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diff --git a/src/backend/storage/freespace/fsmpage.c b/src/backend/storage/freespace/fsmpage.c
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+/*-------------------------------------------------------------------------
+ *
+ * fsmpage.c
+ *	  routines to search and manipulate one FSM page.
+ *
+ *
+ * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *	  src/backend/storage/freespace/fsmpage.c
+ *
+ * NOTES:
+ *
+ *	The public functions in this file form an API that hides the internal
+ *	structure of a FSM page. This allows freespace.c to treat each FSM page
+ *	as a black box with SlotsPerPage "slots". fsm_set_avail() and
+ *	fsm_get_avail() let you get/set the value of a slot, and
+ *	fsm_search_avail() lets you search for a slot with value >= X.
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "storage/bufmgr.h"
+#include "storage/fsm_internals.h"
+
+/* Macros to navigate the tree within a page. Root has index zero. */
+#define leftchild(x)	(2 * (x) + 1)
+#define rightchild(x)	(2 * (x) + 2)
+#define parentof(x)		(((x) - 1) / 2)
+
+/*
+ * Find right neighbor of x, wrapping around within the level
+ */
+static int
+rightneighbor(int x)
+{
+	/*
+	 * Move right. This might wrap around, stepping to the leftmost node at
+	 * the next level.
+	 */
+	x++;
+
+	/*
+	 * Check if we stepped to the leftmost node at next level, and correct if
+	 * so. The leftmost nodes at each level are numbered x = 2^level - 1, so
+	 * check if (x + 1) is a power of two, using a standard
+	 * twos-complement-arithmetic trick.
+	 */
+	if (((x + 1) & x) == 0)
+		x = parentof(x);
+
+	return x;
+}
+
+/*
+ * Sets the value of a slot on page. Returns true if the page was modified.
+ *
+ * The caller must hold an exclusive lock on the page.
+ */
+bool
+fsm_set_avail(Page page, int slot, uint8 value)
+{
+	int			nodeno = NonLeafNodesPerPage + slot;
+	FSMPage		fsmpage = (FSMPage) PageGetContents(page);
+	uint8		oldvalue;
+
+	Assert(slot < LeafNodesPerPage);
+
+	oldvalue = fsmpage->fp_nodes[nodeno];
+
+	/* If the value hasn't changed, we don't need to do anything */
+	if (oldvalue == value && value <= fsmpage->fp_nodes[0])
+		return false;
+
+	fsmpage->fp_nodes[nodeno] = value;
+
+	/*
+	 * Propagate up, until we hit the root or a node that doesn't need to be
+	 * updated.
+	 */
+	do
+	{
+		uint8		newvalue = 0;
+		int			lchild;
+		int			rchild;
+
+		nodeno = parentof(nodeno);
+		lchild = leftchild(nodeno);
+		rchild = lchild + 1;
+
+		newvalue = fsmpage->fp_nodes[lchild];
+		if (rchild < NodesPerPage)
+			newvalue = Max(newvalue,
+						   fsmpage->fp_nodes[rchild]);
+
+		oldvalue = fsmpage->fp_nodes[nodeno];
+		if (oldvalue == newvalue)
+			break;
+
+		fsmpage->fp_nodes[nodeno] = newvalue;
+	} while (nodeno > 0);
+
+	/*
+	 * sanity check: if the new value is (still) higher than the value at the
+	 * top, the tree is corrupt.  If so, rebuild.
+	 */
+	if (value > fsmpage->fp_nodes[0])
+		fsm_rebuild_page(page);
+
+	return true;
+}
+
+/*
+ * Returns the value of given slot on page.
+ *
+ * Since this is just a read-only access of a single byte, the page doesn't
+ * need to be locked.
+ */
+uint8
+fsm_get_avail(Page page, int slot)
+{
+	FSMPage		fsmpage = (FSMPage) PageGetContents(page);
+
+	Assert(slot < LeafNodesPerPage);
+
+	return fsmpage->fp_nodes[NonLeafNodesPerPage + slot];
+}
+
+/*
+ * Returns the value at the root of a page.
+ *
+ * Since this is just a read-only access of a single byte, the page doesn't
+ * need to be locked.
