[PATCH] xrow: print corrupted rows on decoding error.

[PATCH] xrow: print corrupted rows on decoding error.

[Serge Petrenko]

Add row hex printing to log on verbose level. This would be useful
during investigation of errors related to invalid msgpack packet
arrival.

Related to #4040
---
https://github.com/tarantool/tarantool/tree/sp/xrow-decode-verbose-err


 src/box/xrow.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/src/box/xrow.c b/src/box/xrow.c
index bddae1d5b..8a5254902 100644
--- a/src/box/xrow.c
+++ b/src/box/xrow.c
@@ -88,14 +88,29 @@ mp_decode_vclock(const char **data, struct vclock *vclock)
 	return 0;
 }
 
+static inline void
+say_dump_row_hex(const char *start, const char *end)
+{
+	char *buf = (char *)malloc(3 * (end - start) + 1);
+	char *pos = buf;
+	for (const char *cur = start; cur < end; ++cur) {
+		pos += sprintf(pos, "%02hhX ", *cur);
+	}
+	*pos = 0;
+	say_verbose("Corrupted row is: %s", buf);
+	free(buf);
+}
+
 int
 xrow_header_decode(struct xrow_header *header, const char **pos,
 		   const char *end, bool end_is_exact)
 {
 	memset(header, 0, sizeof(struct xrow_header));
 	const char *tmp = *pos;
+	const char * const start = *pos;
 	if (mp_check(&tmp, end) != 0) {
 error:
+		say_dump_row_hex(start, end);
 		diag_set(ClientError, ER_INVALID_MSGPACK, "packet header");
 		return -1;
 	}
@@ -163,6 +178,7 @@ error:
 	if (*pos < end && header->type != IPROTO_NOP) {
 		const char *body = *pos;
 		if (mp_check(pos, end)) {
+			say_dump_row_hex(start, end);
 			diag_set(ClientError, ER_INVALID_MSGPACK, "packet body");
 			return -1;
 		}
@@ -171,6 +187,7 @@ error:
 		header->body[0].iov_len = *pos - body;
 	}
 	if (end_is_exact && *pos < end) {
+		say_dump_row_hex(start,end);
 		diag_set(ClientError, ER_INVALID_MSGPACK, "packet body");
 		return -1;
 	}
@@ -532,6 +549,7 @@ xrow_decode_sql(const struct xrow_header *row, struct sql_request *request)
 
 	if (mp_typeof(*data) != MP_MAP || mp_check_map(data, end) > 0) {
 error:
+		say_dump_row_hex(row->body[0].iov_base, end);
 		diag_set(ClientError, ER_INVALID_MSGPACK, "packet body");
 		return -1;
 	}
@@ -603,6 +621,7 @@ xrow_decode_dml(struct xrow_header *row, struct request *request,
 
 	if (mp_typeof(*data) != MP_MAP || mp_check_map(data, end) > 0) {
 error:
+		say_dump_row_hex(row->body[0].iov_base, end);
 		diag_set(ClientError, ER_INVALID_MSGPACK, "packet body");
 		return -1;
 	}
@@ -662,11 +681,13 @@ error:
 		}
 	}
 	if (data != end) {
+		say_dump_row_hex(row->body[0].iov_base, end);
 		diag_set(ClientError, ER_INVALID_MSGPACK, "packet end");
 		return -1;
 	}
 done:
 	if (key_map) {
+		say_dump_row_hex(row->body[0].iov_base, end);
 		enum iproto_key key = (enum iproto_key) bit_ctz_u64(key_map);
 		diag_set(ClientError, ER_MISSING_REQUEST_FIELD,
 			 iproto_key_name(key));
@@ -809,6 +830,7 @@ xrow_decode_call(const struct xrow_header *row, struct call_request *request)
 
