lang: funcs: Add a new test for readfile and fix a small bug

This adds a new test for readfile. Interestingly, it actually caught a
small bug, which was also fixed with this commit. I think the bug was
actually always masked, because it only occurred on shutdown, and in
this case we often don't care about how the stream exited, but it's a
good example of how a test case focused on just one small aspect can be
important.

As an aside, this test case also would have caught the bug fixed in
94c40909cc and by reverting that patch it
indeed fails.

lang/funcs/core/core_test.go

@@ -454,7 +454,43 @@ func TestLiveFuncExec0(t *testing.T) {
 			cleanup:  func() error { return nil },
 		})
 	}
+	{
+		p := "/tmp/somefiletoread"
+		content := "hello world!\n"
+		timeline := []Step{
+			NewSendInputs([]types.Value{
+				vog(p),
+			}),
 
+			NewWaitForNValues(1, timeout), // more than 1 blocks here
+			NewWaitForNSeconds(5, 10),     // wait longer just to be sure
+
+			// pass in a custom validation function
+			NewRangeExpect(func(args []types.Value) error {
+				//fmt.Printf("range: %+v\n", args) // debugging
+				if c := len(args); c != 1 {
+					return fmt.Errorf("wrong args count, got: %d", c)
+				}
+				if args[0].Type().Kind != types.KindStr {
+					return fmt.Errorf("expected str, got: %+v", args[0])
+				}
+				if err := vog(content).Cmp(args[0]); err != nil {
+					return errwrap.Wrapf(err, "got different expected value: %+v", args[0])
+				}
+				return nil
+			}),
+		}
+
+		testCases = append(testCases, test{
+			name:     "readfile",
+			hostname: "", // not needed for this func
+			funcname: "os.readfile",
+			timeline: timeline,
+			expect:   func() error { return nil },
+			startup:  func() error { return ioutil.WriteFile(p, []byte(content), 0666) },
+			cleanup:  func() error { return os.Remove(p) },
+		})
+	}
 	names := []string{}
 	for index, tc := range testCases { // run all the tests
 		if tc.name == "" {

lang/funcs/core/os/readfile_func.go

@@ -206,7 +206,8 @@ func (obj *ReadFileFunc) Stream() error {
 
 // Close runs some shutdown code for this function and turns off the stream.
 func (obj *ReadFileFunc) Close() error {
-	close(obj.events) // clean up for fun
 	close(obj.closeChan)
+	obj.wg.Wait()     // block so we don't exit by the closure of obj.events
+	close(obj.events) // clean up for fun
 	return nil
 }