internal/outcome/shim: EOF as exit request fallback

In some cases the signal might be delivered before the signal handler is installed, and synchronising against such a case is too expensive. Instead, use the pipe being closed as a fallback to the regular exit request. This change also moves installation of the signal handler early.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/outcome/dispatcher.go

@@ -32,6 +32,8 @@ type syscallDispatcher interface {
 	// just synchronising access is not enough, as this is for test instrumentation.
 	new(f func(k syscallDispatcher, msg message.Msg))
 
+	// getppid provides [os.Getppid].
+	getppid() int
 	// getpid provides [os.Getpid].
 	getpid() int
 	// getuid provides [os.Getuid].
@@ -108,6 +110,7 @@ type direct struct{ msg message.Msg }
 
 func (k direct) new(f func(k syscallDispatcher, msg message.Msg)) { go f(k, k.msg) }
 
+func (direct) getppid() int                               { return os.Getppid() }
 func (direct) getpid() int                                { return os.Getpid() }
 func (direct) getuid() int                                { return os.Getuid() }
 func (direct) getgid() int                                { return os.Getgid() }