Make LazyMultiError only constructable via function

common/errors/multierror.go

Index: common/errors/multierror.go
diff --git a/common/errors/multierror.go b/common/errors/multierror.go
index df812b643251d497f1f01b0a17014acb78b28687..9c02c558f65daca0e753df73842f368b3aedd5a4 100644
--- a/common/errors/multierror.go
+++ b/common/errors/multierror.go
@@ -73,18 +73,26 @@ func MultiErrorFromErrors(ch <-chan error) error {
 	return ret
 }
 
-// LazyMultiError is a lazily-constructed MultiError. You specify the target
-// MultiError size up front (as Size), and then you call Assign for each error
-// encountered, and it's potential index. The MultiError will only be allocated
-// if one of the Assign'd errors is non-nil. Similarly, Get will retrieve either
-// the allocated MultiError, or nil if no error was encountered.
+// LazyMultiError is a lazily-constructed MultiError.
+//
+// LazyMultiError is like MultiError, except that you know the ultimate size up
+// front, and then you call Assign for each error encountered, and it's
+// potential index. The underlying MultiError will only be allocated if one of
+// the Assign'd errors is non-nil. Similarly, Get will retrieve either the
+// allocated MultiError, or nil if no error was encountered.
+// Build one with NewLazy.
 type LazyMultiError struct {

[M-A Ruel, 2015/08/12 19:13:50]

Why doesn't LazyMultiError have method Error() so it would fit the error
interface?

[dnj (Google), 2015/08/14 16:40:00]

If your goal is "only constructable", then LazyMultiError should be an interface
w/ an implementing private struct. Currently, I can instantiate a (admittedly
not so useful) one via LazyMultiError{}.

[iannucci, 2015/08/14 17:00:26]

Done

 	sync.Mutex
 
-	Size int
+	size int

[tandrii_google, 2015/08/14 08:50:36]

wait, this doesn't make what CL description says. ie I can still do:
v := NewLazyMultiError{}

and I tested just that here (pcg is happy):
https://codereview.chromium.org/1297573002 .

So, either fix description, or make LazyMultiError an interface.

[iannucci, 2015/08/14 17:00:26]

done

 	me   MultiError
 }
 
+// NewLazyMultiError makes a new LazyMultiError of the provided size.
+func NewLazyMultiError(size int) *LazyMultiError {
+	return &LazyMultiError{size: size}
+}
+
 // Assign semantically assigns the error to the given index in the MultiError.
 // If the error is nil, no action is taken. Otherwise the MultiError is
 // allocated to its full size (if not already), and the error assigned into it.
@@ -97,7 +105,7 @@ func (e *LazyMultiError) Assign(i int, err error) bool {
 	e.Lock()
 	defer e.Unlock()
 	if e.me == nil {
-		e.me = make(MultiError, e.Size)
+		e.me = make(MultiError, e.size)
 	}
 	e.me[i] = err
 	return true