error)) {
+	ctx, cancel := context.WithCancel(ctx)
+	return func() error {
+			<-ctx.Done()
+			return ctx.Err()
+		}, func(error) {
+			cancel()
+		}
+}
+
 // SignalHandler returns an actor, i.e. an execute and interrupt func, that
-// terminates with SignalError when the process receives one of the provided
-// signals, or the parent context is canceled.
+// terminates with ErrSignal when the process receives one of the provided
+// signals, or with ctx.Error() when the parent context is canceled. If no
+// signals are provided, the actor will terminate on any signal, per
+// [signal.Notify].
 func SignalHandler(ctx context.Context, signals ...os.Signal) (execute func() error, interrupt func(error)) {
 	ctx, cancel := context.WithCancel(ctx)
 	return func() error {
-			c := make(chan os.Signal, 1)
-			signal.Notify(c, signals...)
-			defer signal.Stop(c)
+			testc := getTestSigChan(ctx)
+			sigc := make(chan os.Signal, 1)
+			signal.Notify(sigc, signals...)
+			defer signal.Stop(sigc)
 			select {
-			case sig := <-c:
-				return SignalError{Signal: sig}
+			case sig := <-testc:
+				return &SignalError{Signal: sig}
+			case sig := <-sigc:
+				return &SignalError{Signal: sig}
 			case <-ctx.Done():
 				return ctx.Err()
 			}
@@ -27,13 +45,51 @@ func SignalHandler(ctx context.Context, signals ...os.Signal) (execute func() er
 		}
 }
 
-// SignalError is returned by the signal handler's execute function
-// when it terminates due to a received signal.
+type testSigChanKey struct{}
+
+func getTestSigChan(ctx context.Context) <-chan os.Signal {
+	return ctx.Value(testSigChanKey{}).(<-chan os.Signal) // can be nil
+}
+
+func putTestSigChan(ctx context.Context, c <-chan os.Signal) context.Context {
+	return context.WithValue(ctx, testSigChanKey{}, c)
+}
+
+// SignalError is returned by the signal handler's execute function when it
+// terminates due to a received signal.
+//
+// SignalError has a design error that impacts comparison with errors.As.
+// Callers should prefer using errors.Is(err, ErrSignal) to check for signal
+// errors, and should only use errors.As in the rare case that they need to
+// program against the specific os.Signal value.
 type SignalError struct {
 	Signal os.Signal
 }
 
 // Error implements the error interface.
+//
+// It was a design error to define this method on a value receiver rather than a
+// pointer receiver. For compatibility reasons it won't be changed.
 func (e SignalError) Error() string {
 	return fmt.Sprintf("received signal %s", e.Signal)
 }
+
+// Is addresses a design error in the SignalError type, so that errors.Is with
+// ErrSignal will return true.
+func (e SignalError) Is(err error) bool {
+	return errors.Is(err, ErrSignal)
+}
+
+// As fixes a design error in the SignalError type, so that errors.As with the
+// literal `&SignalError{}` will return true.
+func (e SignalError) As(target interface{}) bool {
+	switch target.(type) {
+	case *SignalError, SignalError:
+		return true
+	default:
+		return false
+	}
+}
+
+// ErrSignal is returned by SignalHandler when a signal triggers termination.
+var ErrSignal = errors.New("signal error")
diff --git a/actors_test.go b/actors_test.go
new file mode 100644
index 0000000..bbb5efa
--- /dev/null
+++ b/actors_test.go
@@ -0,0 +1,59 @@
+package run
+
+import (
+	"context"
+	"errors"
+	"os"
+	"testing"
+	"time"
+)
+
+func TestContextHandler(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	var rg Group
+	rg.Add(ContextHandler(ctx))
+	errc := make(chan error, 1)
+	go func() { errc <- rg.Run() }()
+	cancel()
+	select {
+	case err := <-errc:
+		if want, have := context.Canceled, err; !errors.Is(have, want) {
+			t.Errorf("error: want %v, have %v", want, have)
+		}
+	case <-time.After(time.Second):
+		t.Errorf("timeout waiting for error after cancel")
+	}
+}
+
+func TestSignalError(t *testing.T) {
+	testc := make(chan os.Signal, 1)
+	ctx := putTestSigChan(context.Background(), testc)
+
+	var rg Group
+	rg.Add(SignalHandler(ctx, os.Interrupt))
+	testc <- os.Interrupt
+	err := rg.Run()
+
+	var sigerr *SignalError
+	if want, have := true, errors.As(err, &sigerr); want != have {
+		t.Errorf("errors.As(err, &sigerr): want %v, have %v", want, have)
+	}
+
+	if sigerr != nil {
+		if want, have := os.Interrupt, sigerr.Signal; want != have {
+			t.Errorf("sigerr.Signal: want %v, have %v", want, have)
+		}
+	}
+
+	if sigerr := &(SignalError{}); !errors.As(err, &sigerr) {
+		t.Errorf("errors.As(err, <inline sigerr>): failed")
+	}
+
+	if want, have := true, errors.As(err, &(SignalError{})); want != have {
+		t.Errorf("errors.As(err, &(SignalError{})): want %v, have %v", want, have)
+	}
+
+	if want, have := true, errors.Is(err, ErrSignal); want != have {
+		t.Errorf("errors.Is(err, ErrSignal): want %v, have %v", want, have)
+	}
+}