MDEV-13167 InnoDB key rotation is not skipping unused pages

In key rotation, we must initialize unallocated but previously
initialized pages, so that if encryption is enabled on a table,
all clear-text data for the page will eventually be overwritten.
But we should not rotate keys on pages that were never allocated
after the data file was created.

According to the latching order rules, after acquiring the
tablespace latch, no page latches of previously allocated user pages
may be acquired. So, key rotation should check the page allocation
status after acquiring the page latch, not before. But, the latching
order rules also prohibit accessing pages that were not allocated first,
and then acquiring the tablespace latch. Such behaviour would indeed
result in a deadlock when running the following tests:
encryption.innodb_encryption-page-compression
encryption.innodb-checksum-algorithm

Because the key rotation is accessing potentially unallocated pages, it
cannot reliably check if these pages were allocated. It can only check
the page header. If the page number is zero, we can assume that the
page is unallocated.

fil_crypt_rotate_pages(): Skip pages that are known to be uninitialized.

fil_crypt_rotate_page(): Detect uninitialized pages by FIL_PAGE_OFFSET.
Page 0 is never encrypted, and on other pages that are initialized,
FIL_PAGE_OFFSET must contain the page number.

fil_crypt_is_page_uninitialized(): Remove. It suffices to check the
page number field in fil_crypt_rotate_page().

storage/innobase/fil/fil0crypt.cc

@@ -1596,20 +1596,6 @@ fil_crypt_find_page_to_rotate(
 	return found;
 }
 
-/***********************************************************************
-Check if a page is uninitialized (doesn't need to be rotated)
-@param[in]	frame		Page to check
-@param[in]	page_size	Page size
-@return true if page is uninitialized, false if not. */
-static inline
-bool
-fil_crypt_is_page_uninitialized(
-	const byte	*frame,
-	const page_size_t& page_size)
-{
-	return (buf_page_is_zeroes(frame, page_size));
-}
-
 #define fil_crypt_get_page_throttle(state,offset,mtr,sleeptime_ms) \
 	fil_crypt_get_page_throttle_func(state, offset, mtr, \
 					 sleeptime_ms, __FILE__, __LINE__)
@@ -1770,9 +1756,9 @@ fil_crypt_rotate_page(
 	ulint offset = state->offset;
 	ulint sleeptime_ms = 0;
 	fil_space_crypt_t *crypt_data = space->crypt_data;
-	const page_size_t page_size = page_size_t(space->flags);
 
 	ut_ad(space->n_pending_ops > 0);
+	ut_ad(offset > 0);
 
 	/* In fil_crypt_thread where key rotation is done we have
 	acquired space and checked that this space is not yet
@@ -1787,35 +1773,47 @@ fil_crypt_rotate_page(
 		return;
 	}
 
+	ut_d(const bool was_free = fseg_page_is_free(space, offset));
+
 	mtr_t mtr;
 	mtr.start();
 	if (buf_block_t* block = fil_crypt_get_page_throttle(state,
 							     offset, &mtr,
 							     &sleeptime_ms)) {
-		mtr.set_named_space(space);
-
 		bool modified = false;
 		int needs_scrubbing = BTR_SCRUB_SKIP_PAGE;
 		lsn_t block_lsn = block->page.newest_modification;
 		byte* frame = buf_block_get_frame(block);
 		uint kv =  mach_read_from_4(frame+FIL_PAGE_FILE_FLUSH_LSN_OR_KEY_VERSION);
 
-		/* check if tablespace is closing after reading page */
-		if (!space->is_stopping()) {
-
-			if (kv == 0 &&
-				fil_crypt_is_page_uninitialized(frame, page_size)) {
-				;
+		if (space->is_stopping()) {
+			/* The tablespace is closing (in DROP TABLE or
+			TRUNCATE TABLE or similar): avoid further access */
+		} else if (!*reinterpret_cast<uint32_t*>(FIL_PAGE_OFFSET
+							 + frame)) {
+			/* It looks like this page was never
+			allocated. Because key rotation is accessing
+			pages in a pattern that is unlike the normal
+			B-tree and undo log access pattern, we cannot
+			invoke fseg_page_is_free() here, because that
+			could result in a deadlock. If we invoked
+			fseg_page_is_free() and released the
+			tablespace latch before acquiring block->lock,
+			then the fseg_page_is_free() information
+			could be stale already. */
+			ut_ad(was_free);
+			ut_ad(kv == 0);
+			ut_ad(page_get_space_id(frame) == 0);
 		} else if (fil_crypt_needs_rotation(
 				   crypt_data->encryption,
 				   kv, key_state->key_version,
 				   key_state->rotate_key_age)) {
 
+			mtr.set_named_space(space);
 			modified = true;
 
 			/* force rotation by dummy updating page */
-				mlog_write_ulint(frame +
-					FIL_PAGE_ARCH_LOG_NO_OR_SPACE_ID,
+			mlog_write_ulint(frame + FIL_PAGE_SPACE_ID,
 					 space_id, MLOG_4BYTES, &mtr);
 
 			/* statistics */
@@ -1826,7 +1824,6 @@ fil_crypt_rotate_page(
 					state->min_key_version_found = kv;
 				}
 			}
-			}
 
 			needs_scrubbing = btr_page_needs_scrubbing(
 				&state->scrub_data, block,
@@ -1934,7 +1931,8 @@ fil_crypt_rotate_pages(
 	rotate_thread_t*	state)
 {
 	ulint space = state->space->id;
-	ulint end = state->offset + state->batch;
+	ulint end = std::min(state->offset + state->batch,
+			     state->space->free_limit);
 
 	ut_ad(state->space->n_pending_ops > 0);