branches/zip: Add some clarifying comments.

btr_copy_blob_prefix(), btr_copy_externally_stored_field_prefix_low():
Document the return value as "number of bytes written", not "bytes written".

trx_undo_page_fetch_ext(): Explain the assertion ut_a(ext_len).

row_build_index_entry(): Explain the assertion ut_a(!ext).

btr/btr0cur.c

@@ -4278,7 +4278,7 @@ static
 ulint
 btr_copy_blob_prefix(
 /*=================*/
-				/* out: bytes written to buf */
+				/* out: number of bytes written to buf */
 	byte*		buf,	/* out: the externally stored part of
 				the field, or a prefix of it */
 	ulint		len,	/* in: length of buf, in bytes */
@@ -4458,7 +4458,7 @@ static
 ulint
 btr_copy_externally_stored_field_prefix_low(
 /*========================================*/
-				/* out: bytes written to buf */
+				/* out: number of bytes written to buf */
 	byte*		buf,	/* out: the externally stored part of
 				the field, or a prefix of it */
 	ulint		len,	/* in: length of buf, in bytes */

row/row0row.c

@@ -89,8 +89,10 @@ row_build_index_entry(
 	entry_len = dict_index_get_n_fields(index);
 	entry = dtuple_create(heap, entry_len);
 
-	if (index->type & DICT_UNIVERSAL) {
+	if (UNIV_UNLIKELY(index->type & DICT_UNIVERSAL)) {
 		dtuple_set_n_fields_cmp(entry, entry_len);
+		/* There may only be externally stored columns
+		in a clustered index B-tree of a user table. */
 		ut_a(!ext);
 	} else {
 		dtuple_set_n_fields_cmp(

trx/trx0rec.c

@@ -423,6 +423,7 @@ trx_undo_page_fetch_ext(
 	/* Fetch the BLOB. */
 	ulint	ext_len = btr_copy_externally_stored_field_prefix(
 		ext_buf, REC_MAX_INDEX_COL_LEN, zip_size, field, *len);
+	/* BLOBs should always be nonempty. */
 	ut_a(ext_len);
 	/* Append the BLOB pointer to the prefix. */
 	memcpy(ext_buf + ext_len,