proc.Next: Further improve handling of highly parallel programs

This patch forces Delve to be more mindful of how it handles many
threads and the goroutine context switching that occurs in such cases.

proc/proc.go

@@ -272,64 +272,53 @@ func (dbp *Process) next() (err error) {
 		return err
 	}
 
-	var goroutineExiting bool
-	threadNext := func(thread *Thread) error {
-		if err = thread.setNextBreakpoints(); err != nil {
-			switch t := err.(type) {
-			case ThreadBlockedError, NoReturnAddr: // Noop
-			case GoroutineExitingError:
-				goroutineExiting = t.goid == g.Id
-			default:
-				return err
-			}
-		}
-		return thread.Continue()
-	}
-
 	// Make sure that we halt the process at the end of this
 	// function. We could get into a situation where we have
 	// started some, but not all threads.
 	defer func() { err = dbp.Halt() }()
 
-	// Set next breakpoints and then continue each thread.
+	var goroutineExiting bool
+	if err = dbp.CurrentThread.setNextBreakpoints(); err != nil {
+		switch t := err.(type) {
+		case ThreadBlockedError, NoReturnAddr: // Noop
+		case GoroutineExitingError:
+			goroutineExiting = t.goid == g.Id
+		default:
+			return err
+		}
+	}
+
 	for _, th := range dbp.Threads {
-		if err := threadNext(th); err != nil {
+		if err := th.Continue(); err != nil {
 			return err
 		}
 	}
 
 	for {
-		if _, err := dbp.trapWait(-1); err != nil {
+		th, err := dbp.trapWait(-1)
+		if err != nil {
 			return err
 		}
-		// We need to wait for our goroutine to execute, which may not happen
-		// immediately.
-		//
-		// Loop through all threads, and for each stopped thread
-		// see if it is the thread that we care about (thread.g == original.g).
-		// If so, we're done. Otherwise set next temp breakpoints for
-		// each thread and continue them. The reason we do this is because
-		// if our goroutine is paused, we must execute other threads in order
-		// for them to get to a scheduling point, so they can pick up the
-		// goroutine we care about and begin executing it.
-		for _, thr := range dbp.Threads {
-			if !thr.Stopped() {
-				continue
-			}
-			tg, err := thr.GetG()
-			if err != nil {
-				return err
-			}
-			// Make sure we're on the same goroutine, unless it has exited.
-			if tg.Id == g.Id || goroutineExiting {
-				if dbp.CurrentThread != thr {
-					dbp.SwitchThread(thr.Id)
+		tg, err := th.GetG()
+		if err != nil {
+			return err
+		}
+		// Make sure we're on the same goroutine, unless it has exited.
+		if tg.Id == g.Id || goroutineExiting {
+			// Check to see if the goroutine has switched to another
+			// thread, if so make it the current thread.
+			if dbp.CurrentThread.Id != th.Id {
+				if err := dbp.SwitchThread(th.Id); err != nil {
+					return err
 				}
-				return nil
-			}
-			if err := threadNext(thr); err != nil {
-				return err
 			}
+			return nil
+		}
+		// This thread was not running our goroutine.
+		// We continue it since our goroutine could
+		// potentially be on this threads queue.
+		if err := th.Continue(); err != nil {
+			return err
 		}
 	}
 }

proc/proc_darwin.c

@@ -145,13 +145,12 @@ mach_port_wait(mach_port_t port_set) {
 					return mach_port_wait(port_set);
 				}
 			}
-			break;
+			return thread;
 
 		case 72: // Death
 			return msg.hdr.msgh_local_port;
 	}
-
-	return thread;
+	return 0;
 }
 
 kern_return_t