[PATCH v2] amcheck: Allow interrupting the child-level rightlink walk

bt_child_highkey_check() walks right along the child level following
btpo_next links, reading a page on each iteration, but its loop lacked a
CHECK_FOR_INTERRUPTS(). Every other page-traversal loop in
verify_nbtree.c already has one.

The loop's existing checks do not guarantee a timely exit on a corrupt
index. The in-loop cycle check only fires when a rightlink points back
through the block the walk started from, or to a page whose btpo_prev
points to itself, so a cycle further downstream can go undetected -- for
example a run of pages all flagged P_INCOMPLETE_SPLIT, which skips both
the high-key comparison and bt_downlink_missing_check() and so never
reaches an error. Such an index, or simply a very long rightlink chain,
makes the walk effectively uninterruptible. Since amcheck exists to be
run against possibly-corrupt indexes, make this walk respond to query
cancellation and shutdown requests like the sibling loops do.
---
v2: only the commit message changed -- v1's description of the in-loop
cycle check was imprecise. The code (the one-line CHECK_FOR_INTERRUPTS)
is unchanged.

This is the same shape as commit 191dce109be ("contrib/amcheck: Add
heapam CHECK_FOR_INTERRUPTS()"), which was backpatched.
bt_child_highkey_check() exists in all supported branches, so this looks
like a backpatch candidate as well.

contrib/amcheck/verify_nbtree.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index 3ef2d66f826..486bd7a87cc 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -2191,6 +2191,8 @@ bt_child_highkey_check(BtreeCheckState *state,
/* Move to the right on the child level */
while (true)
{
+ CHECK_FOR_INTERRUPTS();
+
/*
* Did we traverse the whole tree level and this is check for pages to
* the right of rightmost downlink?
--
2.50.1 (Apple Git-155)