 	if (mp_typeof(*data) != MP_MAP || mp_check_map(data, end) > 0) {
 error:
+		say_dump_row_hex(row->body[0].iov_base, end);
 		diag_set(ClientError, ER_INVALID_MSGPACK, "packet body");
 		return -1;
 	}
@@ -848,6 +870,7 @@ error:
 		}
 	}
 	if (data != end) {
+		say_dump_row_hex(row->body[0].iov_base, end);
 		diag_set(ClientError, ER_INVALID_MSGPACK, "packet end");
 		return -1;
 	}
@@ -888,6 +911,7 @@ xrow_decode_auth(const struct xrow_header *row, struct auth_request *request)
 
 	if (mp_typeof(*data) != MP_MAP || mp_check_map(data, end) > 0) {
 error:
+		say_dump_row_hex(row->body[0].iov_base, end);
 		diag_set(ClientError, ER_INVALID_MSGPACK, "packet body");
 		return -1;
 	}
@@ -920,6 +944,7 @@ error:
 		}
 	}
 	if (data != end) {
+		say_dump_row_hex(row->body[0].iov_base, end);
 		diag_set(ClientError, ER_INVALID_MSGPACK, "packet end");
 		return -1;
 	}
@@ -1080,6 +1105,7 @@ xrow_decode_ballot(struct xrow_header *row, struct ballot *ballot)
 	}
 	return 0;
 err:
+	say_dump_row_hex(row->body[0].iov_base, end);
 	diag_set(ClientError, ER_INVALID_MSGPACK, "packet body");
 	return -1;
 }
@@ -1129,6 +1155,7 @@ xrow_decode_subscribe(struct xrow_header *row, struct tt_uuid *replicaset_uuid,
 	const char *end = data + row->body[0].iov_len;
 	const char *d = data;
 	if (mp_check(&d, end) != 0 || mp_typeof(*data) != MP_MAP) {
+		say_dump_row_hex(row->body[0].iov_base, end);
 		diag_set(ClientError, ER_INVALID_MSGPACK, "request body");
 		return -1;
 	}
-- 
2.17.2 (Apple Git-113)

Re: [PATCH] xrow: print corrupted rows on decoding error.

[Vladimir Davydov]

On Fri, Mar 15, 2019 at 07:02:38PM +0300, Serge Petrenko wrote:
> Add row hex printing to log on verbose level. This would be useful
> during investigation of errors related to invalid msgpack packet
> arrival.
> 
> Related to #4040
> ---
> https://github.com/tarantool/tarantool/tree/sp/xrow-decode-verbose-err
> 
> 
>  src/box/xrow.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/src/box/xrow.c b/src/box/xrow.c
> index bddae1d5b..8a5254902 100644
> --- a/src/box/xrow.c
> +++ b/src/box/xrow.c
> @@ -88,14 +88,29 @@ mp_decode_vclock(const char **data, struct vclock *vclock)
>  	return 0;
>  }
>  
> +static inline void
> +say_dump_row_hex(const char *start, const char *end)
> +{
> +	char *buf = (char *)malloc(3 * (end - start) + 1);

Theoretically, malloc() may fail. You'd better check it and print
a warning in this case. Anyway, do we really need to use malloc()
here at all? Won't tt_static_buf be enough for typical cases.

> +	char *pos = buf;
> +	for (const char *cur = start; cur < end; ++cur) {
> +		pos += sprintf(pos, "%02hhX ", *cur);
> +	}
> +	*pos = 0;
> +	say_verbose("Corrupted row is: %s", buf);

say_verbose() will turn into no-op, but you allocate a buffer and format
the message anyway. Please fix that by using say_log_level_is_enabled()
before doing anything.

