split log_t::buf into two buffers

Maybe this patch will help catch problems like buffer overflow.

log_t::first_in_use: removed

log_t::buf: this is where mtr_t are supposed to append data
log_t::flush_buf: this is from server writes to a file

Those two buffers are std::swap()ped when some thread is gonna write
to a file

storage/innobase/buf/buf0buf.cc

@@ -1246,10 +1246,11 @@ buf_madvise_do_dump()
 
 	/* mirrors allocation in log_t::create() */
 	if (log_sys.buf) {
-		ret+= madvise(log_sys.first_in_use
-			      ? log_sys.buf
-			      : log_sys.buf - srv_log_buffer_size,
-			      srv_log_buffer_size * 2,
+		ret += madvise(log_sys.buf,
+			       srv_log_buffer_size,
+			       MADV_DODUMP);
+		ret += madvise(log_sys.flush_buf,
+			       srv_log_buffer_size,
 			       MADV_DODUMP);
 	}
 	/* mirrors recv_sys_init() */

storage/innobase/include/log0log.h

@@ -524,19 +524,11 @@ struct log_t{
 					mtr_commit and still ensure that
 					insertions in the flush_list happen
 					in the LSN order. */
-	byte*		buf;		/*!< Memory of double the
-					srv_log_buffer_size is
-					allocated here. This pointer will change
-					however to either the first half or the
-					second half in turns, so that log
-					write/flush to disk don't block
-					concurrent mtrs which will write
-					log to this buffer. Care to switch back
-					to the first half before freeing/resizing
-					must be undertaken. */
-	bool		first_in_use;	/*!< true if buf points to the first
-					half of the buffer, false
-					if the second half */
+	/** log_buffer, append data here */
+	byte*		buf;
+	/** log_buffer, writing data to file from this buffer.
+	Before flushing write_buf is swapped with flush_buf */
+	byte*		flush_buf;
 	ulong		max_buf_free;	/*!< recommended maximum value of
 					buf_free for the buffer in use, after
 					which the buffer is flushed */

storage/innobase/log/log0log.cc

@@ -162,16 +162,19 @@ log_buf_pool_get_oldest_modification(void)
 void log_buffer_extend(ulong len)
 {
 	const ulong new_buf_size = ut_calc_align(len, srv_page_size);
-	byte* new_buf = static_cast<byte*>(
-		ut_malloc_dontdump(new_buf_size * 2));
-	TRASH_ALLOC(new_buf, new_buf_size * 2);
+	byte* new_buf = static_cast<byte*>(ut_malloc_dontdump(new_buf_size));
+	TRASH_ALLOC(new_buf, new_buf_size);
+	byte* new_flush_buf =
+		static_cast<byte*>(ut_malloc_dontdump(new_buf_size));
+	TRASH_ALLOC(new_flush_buf, new_buf_size);
 
 	log_mutex_enter();
 
 	if (len <= srv_log_buffer_size) {
 		/* Already extended enough by the others */
 		log_mutex_exit();
-		ut_free_dodump(new_buf, new_buf_size * 2);
+		ut_free_dodump(new_buf, new_buf_size);
+		ut_free_dodump(new_flush_buf, new_buf_size);
 		return;
 	}
 
@@ -179,14 +182,14 @@ void log_buffer_extend(ulong len)
 		" exceeds innodb_log_buffer_size="
 		<< srv_log_buffer_size << " / 2). Trying to extend it.";
 
-	const byte* old_buf_begin = log_sys.buf;
+	byte* old_buf = log_sys.buf;
+	byte* old_flush_buf = log_sys.flush_buf;
 	const ulong old_buf_size = srv_log_buffer_size;
-	byte* old_buf = log_sys.first_in_use
-		? log_sys.buf : log_sys.buf - old_buf_size;
+
 	srv_log_buffer_size = new_buf_size;
 	log_sys.buf = new_buf;
-	log_sys.first_in_use = true;
-	memcpy(log_sys.buf, old_buf_begin, log_sys.buf_free);
+	log_sys.flush_buf = new_flush_buf;
+	memcpy(new_buf, old_buf, log_sys.buf_free);
 
 	log_sys.max_buf_free = new_buf_size / LOG_BUF_FLUSH_RATIO
 		- LOG_BUF_FLUSH_MARGIN;
@@ -194,6 +197,7 @@ void log_buffer_extend(ulong len)
 	log_mutex_exit();
 
 	ut_free_dodump(old_buf, old_buf_size);
+	ut_free_dodump(old_flush_buf, old_buf_size);
 
 	ib::info() << "innodb_log_buffer_size was extended to "
 		<< new_buf_size << ".";
@@ -547,10 +551,10 @@ void log_t::create()
   ut_ad(srv_log_buffer_size >= 16 * OS_FILE_LOG_BLOCK_SIZE);
   ut_ad(srv_log_buffer_size >= 4U << srv_page_size_shift);
 
