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 17601bb56b270018c34d6c91bb65f63ceae597cb..77d4a086dd1d1db34f46cf7bf7b0aff813bd4a5e 100644 (file)
--- a/src/backend/access/gin/ginget.c
+++ b/src/backend/access/gin/ginget.c
@@ -1314,6 +1314,8 @@ scanGetItem(IndexScanDesc scan, ItemPointerData advancePast,
     */
    do
    {
+       CHECK_FOR_INTERRUPTS();
+
        ItemPointerSetMin(item);
        match = true;
        for (i = 0; i < so->nkeys && match; i++)
@@ -1953,8 +1955,6 @@ gingetbitmap(IndexScanDesc scan, TIDBitmap *tbm)
 
    for (;;)
    {
-       CHECK_FOR_INTERRUPTS();
-
        if (!scanGetItem(scan, iptr, &iptr, &recheck))
            break;