Fix query-cancel handling in spgdoinsert().

Knowing that a buggy opclass could cause an infinite insertion loop,
spgdoinsert() intended to allow its loop to be interrupted by query
cancel.  However, that never actually worked, because in iterations
after the first, we'd be holding buffer lock(s) which would cause
InterruptHoldoffCount to be positive, preventing servicing of the
interrupt.

To fix, check if an interrupt is pending, and if so fall out of
the insertion loop and service the interrupt after we've released
the buffers.  If it was indeed a query cancel, that's the end of
the matter.  If it was a non-canceling interrupt reason, make use
of the existing provision to retry the whole insertion.  (This isn't
as wasteful as it might seem, since any upper-level index tuples we
already created should be usable in the next attempt.)

While there's no known instance of such a bug in existing release
branches, it still seems like a good idea to back-patch this to
all supported branches, since the behavior is fairly nasty if a
loop does happen --- not only is it uncancelable, but it will
quickly consume memory to the point of an OOM failure.  In any
case, this code is certainly not working as intended.

Per report from Dilip Kumar.

Discussion: https://postgr.es/m/CAFiTN-uxP_soPhVG840tRMQTBmtA_f_Y8N51G7DKYYqDh7XN-A@mail.gmail.com

src/backend/access/spgist/spgdoinsert.c

@@ -1905,13 +1905,14 @@ spgSplitNodeAction(Relation index, SpGistState *state,
  * Insert one item into the index.
  *
  * Returns true on success, false if we failed to complete the insertion
- * because of conflict with a concurrent insert.  In the latter case,
- * caller should re-call spgdoinsert() with the same args.
+ * (typically because of conflict with a concurrent insert).  In the latter
+ * case, caller should re-call spgdoinsert() with the same args.
  */
 bool
 spgdoinsert(Relation index, SpGistState *state,
 			ItemPointer heapPtr, Datum *datums, bool *isnulls)
 {
+	bool		result = true;
 	TupleDesc	leafDescriptor = state->leafTupDesc;
 	bool		isnull = isnulls[spgKeyColumn];
 	int			level = 0;
@@ -2012,6 +2013,14 @@ spgdoinsert(Relation index, SpGistState *state,
 	parent.offnum = InvalidOffsetNumber;
 	parent.node = -1;
 
+	/*
+	 * Before entering the loop, try to clear any pending interrupt condition.
+	 * If a query cancel is pending, we might as well accept it now not later;
+	 * while if a non-canceling condition is pending, servicing it here avoids
+	 * having to restart the insertion and redo all the work so far.
+	 */
+	CHECK_FOR_INTERRUPTS();
+
 	for (;;)
 	{
 		bool		isNew = false;
@@ -2019,9 +2028,18 @@ spgdoinsert(Relation index, SpGistState *state,
 		/*
 		 * Bail out if query cancel is pending.  We must have this somewhere
 		 * in the loop since a broken opclass could produce an infinite
-		 * picksplit loop.
+		 * picksplit loop.  However, because we'll be holding buffer lock(s)
+		 * after the first iteration, ProcessInterrupts() wouldn't be able to
+		 * throw a cancel error here.  Hence, if we see that an interrupt is
+		 * pending, break out of the loop and deal with the situation below.
+		 * Set result = false because we must restart the insertion if the
+		 * interrupt isn't a query-cancel-or-die case.
 		 */
-		CHECK_FOR_INTERRUPTS();
+		if (INTERRUPTS_PENDING_CONDITION())
+		{
+			result = false;
+			break;
+		}
 
 		if (current.blkno == InvalidBlockNumber)
 		{
@@ -2140,10 +2158,14 @@ spgdoinsert(Relation index, SpGistState *state,
 			 * spgAddNode and spgSplitTuple cases will loop back to here to
 			 * complete the insertion operation.  Just in case the choose
 			 * function is broken and produces add or split requests
-			 * repeatedly, check for query cancel.
+			 * repeatedly, check for query cancel (see comments above).
 			 */
 	process_inner_tuple:
-			CHECK_FOR_INTERRUPTS();
+			if (INTERRUPTS_PENDING_CONDITION())
+			{
+				result = false;
+				break;
+			}
 
 			innerTuple = (SpGistInnerTuple) PageGetItem(current.page,
 														PageGetItemId(current.page, current.offnum));
@@ -2267,5 +2289,21 @@ spgdoinsert(Relation index, SpGistState *state,
 		UnlockReleaseBuffer(parent.buffer);
 	}
 
-	return true;
+	/*
+	 * We do not support being called while some outer function is holding a
+	 * buffer lock (or any other reason to postpone query cancels).  If that
+	 * were the case, telling the caller to retry would create an infinite
+	 * loop.
+	 */
+	Assert(INTERRUPTS_CAN_BE_PROCESSED());
+
+	/*
+	 * Finally, check for interrupts again.  If there was a query cancel,
+	 * ProcessInterrupts() will be able to throw the error here.  If it was
+	 * some other kind of interrupt that can just be cleared, return false to
+	 * tell our caller to retry.
+	 */
+	CHECK_FOR_INTERRUPTS();
+
+	return result;
 }