Prevent hangs in OncePer when the callback panics

If the callback passed to Once panics it was leaving the waiter
in place that would never be completed.  Move writing the value
and signalling the waiter to defer.

Test: TestOncePerPanic
Change-Id: Icc4d3b779a79914fcd881d61d38dffcc2f591c39

android/onceper_test.go

@@ -175,3 +175,43 @@ func TestOncePerReentrant(t *testing.T) {
 		t.Errorf(`reentrant Once should return "a": %q`, a)
 	}
 }
+
+// Test that a recovered panic in a Once function doesn't deadlock
+func TestOncePerPanic(t *testing.T) {
+	once := OncePer{}
+	key := NewOnceKey("key")
+
+	ch := make(chan interface{})
+
+	var a interface{}
+
+	go func() {
+		defer func() {
+			ch <- recover()
+		}()
+
+		a = once.Once(key, func() interface{} {
+			panic("foo")
+		})
+	}()
+
+	p := <-ch
+
+	if p.(string) != "foo" {
+		t.Errorf(`expected panic with "foo", got %#v`, p)
+	}
+
+	if a != nil {
+		t.Errorf(`expected a to be nil, got %#v`, a)
+	}
+
+	// If the call to Once that panicked leaves the key in a bad state this will deadlock
+	b := once.Once(key, func() interface{} {
+		return "bar"
+	})
+
+	// The second call to Once should return nil inserted by the first call that panicked.
+	if b != nil {
+		t.Errorf(`expected b to be nil, got %#v`, b)
+	}
+}