[PATCH v2 2/2] recovery: fix incorrect handling of empty-body requests.

[Serge Petrenko <sergepetrenko@tarantool.org>]

In some cases no-ops are written to xlog. They have no effect but are
needed to bump lsn.
Some time ago (see commit 89e5b7846c9de968f8250ad03a128fc3b048271f)
such ops were made bodiless, and empty body requests are not handled in xrow_header_decode(). This leads to recovery errors in special case: when
we have a multi-statement transaction containing no-ops written to xlog,
upon recovering from such xlog, all data after the no-op end till the start
of new transaction will become no-op's body, so, effectively, it will be
ignored. Here's example `tarantoolctl cat` output showing this
(BODY contains next request data):
```
    ---
    HEADER:
      lsn: 5
      replica_id: 1
      type: NOP
      timestamp: 1536656270.5092
    BODY:
      type: 3
      timestamp: 1536656270.5092
      lsn: 6
      replica_id: 1
    ---
    HEADER:
      type: 0
    ...
```
This patch handles no-ops correctly in xrow_header_decode().

Closes #3678
---
 src/box/xrow.c                   |  3 ++-
 test/box/before_replace.result   | 25 ++++++++++++++++++++++---
 test/box/before_replace.test.lua | 14 +++++++++++---
 3 files changed, 35 insertions(+), 7 deletions(-)

diff --git a/src/box/xrow.c b/src/box/xrow.c
index 7a35d0db1..4473acfe3 100644
--- a/src/box/xrow.c
+++ b/src/box/xrow.c
@@ -135,7 +135,8 @@ error:
 		}
 	}
 	assert(*pos <= end);
-	if (*pos < end) {
+	/* Nop requests aren't supposed to have a body. */
+	if (*pos < end && header->type != IPROTO_NOP) {
 		const char *body = *pos;
 		if (mp_check(pos, end)) {
 			diag_set(ClientError, ER_INVALID_MSGPACK, "packet body");
diff --git a/test/box/before_replace.result b/test/box/before_replace.result
index cd0a3be1b..0adab4dd6 100644
--- a/test/box/before_replace.result
+++ b/test/box/before_replace.result
@@ -640,14 +640,17 @@ fio = require('fio')
 xlog = require('xlog')
 ---
 ...
-type(s:before_replace(function(old, new) return old end))
+f = function(old, new) return old end
+---
+...
+type(s:before_replace(f))
 ---
 - function
 ...
-s:insert{1, 1}
+box.begin() s:insert{1, 1} s:replace{1,2} s:before_replace(nil, f) s:replace{1,3} box.commit()
 ---
 ...
-path = fio.pathjoin(box.cfg.wal_dir, string.format('%020d.xlog', box.info.lsn - 1))
+path = fio.pathjoin(box.cfg.wal_dir, string.format('%020d.xlog', box.info.lsn - 3))
 ---
 ...
 fun, param, state = xlog.pairs(path)
@@ -664,6 +667,22 @@ row.BODY == nil
 ---
 - true
 ...
+-- gh-3678 recovery after IPROTO_NOP in a multi-statement transaction:
+test_run:cmd('restart server default')
+s = box.space.test
+---
+...
+s:select()
+---
+- - [1, 3]
+...
+s:truncate()
+---
+...
+type(s:before_replace(function(old, new) return old end))
+---
+- function
+...
 -- gh-3128 before_replace with run_triggers
 s2 = box.schema.space.create("test2")
 ---
diff --git a/test/box/before_replace.test.lua b/test/box/before_replace.test.lua
index dd4643844..919d2ccbb 100644
--- a/test/box/before_replace.test.lua
+++ b/test/box/before_replace.test.lua
@@ -214,15 +214,23 @@ s:select()
 fio = require('fio')
 xlog = require('xlog')
 
-type(s:before_replace(function(old, new) return old end))
-s:insert{1, 1}
+f = function(old, new) return old end
+type(s:before_replace(f))
+box.begin() s:insert{1, 1} s:replace{1,2} s:before_replace(nil, f) s:replace{1,3} box.commit()
 
-path = fio.pathjoin(box.cfg.wal_dir, string.format('%020d.xlog', box.info.lsn - 1))
+path = fio.pathjoin(box.cfg.wal_dir, string.format('%020d.xlog', box.info.lsn - 3))
 fun, param, state = xlog.pairs(path)
 state, row = fun(param, state)
 row.HEADER.type
 row.BODY == nil
 
+-- gh-3678 recovery after IPROTO_NOP in a multi-statement transaction:
+test_run:cmd('restart server default')
+s = box.space.test
+s:select()
+s:truncate()
+type(s:before_replace(function(old, new) return old end))
+
 -- gh-3128 before_replace with run_triggers
 s2 = box.schema.space.create("test2")
 _ = s2:create_index("prim")
-- 
2.17.1 (Apple Git-112)