/*
* 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;
}
/*
* 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_p page, int slot, uint8 value)
{
int nodeno = NON_LEAF_NODES_PER_PAGE + slot;
struct fsm_page* fsmpage = (struct fsm_page *) PAGE_CONTENTS(page);
uint8 oldvalue;
ASSERT(slot < LEAF_NODES_PER_PAGE);
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 < NODES_PER_PAGE)
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;
}
/*
* 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;
}
/*
* 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);
MarkBufferDirty(buf);
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;
}
