Add the missing URI extension size offset field

docs/specification.rst

@@ -151,6 +151,7 @@ For v2 the values are represented as 8 byte big-endian.
 #. The offset into the share of the ciphertext hash tree
 #. The offset into the share of the block hash tree
 #. The offset into the share of the needed share hashes.
+#. The offset into the share of the URI extension size
 
 Following the header is the concatenation of all of the blocks that belong to the share.
 Following the blocks are:
@@ -159,8 +160,8 @@ Following the blocks are:
 #. The ciphertext hash tree
 #. The block hash tree
 #. The needed share hashes
-#. A 4 or 8 byte (v1 or v2) big-endian representation of the length of the URI extension.
-#. The URI extension.
+#. The URI extension size
+#. The URI extension
 
 Capability
 ----------

src/CHK.hs

@@ -200,7 +200,9 @@ data CHKStage
   | CrypttextHashTreeOffset  -- offset of crypttext hash tree
   | BlockHashesOffset        -- offset of block hash tree
   | ShareHashesOffset        -- offset of share hashes
-  | URIExtensionOffset       -- offset ofURI extension
+  | URIExtensionOffset       -- offset of the *length prefix* of the URI
+                             -- extension; the URI extension follows
+                             -- immediately.
   | ShareBlock SegmentNum    -- A block of share data.  The segment number
                              -- indicates which source segment needs to be
                              -- encoded to produce the block of data.  These