> +	free(buf);
> +}
> +
>  int
>  xrow_header_decode(struct xrow_header *header, const char **pos,
>  		   const char *end, bool end_is_exact)
>  {
>  	memset(header, 0, sizeof(struct xrow_header));
>  	const char *tmp = *pos;
> +	const char * const start = *pos;
>  	if (mp_check(&tmp, end) != 0) {
>  error:
> +		say_dump_row_hex(start, end);
>  		diag_set(ClientError, ER_INVALID_MSGPACK, "packet header");
>  		return -1;
>  	}
> @@ -163,6 +178,7 @@ error:
>  	if (*pos < end && header->type != IPROTO_NOP) {
>  		const char *body = *pos;
>  		if (mp_check(pos, end)) {
> +			say_dump_row_hex(start, end);
>  			diag_set(ClientError, ER_INVALID_MSGPACK, "packet body");
>  			return -1;
>  		}
> @@ -171,6 +187,7 @@ error:
>  		header->body[0].iov_len = *pos - body;
>  	}
>  	if (end_is_exact && *pos < end) {
> +		say_dump_row_hex(start,end);
>  		diag_set(ClientError, ER_INVALID_MSGPACK, "packet body");

It would be better to add a helper function that would set diag and
print a corrupted row so that we don't miss it in case we add another
point of failure. Dunno, may be it's even worth adding a new exception
class with logging built in.

Re: [tarantool-patches] [PATCH] xrow: print corrupted rows on decoding error.

[Serge Petrenko]

Hi!
Thankyou for review!
The fixes are on the branch, and an incremental diff is below.

> 18 марта 2019 г., в 20:58, Vladimir Davydov <vdavydov.dev@gmail.com> написал(а):
> 
> On Fri, Mar 15, 2019 at 07:02:38PM +0300, Serge Petrenko wrote:
>> Add row hex printing to log on verbose level. This would be useful
>> during investigation of errors related to invalid msgpack packet
>> arrival.
>> 
>> Related to #4040
>> ---
>> https://github.com/tarantool/tarantool/tree/sp/xrow-decode-verbose-err
>> 
>> 
>> src/box/xrow.c | 27 +++++++++++++++++++++++++++
>> 1 file changed, 27 insertions(+)
>> 
>> diff --git a/src/box/xrow.c b/src/box/xrow.c
>> index bddae1d5b..8a5254902 100644
>> --- a/src/box/xrow.c
>> +++ b/src/box/xrow.c
>> @@ -88,14 +88,29 @@ mp_decode_vclock(const char **data, struct vclock *vclock)
>> 	return 0;
>> }
>> 
>> +static inline void
>> +say_dump_row_hex(const char *start, const char *end)
>> +{
>> +	char *buf = (char *)malloc(3 * (end - start) + 1);
> 
> Theoretically, malloc() may fail. You'd better check it and print
> a warning in this case. Anyway, do we really need to use malloc()
> here at all? Won't tt_static_buf be enough for typical cases.

Done, please see the incremental diff below. I still use malloc for sizes greater than TT_STATIC_BUF_LEN,
but check for an allocation error.

> 
>> +	char *pos = buf;
>> +	for (const char *cur = start; cur < end; ++cur) {
>> +		pos += sprintf(pos, "%02hhX ", *cur);
>> +	}
>> +	*pos = 0;
>> +	say_verbose("Corrupted row is: %s", buf);
> 
> say_verbose() will turn into no-op, but you allocate a buffer and format
> the message anyway. Please fix that by using say_log_level_is_enabled()
> before doing anything.

Fixed.
> 
>> +	free(buf);
>> +}
>> +
>> int
>> xrow_header_decode(struct xrow_header *header, const char **pos,
>> 		   const char *end, bool end_is_exact)
>> {
>> 	memset(header, 0, sizeof(struct xrow_header));
>> 	const char *tmp = *pos;
>> +	const char * const start = *pos;
>> 	if (mp_check(&tmp, end) != 0) {
>> error:
>> +		say_dump_row_hex(start, end);
>> 		diag_set(ClientError, ER_INVALID_MSGPACK, "packet header");
>> 		return -1;
>> 	}
>> @@ -163,6 +178,7 @@ error:
>> 	if (*pos < end && header->type != IPROTO_NOP) {
>> 		const char *body = *pos;
>> 		if (mp_check(pos, end)) {
>> +			say_dump_row_hex(start, end);
>> 			diag_set(ClientError, ER_INVALID_MSGPACK, "packet body");
>> 			return -1;
>> 		}
>> @@ -171,6 +187,7 @@ error:
>> 		header->body[0].iov_len = *pos - body;
>> 	}
>> 	if (end_is_exact && *pos < end) {
>> +		say_dump_row_hex(start,end);
>> 		diag_set(ClientError, ER_INVALID_MSGPACK, "packet body");
> 
> It would be better to add a helper function that would set diag and
> print a corrupted row so that we don't miss it in case we add another
> point of failure. Dunno, may be it's even worth adding a new exception
> class with logging built in.

