branches/zip: Minor improvements.

page_dir_set_n_slots(): Add debug assertion that there will be enough space
available in the modification log of the compressed page.
page_trailer_get_len(): Simplify the formula.  Make more use of the function.

include/page0page.ic

@@ -433,6 +433,17 @@ page_dir_set_n_slots(
 				uncompressed part will be updated, or NULL */
 	ulint		n_slots)/* in: number of slots */
 {
+#ifdef UNIV_DEBUG
+	if (UNIV_LIKELY_NULL(page_zip)) {
+		/* Ensure that the modification log will not be overwritten. */
+		ulint	n_slots_old = page_dir_get_n_slots(page);
+		if (n_slots > n_slots_old) {
+			ut_ad(page_zip_available(page_zip,
+					(n_slots - n_slots_old)
+					* PAGE_DIR_SLOT_SIZE));
+		}
+	}
+#endif /* UNIV_DEBUG */
 	page_header_set_field(page, page_zip, PAGE_N_DIR_SLOTS, n_slots);
 }
 
@@ -445,9 +456,8 @@ page_trailer_get_len(
 				/* out: length of page trailer, in bytes */
 	const page_t*	page)	/* in: index page */
 {
-	return(page + UNIV_PAGE_SIZE
-			- page_dir_get_nth_slot((page_t*) page,
-				page_dir_get_n_slots((page_t*) page) - 1));
+	return(PAGE_DIR + PAGE_DIR_SLOT_SIZE
+		* page_dir_get_n_slots((page_t*) page));
 }
 
 /*****************************************************************

include/page0zip.ic

@@ -122,8 +122,7 @@ page_zip_alloc(
 	const page_t*	page,	/* in: uncompressed page */
 	ulint		size)	/* in: size of modification log entries */
 {
-	ulint	trailer_len = PAGE_DIR + PAGE_DIR_SLOT_SIZE
-			* page_dir_get_n_slots((page_t*) page_zip->data);
+	ulint	trailer_len = page_trailer_get_len(page_zip->data);
 
 	ut_ad(page_zip_simple_validate(page_zip));
 	ut_ad(page_zip->m_end + trailer_len < page_zip->size);
@@ -155,12 +154,10 @@ page_zip_available(
 	ulint			size)	/* in: requested size of
 					modification log entries */
 {
-	ulint	trailer_len = PAGE_DIR + PAGE_DIR_SLOT_SIZE
-			* page_dir_get_n_slots((page_t*) page_zip->data);
+	ulint	trailer_len = page_trailer_get_len(page_zip->data);
 
 	ut_ad(page_zip_simple_validate(page_zip));
 	ut_ad(page_zip->m_end + trailer_len < page_zip->size);
-	ut_ad(size >= 3); /* modification log entries are >= 1+1+1 bytes */
 	ut_ad(size < page_zip->size);
 
 	return(UNIV_LIKELY(

page/page0zip.c

@@ -159,9 +159,7 @@ page_zip_decompress(
 	ulint		trailer_len;
 
 	ut_ad(page_zip_simple_validate(page_zip));
-	trailer_len = PAGE_DIR
-			+ PAGE_DIR_SLOT_SIZE
-			* page_dir_get_n_slots((page_t*) page_zip->data);
+	trailer_len = page_trailer_get_len(page_zip->data);
 	ut_ad(trailer_len < page_zip->size - PAGE_DATA);
 	ut_ad(page_header_get_field((page_t*) page_zip->data, PAGE_HEAP_TOP)
 				<= UNIV_PAGE_SIZE - trailer_len);
@@ -278,9 +276,7 @@ page_zip_write(
 {
 	ulint	pos = ut_align_offset(str, UNIV_PAGE_SIZE);
 #ifdef UNIV_DEBUG
-	ulint	trailer_len = PAGE_DIR
-			+ PAGE_DIR_SLOT_SIZE
-			* page_dir_get_n_slots((page_t*) page_zip->data);
+	ulint	trailer_len = page_trailer_get_len(page_zip->data);
 #endif /* UNIV_DEBUG */
 
 	ut_ad(buf_block_get_page_zip(buf_block_align((byte*)str)) == page_zip);