+ */
+uint8
+fsm_get_max_avail(Page page)
+{
+	FSMPage		fsmpage = (FSMPage) PageGetContents(page);
+
+	return fsmpage->fp_nodes[0];
+}
+
+/*
+ * Searches for a slot with category at least minvalue.
+ * Returns slot number, or -1 if none found.
+ *
+ * The caller must hold at least a shared lock on the page, and this
+ * function can unlock and lock the page again in exclusive mode if it
+ * needs to be updated. exclusive_lock_held should be set to true if the
+ * caller is already holding an exclusive lock, to avoid extra work.
+ *
+ * If advancenext is false, fp_next_slot is set to point to the returned
+ * slot, and if it's true, to the slot after the returned slot.
+ */
+int
+fsm_search_avail(Buffer buf, uint8 minvalue, bool advancenext,
+				 bool exclusive_lock_held)
+{
+	Page		page = BufferGetPage(buf);
+	FSMPage		fsmpage = (FSMPage) PageGetContents(page);
+	int			nodeno;
+	int			target;
+	uint16		slot;
+
+restart:
+
+	/*
+	 * Check the root first, and exit quickly if there's no leaf with enough
+	 * free space
+	 */
+	if (fsmpage->fp_nodes[0] < minvalue)
+		return -1;
+
+	/*
+	 * Start search using fp_next_slot.  It's just a hint, so check that it's
+	 * sane.  (This also handles wrapping around when the prior call returned
+	 * the last slot on the page.)
+	 */
+	target = fsmpage->fp_next_slot;
+	if (target < 0 || target >= LeafNodesPerPage)
+		target = 0;
+	target += NonLeafNodesPerPage;
+
+	/*----------
+	 * Start the search from the target slot.  At every step, move one
+	 * node to the right, then climb up to the parent.  Stop when we reach
+	 * a node with enough free space (as we must, since the root has enough
+	 * space).
+	 *
+	 * The idea is to gradually expand our "search triangle", that is, all
+	 * nodes covered by the current node, and to be sure we search to the
+	 * right from the start point.  At the first step, only the target slot
+	 * is examined.  When we move up from a left child to its parent, we are
+	 * adding the right-hand subtree of that parent to the search triangle.
+	 * When we move right then up from a right child, we are dropping the
+	 * current search triangle (which we know doesn't contain any suitable
+	 * page) and instead looking at the next-larger-size triangle to its
+	 * right.  So we never look left from our original start point, and at
+	 * each step the size of the search triangle doubles, ensuring it takes
+	 * only log2(N) work to search N pages.
+	 *
+	 * The "move right" operation will wrap around if it hits the right edge
+	 * of the tree, so the behavior is still good if we start near the right.
+	 * Note also that the move-and-climb behavior ensures that we can't end
+	 * up on one of the missing nodes at the right of the leaf level.
+	 *
+	 * For example, consider this tree:
+	 *
+	 *		   7
+	 *	   7	   6
+	 *	 5	 7	 6	 5
+	 *	4 5 5 7 2 6 5 2
+	 *				T
+	 *
+	 * Assume that the target node is the node indicated by the letter T,
+	 * and we're searching for a node with value of 6 or higher. The search
+	 * begins at T. At the first iteration, we move to the right, then to the
+	 * parent, arriving at the rightmost 5. At the second iteration, we move
+	 * to the right, wrapping around, then climb up, arriving at the 7 on the
+	 * third level.  7 satisfies our search, so we descend down to the bottom,
+	 * following the path of sevens.  This is in fact the first suitable page
+	 * to the right of (allowing for wraparound) our start point.
+	 *----------
+	 */
+	nodeno = target;
+	while (nodeno > 0)
+	{
+		if (fsmpage->fp_nodes[nodeno] >= minvalue)
+			break;
+
+		/*
+		 * Move to the right, wrapping around on same level if necessary, then
+		 * climb up.
+		 */
+		nodeno = parentof(rightneighbor(nodeno));
+	}
+
+	/*
+	 * We're now at a node with enough free space, somewhere in the middle of
+	 * the tree. Descend to the bottom, following a path with enough free
+	 * space, preferring to move left if there's a choice.
