internal/impl: fix size cache semantics with lazy decoding

When a message (within an extension) is lazily decoded, its size cache is
initialized to 0 (the zero value for an int32). This doesn’t mean the size cache
reads 0, but rather that it was not initialized.

This fixes TestExtensionGetRace being flaky since CL 580015.

related to golang/protobuf#1609

Change-Id: Ia305badadd300679975f230005c3e33c94050e4a
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/586396
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Lasse Folger <lassefolger@google.com>

internal/impl/encode.go

@@ -49,8 +49,11 @@ func (mi *MessageInfo) sizePointer(p pointer, opts marshalOptions) (size int) {
 		return 0
 	}
 	if opts.UseCachedSize() && mi.sizecacheOffset.IsValid() {
-		if size := atomic.LoadInt32(p.Apply(mi.sizecacheOffset).Int32()); size >= 0 {
-			return int(size)
+		// The size cache contains the size + 1, to allow the
+		// zero value to be invalid, while also allowing for a
+		// 0 size to be cached.
+		if size := atomic.LoadInt32(p.Apply(mi.sizecacheOffset).Int32()); size > 0 {
+			return int(size - 1)
 		}
 	}
 	return mi.sizePointerSlow(p, opts)
@@ -60,7 +63,7 @@ func (mi *MessageInfo) sizePointerSlow(p pointer, opts marshalOptions) (size int
 	if flags.ProtoLegacy && mi.isMessageSet {
 		size = sizeMessageSet(mi, p, opts)
 		if mi.sizecacheOffset.IsValid() {
-			atomic.StoreInt32(p.Apply(mi.sizecacheOffset).Int32(), int32(size))
+			atomic.StoreInt32(p.Apply(mi.sizecacheOffset).Int32(), int32(size+1))
 		}
 		return size
 	}
@@ -84,13 +87,16 @@ func (mi *MessageInfo) sizePointerSlow(p pointer, opts marshalOptions) (size int
 		}
 	}
 	if mi.sizecacheOffset.IsValid() {
-		if size > math.MaxInt32 {
+		if size > (math.MaxInt32 - 1) {
 			// The size is too large for the int32 sizecache field.
 			// We will need to recompute the size when encoding;
 			// unfortunately expensive, but better than invalid output.
-			atomic.StoreInt32(p.Apply(mi.sizecacheOffset).Int32(), -1)
+			atomic.StoreInt32(p.Apply(mi.sizecacheOffset).Int32(), 0)
 		} else {
-			atomic.StoreInt32(p.Apply(mi.sizecacheOffset).Int32(), int32(size))
+			// The size cache contains the size + 1, to allow the
+			// zero value to be invalid, while also allowing for a
+			// 0 size to be cached.
+			atomic.StoreInt32(p.Apply(mi.sizecacheOffset).Int32(), int32(size+1))
 		}
 	}
 	return size