Done, please, see below.
I don’t like the idea with a new exception class.
If we introduce it, we’ll have to store the whole
corrupted row, which can be quite big, until logging happens.

diff --git a/src/box/xrow.c b/src/box/xrow.c
index 3854874cc..000647111 100644
--- a/src/box/xrow.c
+++ b/src/box/xrow.c
@@ -88,17 +88,44 @@ mp_decode_vclock(const char **data, struct vclock *vclock)
 	return 0;
 }
 
+/**
+ * Set diagnostics with an error and log the corrupted row
+ * which caused the error.
+ * @param what - exception to set.
+ * @param desc_str - error description string.
+ */
 static inline void
-say_dump_row_hex(const char *start, const char *end)
+xrow_on_decode_err(const char *start, const char *end, uint32_t what,
+		   const char *desc_str)
 {
-	char *buf = (char *)malloc(3 * (end - start) + 1);
+	diag_set(ClientError, what, desc_str);
+
+	if (!say_log_level_is_enabled(S_VERBOSE))
+		return;
+
+	size_t len = 3 * (end - start + 1);
+	char *buf;
+
+	if (len <= TT_STATIC_BUF_LEN) {
+		buf = tt_static_buf();
+	} else {
+		buf = (char *)malloc(len);
+		if (!buf) {
+			say_verbose("Got a corrupted row during decoding. "\
+				    "Not enough memory to dump row contents.");
+			return;
+		}
+	}
+
 	char *pos = buf;
 	for (const char *cur = start; cur < end; ++cur) {
 		pos += sprintf(pos, "%02hhX ", *cur);
 	}
 	*pos = 0;
 	say_verbose("Corrupted row is: %s", buf);
-	free(buf);
+
+	if (len > TT_STATIC_BUF_LEN)
+		free(buf);
 }
 
 int
@@ -110,8 +137,7 @@ xrow_header_decode(struct xrow_header *header, const char **pos,
 	const char * const start = *pos;
 	if (mp_check(&tmp, end) != 0) {
 error:
-		say_dump_row_hex(start, end);
-		diag_set(ClientError, ER_INVALID_MSGPACK, "packet header");
+		xrow_on_decode_err(start, end, ER_INVALID_MSGPACK, "packet header");
 		return -1;
 	}
 
@@ -178,8 +204,7 @@ error:
 	if (*pos < end && header->type != IPROTO_NOP) {
 		const char *body = *pos;
 		if (mp_check(pos, end)) {
-			say_dump_row_hex(start, end);
-			diag_set(ClientError, ER_INVALID_MSGPACK, "packet body");
+			xrow_on_decode_err(start, end, ER_INVALID_MSGPACK, "packet body");
 			return -1;
 		}
 		header->bodycnt = 1;
@@ -187,8 +212,7 @@ error:
 		header->body[0].iov_len = *pos - body;
 	}
 	if (end_is_exact && *pos < end) {
-		say_dump_row_hex(start,end);
-		diag_set(ClientError, ER_INVALID_MSGPACK, "packet body");
+		xrow_on_decode_err(start,end, ER_INVALID_MSGPACK, "packet body");
 		return -1;
 	}
 	return 0;
@@ -549,8 +573,8 @@ xrow_decode_sql(const struct xrow_header *row, struct sql_request *request)
 
 	if (mp_typeof(*data) != MP_MAP || mp_check_map(data, end) > 0) {
 error:
-		say_dump_row_hex(row->body[0].iov_base, end);
-		diag_set(ClientError, ER_INVALID_MSGPACK, "packet body");
+		xrow_on_decode_err(row->body[0].iov_base, end, ER_INVALID_MSGPACK,
+				   "packet body");
 		return -1;
 	}
 
