[server 3/4] iproto: add IPROTO_NOP request type

[Vladimir Davydov]

To implement space:before_replace trigger, we need to introduce a new
request type for bumping LSN, because the new trigger may turn any DML
operation into a no-op. Let's call it IPROTO_NOP. It is treated as DML
(passed to apply_row, etc), but it is ignored by space_execute_dml() and
so doesn't actually modify anyting, only bumps LSN on the server. The
new request type has name "NOP" (for xlog reader), however it isn't
reported via box.stat().

Needed for #2993
---
 src/box/iproto.cc          |  3 ++-
 src/box/iproto_constants.c |  5 ++++-
 src/box/iproto_constants.h | 11 ++++++++++-
 3 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/src/box/iproto.cc b/src/box/iproto.cc
index 13594a46..5dfb2b83 100644
--- a/src/box/iproto.cc
+++ b/src/box/iproto.cc
@@ -913,7 +913,8 @@ static const struct cmsg_hop *dml_route[IPROTO_TYPE_STAT_MAX] = {
 	misc_route,                             /* IPROTO_AUTH */
 	call_route,                             /* IPROTO_EVAL */
 	process1_route,                         /* IPROTO_UPSERT */
-	call_route                              /* IPROTO_CALL */
+	call_route,                             /* IPROTO_CALL */
+	NULL,                                   /* IPROTO_NOP */
 };
 
 static const struct cmsg_hop join_route[] = {
diff --git a/src/box/iproto_constants.c b/src/box/iproto_constants.c
index ff7a755f..7710196d 100644
--- a/src/box/iproto_constants.c
+++ b/src/box/iproto_constants.c
@@ -101,7 +101,8 @@ const char *iproto_type_strs[] =
 	"AUTH",
 	"EVAL",
 	"UPSERT",
-	"CALL"
+	"CALL",
+	NULL, /* NOP */
 };
 
 #define bit(c) (1ULL<<IPROTO_##c)
@@ -116,6 +117,8 @@ const uint64_t iproto_body_key_map[IPROTO_TYPE_STAT_MAX] = {
 	0,                                                     /* AUTH */
 	0,                                                     /* EVAL */
 	bit(SPACE_ID) | bit(OPS) | bit(TUPLE),                 /* UPSERT */
+	0,                                                     /* CALL */
+	bit(SPACE_ID),                                         /* NOP */
 };
 #undef bit
 
diff --git a/src/box/iproto_constants.h b/src/box/iproto_constants.h
index e1192e91..94b111ad 100644
--- a/src/box/iproto_constants.h
+++ b/src/box/iproto_constants.h
@@ -141,6 +141,8 @@ enum iproto_type {
 	IPROTO_UPSERT = 9,
 	/** CALL request - returns arbitrary MessagePack */
 	IPROTO_CALL = 10,
+	/** No operation. Treated as DML, used to bump LSN. */
+	IPROTO_NOP = 11,
 	/** The maximum typecode used for box.stat() */
 	IPROTO_TYPE_STAT_MAX,
 
@@ -174,6 +176,13 @@ extern const char *iproto_type_strs[];
 static inline const char *
 iproto_type_name(uint32_t type)
 {
+	/*
+	 * Sic: iptoto_type_strs[IPROTO_NOP] is NULL
+	 * to suppress box.stat() output.
+	 */
+	if (type == IPROTO_NOP)
+		return "NOP";
+
 	if (type < IPROTO_TYPE_STAT_MAX)
 		return iproto_type_strs[type];
 
@@ -207,7 +216,7 @@ static inline bool
 iproto_type_is_dml(uint32_t type)
 {
 	return (type >= IPROTO_SELECT && type <= IPROTO_DELETE) ||
-		type == IPROTO_UPSERT;
+		type == IPROTO_UPSERT || type == IPROTO_NOP;
 }
 
 /**
-- 
2.11.0