Add pessimistic locking. Closes etrepat#89.

Use pessimistic locks when performing certain operations which
may result in a race condition. Mainly:

+ When moving nodes (updates bounds and depth for whole subbranches).

+ While looking for rightmost node (at every node creation) which may
result in some error, mainly if it gets updated while doing this.

+ When pruning whole subtrees.

src/Baum/Move.php

@@ -107,10 +107,7 @@ public function perform() {
 
       $this->target->reload();
 
-      $this->node->setDepth();
-
-      foreach($this->node->getDescendants() as $descendant)
-        $descendant->save();
+      $this->node->setDepthWithSubtree();
 
       $this->node->reload();
     }
@@ -129,6 +126,9 @@ public function perform() {
   public function updateStructure() {
     list($a, $b, $c, $d) = $this->boundaries();
 
+    // select the rows between the leftmost & the rightmost boundaries and apply a lock
+    $this->applyLockBetween($a, $d);
+
     $connection = $this->node->getConnection();
     $grammar    = $connection->getQueryGrammar();
 
@@ -348,4 +348,19 @@ protected function quoteIdentifier($value) {
     return $pdo->quote($value);
   }
 
+  /**
+   * Applies a lock to the rows between the supplied index boundaries.
+   *
+   * @param   int   $lft
+   * @param   int   $rgt
+   * @return  void
+   */
+  protected function applyLockBetween($lft, $rgt) {
+    $this->node->newQuery()
+      ->where($this->node->getLeftColumnName(), '>=', $lft)
+      ->where($this->node->getRightColumnName(), '<=', $rgt)
+      ->select($this->node->getKeyName())
+      ->lockForUpdate()
+      ->get();
+  }
 }

src/Baum/Node.php

@@ -974,7 +974,7 @@ public function insideSubtree($node) {
    * @return void
    */
   public function setDefaultLeftAndRight() {
-    $withHighestRight = $this->newNestedSetQuery()->reOrderBy($this->getRightColumnName(), 'desc')->take(1)->first();
+    $withHighestRight = $this->newNestedSetQuery()->reOrderBy($this->getRightColumnName(), 'desc')->take(1)->sharedLock()->first();
 
     $maxRgt = 0;
     if ( !is_null($withHighestRight) ) $maxRgt = $withHighestRight->getRight();
@@ -1030,6 +1030,34 @@ public function setDepth() {
     return $this;
   }
 
+  /**
+   * Sets the depth attribute for the current node and all of its descendants.
+   *
+   * @return \Baum\Node
+   */
+  public function setDepthWithSubtree() {
+    $self = $this;
+
+    $this->getConnection()->transaction(function() use ($self) {
+      $self->reload();
+
+      $self->descendantsAndSelf()->select($self->getKeyName())->lockForUpdate()->get();
+
+      $oldDepth = !is_null($self->getDepth()) ?: 0;
+
+      $newDepth = $self->getLevel();
+
+      $self->newNestedSetQuery()->where($self->getKeyName(), '=', $self->getKey())->update(array($self->getDepthColumnName() => $newDepth));
+      $self->setAttribute($self->getDepthColumnName(), $newDepth);
+
+      $diff = $newDepth - $oldDepth;
+      if ( !$self->isLeaf() && $diff != 0 )
+        $self->descendants()->increment($self->getDepthColumnName(), $diff);
+    });
+
+    return $this;
+  }
+
   /**
    * Prunes a branch off the tree, shifting all the elements on the right
    * back to the left so the counts work.
@@ -1049,6 +1077,9 @@ public function destroyDescendants() {
       $lft    = $self->getLeft();
       $rgt    = $self->getRight();
 
+      // Apply a lock to the rows which fall past the deletion point
+      $self->newNestedSetQuery()->where($lftCol, '>=', $lft)->select($self->getKeyName())->lockForUpdate()->get();
+
       // Prune children
       $self->newNestedSetQuery()->where($lftCol, '>', $lft)->where($rgtCol, '<', $rgt)->delete();