+	 */
+	while (nodeno < NonLeafNodesPerPage)
+	{
+		int			childnodeno = leftchild(nodeno);
+
+		if (childnodeno < NodesPerPage &&
+			fsmpage->fp_nodes[childnodeno] >= minvalue)
+		{
+			nodeno = childnodeno;
+			continue;
+		}
+		childnodeno++;			/* point to right child */
+		if (childnodeno < NodesPerPage &&
+			fsmpage->fp_nodes[childnodeno] >= minvalue)
+		{
+			nodeno = childnodeno;
+		}
+		else
+		{
+			/*
+			 * Oops. The parent node promised that either left or right child
+			 * has enough space, but neither actually did. This can happen in
+			 * case of a "torn page", IOW if we crashed earlier while writing
+			 * the page to disk, and only part of the page made it to disk.
+			 *
+			 * Fix the corruption and restart.
+			 */
+			RelFileNode rnode;
+			ForkNumber	forknum;
+			BlockNumber blknum;
+
+			BufferGetTag(buf, &rnode, &forknum, &blknum);
+			elog(DEBUG1, "fixing corrupt FSM block %u, relation %u/%u/%u",
+				 blknum, rnode.spcNode, rnode.dbNode, rnode.relNode);
+
+			/* make sure we hold an exclusive lock */
+			if (!exclusive_lock_held)
+			{
+				LockBuffer(buf, BUFFER_LOCK_UNLOCK);
+				LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
+				exclusive_lock_held = true;
+			}
+			fsm_rebuild_page(page);
+			MarkBufferDirtyHint(buf, false);
+			goto restart;
+		}
+	}
+
+	/* We're now at the bottom level, at a node with enough space. */
+	slot = nodeno - NonLeafNodesPerPage;
+
+	/*
+	 * Update the next-target pointer. Note that we do this even if we're only
+	 * holding a shared lock, on the grounds that it's better to use a shared
+	 * lock and get a garbled next pointer every now and then, than take the
+	 * concurrency hit of an exclusive lock.
+	 *
+	 * Wrap-around is handled at the beginning of this function.
+	 */
+	fsmpage->fp_next_slot = slot + (advancenext ? 1 : 0);
+
+	return slot;
+}
+
+/*
+ * Sets the available space to zero for all slots numbered >= nslots.
+ * Returns true if the page was modified.
+ */
+bool
+fsm_truncate_avail(Page page, int nslots)
+{
+	FSMPage		fsmpage = (FSMPage) PageGetContents(page);
+	uint8	   *ptr;
+	bool		changed = false;
+
+	Assert(nslots >= 0 && nslots < LeafNodesPerPage);
+
+	/* Clear all truncated leaf nodes */
+	ptr = &fsmpage->fp_nodes[NonLeafNodesPerPage + nslots];
+	for (; ptr < &fsmpage->fp_nodes[NodesPerPage]; ptr++)
+	{
+		if (*ptr != 0)
+			changed = true;
+		*ptr = 0;
+	}
+
+	/* Fix upper nodes. */
+	if (changed)
+		fsm_rebuild_page(page);
+
+	return changed;
+}
+
+/*
+ * Reconstructs the upper levels of a page. Returns true if the page
+ * was modified.
+ */
+bool
+fsm_rebuild_page(Page page)
+{
+	FSMPage		fsmpage = (FSMPage) PageGetContents(page);
+	bool		changed = false;
+	int			nodeno;
+
+	/*
+	 * Start from the lowest non-leaf level, at last node, working our way
+	 * backwards, through all non-leaf nodes at all levels, up to the root.
+	 */
+	for (nodeno = NonLeafNodesPerPage - 1; nodeno >= 0; nodeno--)
+	{
+		int			lchild = leftchild(nodeno);
+		int			rchild = lchild + 1;
+		uint8		newvalue = 0;
+
+		/* The first few nodes we examine might have zero or one child. */
+		if (lchild < NodesPerPage)
+			newvalue = fsmpage->fp_nodes[lchild];
+
+		if (rchild < NodesPerPage)
+			newvalue = Max(newvalue,
+						   fsmpage->fp_nodes[rchild]);
+
+		if (fsmpage->fp_nodes[nodeno] != newvalue)
+		{
+			fsmpage->fp_nodes[nodeno] = newvalue;
+			changed = true;
+		}
+	}
+
+	return changed;
+}