container/dispatcher: instrument each goroutine individually

Scheduler nondeterminism cannot be accounted for, so do this instead.

There should not be any performance penalty as these calls are optimised out for direct.

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

container/dispatcher.go

@@ -22,6 +22,11 @@ type osFile interface {
 
 // syscallDispatcher provides methods that make state-dependent system calls as part of their behaviour.
 type syscallDispatcher interface {
+	// new returns a new instance of syscallDispatcher for use in another goroutine.
+	// A syscallDispatcher must never be used in any goroutine other than the one owning it,
+	// just synchronising access is not enough, as this is for test instrumentation.
+	new() syscallDispatcher
+
 	// lockOSThread provides [runtime.LockOSThread].
 	lockOSThread()
 
@@ -140,6 +145,8 @@ type syscallDispatcher interface {
 // direct implements syscallDispatcher on the current kernel.
 type direct struct{}
 
+func (k direct) new() syscallDispatcher { return k }
+
 func (direct) lockOSThread() { runtime.LockOSThread() }
 
 func (direct) setPtracer(pid uintptr) error       { return SetPtracer(pid) }