Process: handle a possible case of a nil runloop source

If the process terminates before the runloop source being woken up, we
could end up with the callout from CoreFoundation where the
`process.runLoopSource` returns a `nil`, breaking an invariant for the
`CFRunLoopSourceInvalidate`.  This would then cascade into a null
pointer dereference when trying to invalidate a non-existent runloop
source.  Guard against such a case by ensuring that we have a source
before invalidation.

We additionally assert that if we do not have a runloop source, the
process has finished execution as we always expect to have a runloop
source while the process is running.  The runloop source is created
when the process is launched, so it is not possible for the process to
not have started before the runloop is awoken.

Author: compnerd

Sources/Foundation/Process.swift

@@ -649,8 +649,16 @@ open class Process: NSObject {
 
             process.isRunning = false
 
-            // Invalidate the source and wake up the run loop, if it is available
-            CFRunLoopSourceInvalidate(process.runLoopSource)
+            // Invalidate the source and wake up the run loop, if it is
+            // available.  The runloop source may no longer be available if the
+            // process has exited.  Failure to verify that we have a runloop
+            // source breaks an invariant that `null` not be passed to the
+            // function.
+            if let source = process.runLoopSource {
+                CFRunLoopSourceInvalidate(source)
+            } else {
+                assert(process.hasFinished)
+            }
             if let runloop = process.runLoop {
                 CFRunLoopWakeUp(runloop._cfRunLoop)
             }