Fix error message if PS connection is lost while receiving prefetch

If the pageserver connection is lost while receiving the prefetch
request, the prefetch queue is cleared. The error message prints the
values from the prefetch slot, but because the slot was already
cleared, they're all zeros:

    LOG:  [NEON_SMGR] [shard 0] No response from reading prefetch entry 0: 0/0/0.0 block 0. This can be caused by a concurrent disconnect

To fix, make local copies of the values.

In the passing, also add a sanity check that if the receive() call
succeeds, the prefetch slot is still intact.

pgxn/neon/pagestore_smgr.c

@@ -604,6 +604,9 @@ prefetch_read(PrefetchRequest *slot)
 {
 	NeonResponse *response;
 	MemoryContext old;
+	BufferTag	buftag;
+	shardno_t	shard_no;
+	uint64		my_ring_index;
 
 	Assert(slot->status == PRFS_REQUESTED);
 	Assert(slot->response == NULL);
@@ -617,11 +620,29 @@ prefetch_read(PrefetchRequest *slot)
 					   slot->status, slot->response,
 					   (long)slot->my_ring_index, (long)MyPState->ring_receive);
 
+	/*
+	 * Copy the request info so that if an error happens and the prefetch
+	 * queue is flushed during the receive call, we can print the original
+	 * values in the error message
+	 */
+	buftag = slot->buftag;
+	shard_no = slot->shard_no;
+	my_ring_index = slot->my_ring_index;
+
 	old = MemoryContextSwitchTo(MyPState->errctx);
-	response = (NeonResponse *) page_server->receive(slot->shard_no);
+	response = (NeonResponse *) page_server->receive(shard_no);
 	MemoryContextSwitchTo(old);
 	if (response)
 	{
+		/* The slot should still be valid */
+		if (slot->status != PRFS_REQUESTED ||
+			slot->response != NULL ||
+			slot->my_ring_index != MyPState->ring_receive)
+			neon_shard_log(shard_no, ERROR,
+						   "Incorrect prefetch slot state after receive: status=%d response=%p my=%lu receive=%lu",
+						   slot->status, slot->response,
+						   (long) slot->my_ring_index, (long) MyPState->ring_receive);
+
 		/* update prefetch state */
 		MyPState->n_responses_buffered += 1;
 		MyPState->n_requests_inflight -= 1;
@@ -636,11 +657,15 @@ prefetch_read(PrefetchRequest *slot)
 	}
 	else
 	{
-		neon_shard_log(slot->shard_no, LOG,
+		/*
+		 * Note: The slot might no longer be valid, if the connection was lost
+		 * and the prefetch queue was flushed during the receive call
+		 */
+		neon_shard_log(shard_no, LOG,
 					   "No response from reading prefetch entry %lu: %u/%u/%u.%u block %u. This can be caused by a concurrent disconnect",
-					   (long)slot->my_ring_index,
-					   RelFileInfoFmt(BufTagGetNRelFileInfo(slot->buftag)),
-					   slot->buftag.forkNum, slot->buftag.blockNum);
+					   (long) my_ring_index,
+					   RelFileInfoFmt(BufTagGetNRelFileInfo(buftag)),
+					   buftag.forkNum, buftag.blockNum);
 		return false;
 	}
 }