Do CHECK_FOR_INTERRUPTS inside, not before, scanGetItem.

The CHECK_FOR_INTERRUPTS call in gingetbitmap turns out to be
inadequate to prevent a long uninterruptible loop, because
we now know a case where looping occurs within scanGetItem.
While the next patch will fix the bug that caused that, it
seems foolish to assume that no similar patterns are possible.
Let's do the CFI within scanGetItem's retry loop, instead.
This demonstrably allows canceling out of the loop exhibited
in bug #19031.

Bug: #19031
Reported-by: Tim Wood <washwithcare@gmail.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/19031-0638148643d25548@postgresql.org
Backpatch-through: 13

diff --git a/src/backend/access/gin/ginget.c b/src/backend/access/gin/ginget.c
index 84d3a8f11207bdb17835fcb4588349b75563f98d..2422e991521ccb377696f018760e280b7f2b9b46 100644 (file)
--- a/src/backend/access/gin/ginget.c
+++ b/src/backend/access/gin/ginget.c
@@ -1316,6 +1316,8 @@ scanGetItem(IndexScanDesc scan, ItemPointerData advancePast,
     */
    do
    {
+       CHECK_FOR_INTERRUPTS();
+
        ItemPointerSetMin(item);
        match = true;
        for (i = 0; i < so->nkeys && match; i++)
@@ -1959,8 +1961,6 @@ gingetbitmap(IndexScanDesc scan, TIDBitmap *tbm)
 
    for (;;)
    {
-       CHECK_FOR_INTERRUPTS();
-
        if (!scanGetItem(scan, iptr, &iptr, &recheck))
            break;