Function _bitmap_getbuf

src/backend/access/bitmap/bitmappages.c:60–105

* _bitmap_getbuf() -- return the buffer for the given block number and * the access method. */

 * 					   the access method.
 */
Buffer
_bitmap_getbuf(Relation rel, BlockNumber blkno, int access)
{
	Buffer buf;

	if (blkno != P_NEW)
	{
		buf = ReadBuffer(rel, blkno);
		if (access != BM_NOLOCK)
			LockBuffer(buf, access);
	}

	else
	{
		bool		needLock;

		Assert(access == BM_WRITE);

		/*
		 * Extend the relation by one page.
		 *
		 * We have to use a lock to ensure no one else is extending the rel at
		 * the same time, else we will both try to initialize the same new
		 * page.  We can skip locking for new or temp relations, however,
		 * since no one else could be accessing them.
		 */
		needLock = !RELATION_IS_LOCAL(rel);

		if (needLock)
			LockRelationForExtension(rel, ExclusiveLock);

		buf = ReadBuffer(rel, P_NEW);

		/* Acquire buffer lock on new page */
		LockBuffer(buf, BM_WRITE);

		/*
		 * Release the file-extension lock; it's now OK for someone else to
		 * extend the relation some more.
		 */
		if (needLock)
			UnlockRelationForExtension(rel, ExclusiveLock);
	}

	return buf;
}

/*
 * _bitmap_wrtbuf() -- write a buffer page to disk.