Re: [Tarantool-patches] [PATCH v2 2/9] xrow: introduce a PROMOTE entry

[Cyrill Gorcunov via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

On Mon, Apr 12, 2021 at 10:40:15PM +0300, Serge Petrenko wrote:
> diff --git a/src/box/xrow.c b/src/box/xrow.c
> index cc8e43ed4..70ba075f8 100644
> --- a/src/box/xrow.c
> +++ b/src/box/xrow.c
> @@ -890,11 +890,11 @@ xrow_encode_synchro(struct xrow_header *row,
>  		    const struct synchro_request *req)
>  {
>  	/*
> -	 * A map with two elements. We don't compress
> +	 * A map with two or three elements. We don't compress
>  	 * numbers to have this structure constant in size,
>  	 * which allows us to preallocate it on stack.
>  	 */
> -	body->m_body = 0x80 | 2;
> +	body->m_body = 0x80 | (req->type == IPROTO_PROMOTE ? 3 : 2);
>  	body->k_replica_id = IPROTO_REPLICA_ID;
>  	body->m_replica_id = 0xce;
>  	body->v_replica_id = mp_bswap_u32(req->replica_id);
> @@ -903,10 +903,24 @@ xrow_encode_synchro(struct xrow_header *row,
>  	body->v_lsn = mp_bswap_u64(req->lsn);
>  
>  	memset(row, 0, sizeof(*row));
> -
>  	row->type = req->type;
> -	row->body[0].iov_base = (void *)body;
> -	row->body[0].iov_len = sizeof(*body);
> +
> +	/* Promote body is longer. It has an additional IPROTO_TERM field. */
> +	if (req->type == IPROTO_PROMOTE) {
> +		struct promote_body_bin *promote_body =
> +			(struct promote_body_bin *)body;
> +
> +		promote_body->k_term = IPROTO_TERM;
> +		promote_body->m_term = 0xcf;
> +		promote_body->v_term = mp_bswap_u64(req->term);
> +
> +		row->body[0].iov_base = (void *)promote_body;
> +		row->body[0].iov_len = sizeof(*promote_body);
> +	} else {
> +		row->body[0].iov_base = (void *)body;
> +		row->body[0].iov_len = sizeof(*body);
> +	}
> +
>  	row->bodycnt = 1;
>  }

You know, while I understand that we're trying to reuse code flow
here but I really don't like that this function unaware of type passed.
IOW the function may easily overwire caller's stack if you occasionally
pass synchro_body_bin instead of promote request.

Actually I've sevaral options:

1) make the caller to provide a size and use assert() inside
   this encoder to make sure the caller passer proper amount
   of data from stack;
2) provide own helper for promote packet encoding (see below).

Still both approaches somehow *ugly* I think. Since there only a
few use of this encodings it is easy to remember where and what
and don't make a mistake.

So lets leave it as it now.
---
diff --git a/src/box/txn_limbo.c b/src/box/txn_limbo.c
index f119c35b6..87fa220ea 100644
--- a/src/box/txn_limbo.c
+++ b/src/box/txn_limbo.c
@@ -333,8 +333,7 @@ txn_limbo_write_synchro(struct txn_limbo *limbo, uint32_t type, int64_t lsn,
 	 * This is a synchronous commit so we can allocate everything on a
 	 * stack. Promote body includes synchro body.
 	 */
-	struct promote_body_bin body;
-	struct synchro_body_bin *base = &body.base;
+	struct promote_body_bin body_bin;
 
 	struct xrow_header row;
 	char buf[sizeof(struct journal_entry) +
@@ -343,7 +342,7 @@ txn_limbo_write_synchro(struct txn_limbo *limbo, uint32_t type, int64_t lsn,
 	struct journal_entry *entry = (struct journal_entry *)buf;
 	entry->rows[0] = &row;
 
-	xrow_encode_synchro(&row, base, &req);
+	xrow_encode_promote(&row, &body_bin, &req);
 
 	journal_entry_create(entry, 1, xrow_approx_len(&row),
 			     txn_limbo_write_cb, fiber());
diff --git a/src/box/xrow.c b/src/box/xrow.c
index 70ba075f8..df76b104b 100644
--- a/src/box/xrow.c
+++ b/src/box/xrow.c
@@ -884,16 +884,11 @@ xrow_encode_dml(const struct request *request, struct region *region,
 	return iovcnt;
 }
 
-void
-xrow_encode_synchro(struct xrow_header *row,
-		    struct synchro_body_bin *body,
-		    const struct synchro_request *req)
+static void
+xrow_encode_synchro_common(struct xrow_header *row,
+			   struct synchro_body_bin *body,
+			   const struct synchro_request *req)
 {
-	/*
-	 * A map with two or three elements. We don't compress
-	 * numbers to have this structure constant in size,
-	 * which allows us to preallocate it on stack.
-	 */
 	body->m_body = 0x80 | (req->type == IPROTO_PROMOTE ? 3 : 2);
 	body->k_replica_id = IPROTO_REPLICA_ID;
 	body->m_replica_id = 0xce;
@@ -904,24 +899,32 @@ xrow_encode_synchro(struct xrow_header *row,
 
 	memset(row, 0, sizeof(*row));
 	row->type = req->type;
+	row->bodycnt = 1;
+}
 
-	/* Promote body is longer. It has an additional IPROTO_TERM field. */
-	if (req->type == IPROTO_PROMOTE) {
-		struct promote_body_bin *promote_body =
-			(struct promote_body_bin *)body;
+void
+xrow_encode_promote(struct xrow_header *row, struct promote_body_bin *body,
+		    const struct synchro_request *req)
+{
+	xrow_encode_synchro_common(row, body, req);
 
-		promote_body->k_term = IPROTO_TERM;
-		promote_body->m_term = 0xcf;
-		promote_body->v_term = mp_bswap_u64(req->term);
+	promote_body->k_term = IPROTO_TERM;
+	promote_body->m_term = 0xcf;
+	promote_body->v_term = mp_bswap_u64(req->term);
 
-		row->body[0].iov_base = (void *)promote_body;
-		row->body[0].iov_len = sizeof(*promote_body);
-	} else {
-		row->body[0].iov_base = (void *)body;
-		row->body[0].iov_len = sizeof(*body);
-	}
+	row->body[0].iov_base = body;
+	row->body[0].iov_len = sizeof(*body);
+}
+
+void
+xrow_encode_synchro(struct xrow_header *row, struct synchro_body_bin *body,
+		    const struct synchro_request *req)
+{
+	xrow_encode_synchro_common(row, body, req);
+
+	row->body[0].iov_base = body;
+	row->body[0].iov_len = sizeof(*body);
 
-	row->bodycnt = 1;
 }
 
 int