-  buf= static_cast<byte*>(ut_malloc_dontdump(srv_log_buffer_size * 2));
-  TRASH_ALLOC(buf, srv_log_buffer_size * 2);
-
-  first_in_use= true;
+  buf= static_cast<byte*>(ut_malloc_dontdump(srv_log_buffer_size));
+  TRASH_ALLOC(buf, srv_log_buffer_size);
+  flush_buf= static_cast<byte*>(ut_malloc_dontdump(srv_log_buffer_size));
+  TRASH_ALLOC(flush_buf, srv_log_buffer_size);
 
   max_buf_free= srv_log_buffer_size / LOG_BUF_FLUSH_RATIO -
     LOG_BUF_FLUSH_MARGIN;
@@ -824,8 +828,8 @@ log_write_flush_to_disk_low()
 	os_event_set(log_sys.flush_event);
 }
 
-/** Switch the log buffer in use, and copy the content of last block
-from old log buffer to the head of the to be used one. Thus, buf_free and
+/** Swap log buffers, and copy the content of last block
+from old buf to the head of the new buf. Thus, buf_free and
 buf_next_to_write would be changed accordingly */
 static inline
 void
@@ -834,27 +838,16 @@ log_buffer_switch()
 	ut_ad(log_mutex_own());
 	ut_ad(log_write_mutex_own());
 
-	const byte*	old_buf = log_sys.buf;
 	ulong		area_end = ut_calc_align(
 		log_sys.buf_free, ulong(OS_FILE_LOG_BLOCK_SIZE));
 
-	if (log_sys.first_in_use) {
-		log_sys.first_in_use = false;
-		ut_ad(log_sys.buf == ut_align(log_sys.buf,
-					       OS_FILE_LOG_BLOCK_SIZE));
-		log_sys.buf += srv_log_buffer_size;
-	} else {
-		log_sys.first_in_use = true;
-		log_sys.buf -= srv_log_buffer_size;
-		ut_ad(log_sys.buf == ut_align(log_sys.buf,
-					       OS_FILE_LOG_BLOCK_SIZE));
-	}
-
 	/* Copy the last block to new buf */
-	ut_memcpy(log_sys.buf,
-		  old_buf + area_end - OS_FILE_LOG_BLOCK_SIZE,
+	ut_memcpy(log_sys.flush_buf,
+		  log_sys.buf + area_end - OS_FILE_LOG_BLOCK_SIZE,
 		  OS_FILE_LOG_BLOCK_SIZE);
 
+	std::swap(log_sys.buf, log_sys.flush_buf);
+
 	log_sys.buf_free %= OS_FILE_LOG_BLOCK_SIZE;
 	log_sys.buf_next_to_write = log_sys.buf_free;
 }
@@ -1942,10 +1935,10 @@ void log_t::close()
   m_initialised = false;
   log.close();
 
-  if (!first_in_use)
-    buf -= srv_log_buffer_size;
-  ut_free_dodump(buf, srv_log_buffer_size * 2);
+  ut_free_dodump(buf, srv_log_buffer_size);
   buf = NULL;
+  ut_free_dodump(flush_buf, srv_log_buffer_size);
+  flush_buf = NULL;
 
   os_event_destroy(flush_event);
   rw_lock_free(&checkpoint_lock);

storage/innobase/srv/srv0start.cc

@@ -502,6 +502,7 @@ create_log_files(
 	memset(log_sys.buf, 0, srv_log_buffer_size);
 	log_block_init(log_sys.buf, log_sys.lsn);
 	log_block_set_first_rec_group(log_sys.buf, LOG_BLOCK_HDR_SIZE);
+	memset(log_sys.flush_buf, 0, srv_log_buffer_size);
 
 	log_sys.buf_free = LOG_BLOCK_HDR_SIZE;
 	log_sys.lsn += LOG_BLOCK_HDR_SIZE;