proc: bugfix: race condition between termination and Continue (linux)

resume loops in continueOnce moved to a OS specific resume function,
this makes the problem easier to deal with and seems to be more
appropriate to a windows port given what transpired from discussion
of Pull Request go-delve#276

proc/proc.go

@@ -394,20 +394,8 @@ func (dbp *Process) runBreakpointConditions() error {
 
 // Resume process, does not evaluate breakpoint conditionals
 func (dbp *Process) continueOnce() error {
-	// all threads stopped over a breakpoint are made to step over it
-	for _, thread := range dbp.Threads {
-		if thread.CurrentBreakpoint != nil {
-			if err := thread.Step(); err != nil {
-				return err
-			}
-			thread.CurrentBreakpoint = nil
-		}
-	}
-	// everything is resumed
-	for _, thread := range dbp.Threads {
-		if err := thread.resume(); err != nil {
-			return dbp.exitGuard(err)
-		}
+	if err := dbp.resume(); err != nil {
+		return err
 	}
 	return dbp.run(func() error {
 		thread, err := dbp.trapWait(-1)

proc/proc_darwin.go

@@ -356,3 +356,22 @@ func (dbp *Process) exitGuard(err error) error {
 	}
 	return err
 }
+
+func (dbp *Process) resume() error {
+	// all threads stopped over a breakpoint are made to step over it
+	for _, thread := range dbp.Threads {
+		if thread.CurrentBreakpoint != nil {
+			if err := thread.Step(); err != nil {
+				return err
+			}
+			thread.CurrentBreakpoint = nil
+		}
+	}
+	// everything is resumed
+	for _, thread := range dbp.Threads {
+		if err := thread.resume(); err != nil {
+			return dbp.exitGuard(err)
+		}
+	}
+	return nil
+}

proc/proc_linux.go

@@ -286,7 +286,9 @@ func (dbp *Process) trapWait(pid int) (*Thread, error) {
 				return nil, fmt.Errorf("could not continue new thread %d %s", cloned, err)
 			}
 			if err = dbp.Threads[int(wpid)].Continue(); err != nil {
-				return nil, fmt.Errorf("could not continue existing thread %d %s", cloned, err)
+				if err != sys.ESRCH {
+					return nil, fmt.Errorf("could not continue existing thread %d %s", wpid, err)
+				}
 			}
 			continue
 		}
@@ -401,3 +403,22 @@ func (dbp *Process) exitGuard(err error) error {
 
 	return err
 }
+
+func (dbp *Process) resume() error {
+	// all threads stopped over a breakpoint are made to step over it
+	for _, thread := range dbp.Threads {
+		if thread.CurrentBreakpoint != nil {
+			if err := thread.Step(); err != nil {
+				return err
+			}
+			thread.CurrentBreakpoint = nil
+		}
+	}
+	// everything is resumed
+	for _, thread := range dbp.Threads {
+		if err := thread.resume(); err != nil && err != sys.ESRCH {
+			return err
+		}
+	}
+	return nil
+}