Fix another minor page deletion buffer lock issue.

Avoid accessing the leaf page's top parent tuple without a buffer lock
held during the second phase of nbtree page deletion.  The old approach
was safe, though only because VACUUM never drops its buffer pin (and
because only VACUUM itself can modify a half-dead page).  Even still, it
seems like a good idea to be strict here.  Tighten things up by copying
the top parent page's block number to a local variable before releasing
the buffer lock on the leaf page -- not after.

This is a follow-up to commit fa7ff642, which fixed a similar issue in
the first phase of nbtree page deletion.

Update some related comments in passing.

Discussion: https://postgr.es/m/CAH2-WzkLgyN3zBvRZ1pkNJThC=xi_0gpWRUb_45eexLH1+k2_Q@mail.gmail.com

src/backend/access/nbtree/nbtpage.c

@@ -1951,6 +1951,7 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, bool *rightsib_empty)
 	 */
 	itemid = PageGetItemId(page, P_HIKEY);
 	leafhikey = (IndexTuple) PageGetItem(page, itemid);
+	target = BTreeTupleGetTopParent(leafhikey);
 	leafleftsib = opaque->btpo_prev;
 	leafrightsib = opaque->btpo_next;
 
@@ -1965,21 +1966,29 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, bool *rightsib_empty)
 	/*
 	 * If the leaf page still has a parent pointing to it (or a chain of
 	 * parents), we don't unlink the leaf page yet, but the topmost remaining
-	 * parent in the branch.  Set 'target' and 'buf' to reference the page
+	 * parent in the branch (i.e. the "top parent")
-	 * actually being unlinked.
 	 */
-	target = BTreeTupleGetTopParent(leafhikey);
+	if (!BlockNumberIsValid(target))
+	{
+		/* No top parent, so target is leaf page */
+		target = leafblkno;
 
-	if (target != InvalidBlockNumber)
+		buf = leafbuf;
+		leftsib = leafleftsib;
+		targetlevel = 0;
+	}
+	else
 	{
+		/* Target is the internal page taken from leaf's top parent */
 		Assert(target != leafblkno);
 
-		/* fetch the block number of the topmost parent's left sibling */
+		/* Fetch the block number of the target's left sibling */
 		buf = _bt_getbuf(rel, target, BT_READ);
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		leftsib = opaque->btpo_prev;
 		targetlevel = opaque->btpo.level;
+		Assert(targetlevel > 0);
 
 		/*
 		 * To avoid deadlocks, we'd better drop the target page lock before
@@ -1987,14 +1996,6 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, bool *rightsib_empty)
 		 */
 		LockBuffer(buf, BUFFER_LOCK_UNLOCK);
 	}
-	else
-	{
-		target = leafblkno;
-
-		buf = leafbuf;
-		leftsib = leafleftsib;
-		targetlevel = 0;
-	}
 
 	/*
 	 * We have to lock the pages we need to modify in the standard order:
@@ -2181,6 +2182,12 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, bool *rightsib_empty)
 	 * If we deleted a parent of the targeted leaf page, instead of the leaf
 	 * itself, update the leaf to point to the next remaining child in the
 	 * branch.
+	 *
+	 * Note: We rely on the fact that a buffer pin on the leaf page has been
+	 * held since leafhikey was initialized.  This is safe, though only
+	 * because the page was already half-dead at that point.  The leaf page
+	 * cannot have been modified by any other backend during the period when
+	 * no lock was held.
 	 */
 	if (target != leafblkno)
 		BTreeTupleSetTopParent(leafhikey, nextchild);