Add per attachment metadata records.

sync/internal_api/attachments/proto/attachment_store.proto

Index: sync/internal_api/attachments/proto/attachment_store.proto
diff --git a/sync/internal_api/attachments/proto/attachment_store.proto b/sync/internal_api/attachments/proto/attachment_store.proto
index 533332d5d011417ff241c0e86d97803319c4fb30..b233c8ac9ebf967069a37179aa44cc7f00165eb6 100644
--- a/sync/internal_api/attachments/proto/attachment_store.proto
+++ b/sync/internal_api/attachments/proto/attachment_store.proto
@@ -16,3 +16,12 @@ message AttachmentStoreMetadata {
   // older code that doesn't understand it.
   optional int32 schema_version = 1;
 }
+
+// Metadata for attachment in attachemnt store. Storing metadata in separate
+// record from actual data allows to enumerate attachments in the store without

[maniscalco, 2014/10/30 16:40:20]

allows to -> allows us to (and again later in this paragraph)

[pavely, 2014/10/30 18:28:36]

Done.

+// incurring cost to read actual data. It also allows to update attachment
+// metadata independent of the data.
+message AttachmentRecordMetadata {

[maniscalco, 2014/10/30 16:40:20]

I remember that we already have a proto called AttachmentMetadataRecord (stored
in sync entities and Directory).  Naming is hard.

Are there other good names we could use?  Some ideas:

1. Drop Attachment from the message names (StoreMetadata, RecordMetadata).

2. Record -> Entry?

3. ?

[pavely, 2014/10/30 18:28:36]

This proto file already declares attachment_store_pb namespace. I'm dropping
Metadata from both names.

[maniscalco, 2014/10/30 18:33:09]

SGTM

+  // Size of attachment data. Useful for attachment store space management.
+  optional int64 attachment_size = 2;

[maniscalco, 2014/10/30 16:40:20]

2 not 1?

[pavely, 2014/10/30 18:28:36]

Sorry, I started with CRC, but reverted it.

+}