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 d23100be1072b37dd4410f234090919769359b42..24458c564c6286b59748e8bcf6c70e5b7564d35a 100644 (file)
--- a/src/backend/access/gin/ginget.c
+++ b/src/backend/access/gin/ginget.c
@@ -1315,6 +1315,8 @@ scanGetItem(IndexScanDesc scan, ItemPointerData advancePast,
         */
        do
        {
+               CHECK_FOR_INTERRUPTS();
+
                ItemPointerSetMin(item);
                match = true;
                for (i = 0; i < so->nkeys && match; i++)
@@ -1958,8 +1960,6 @@ gingetbitmap(IndexScanDesc scan, TIDBitmap *tbm)
 
        for (;;)
        {
-               CHECK_FOR_INTERRUPTS();
-
                if (!scanGetItem(scan, iptr, &iptr, &recheck))
                        break;