@@ -621,8 +645,8 @@ xrow_decode_dml(struct xrow_header *row, struct request *request,
 
 	if (mp_typeof(*data) != MP_MAP || mp_check_map(data, end) > 0) {
 error:
-		say_dump_row_hex(row->body[0].iov_base, end);
-		diag_set(ClientError, ER_INVALID_MSGPACK, "packet body");
+		xrow_on_decode_err(row->body[0].iov_base, end, ER_INVALID_MSGPACK,
+				   "packet body");
 		return -1;
 	}
 
@@ -681,16 +705,15 @@ error:
 		}
 	}
 	if (data != end) {
-		say_dump_row_hex(row->body[0].iov_base, end);
-		diag_set(ClientError, ER_INVALID_MSGPACK, "packet end");
+		xrow_on_decode_err(row->body[0].iov_base, end, ER_INVALID_MSGPACK,
+				   "packet end");
 		return -1;
 	}
 done:
 	if (key_map) {
-		say_dump_row_hex(row->body[0].iov_base, end);
 		enum iproto_key key = (enum iproto_key) bit_ctz_u64(key_map);
-		diag_set(ClientError, ER_MISSING_REQUEST_FIELD,
-			 iproto_key_name(key));
+		xrow_on_decode_err(row->body[0].iov_base, end,
+				   ER_MISSING_REQUEST_FIELD, iproto_key_name(key));
 		return -1;
 	}
 	return 0;
@@ -837,8 +860,8 @@ xrow_decode_call(const struct xrow_header *row, struct call_request *request)
 
 	if (mp_typeof(*data) != MP_MAP || mp_check_map(data, end) > 0) {
 error:
-		say_dump_row_hex(row->body[0].iov_base, end);
-		diag_set(ClientError, ER_INVALID_MSGPACK, "packet body");
+		xrow_on_decode_err(row->body[0].iov_base, end, ER_INVALID_MSGPACK,
+				   "packet body");
 		return -1;
 	}
 
@@ -877,8 +900,8 @@ error:
 		}
 	}
 	if (data != end) {
-		say_dump_row_hex(row->body[0].iov_base, end);
-		diag_set(ClientError, ER_INVALID_MSGPACK, "packet end");
+		xrow_on_decode_err(row->body[0].iov_base, end, ER_INVALID_MSGPACK,
+				   "packet end");
 		return -1;
 	}
 	if (row->type == IPROTO_EVAL) {
@@ -918,8 +941,8 @@ xrow_decode_auth(const struct xrow_header *row, struct auth_request *request)
 
 	if (mp_typeof(*data) != MP_MAP || mp_check_map(data, end) > 0) {
 error:
-		say_dump_row_hex(row->body[0].iov_base, end);
-		diag_set(ClientError, ER_INVALID_MSGPACK, "packet body");
+		xrow_on_decode_err(row->body[0].iov_base, end, ER_INVALID_MSGPACK,
+				   "packet body");
 		return -1;
 	}
 
@@ -951,8 +974,8 @@ error:
 		}
 	}
 	if (data != end) {
-		say_dump_row_hex(row->body[0].iov_base, end);
-		diag_set(ClientError, ER_INVALID_MSGPACK, "packet end");
+		xrow_on_decode_err(row->body[0].iov_base, end, ER_INVALID_MSGPACK,
+				   "packet end");
 		return -1;
 	}
 	if (request->user_name == NULL) {
@@ -1112,8 +1135,8 @@ xrow_decode_ballot(struct xrow_header *row, struct ballot *ballot)
 	}
 	return 0;
 err:
-	say_dump_row_hex(row->body[0].iov_base, end);
-	diag_set(ClientError, ER_INVALID_MSGPACK, "packet body");
+	xrow_on_decode_err(row->body[0].iov_base, end, ER_INVALID_MSGPACK,
+			   "packet body");
 	return -1;
 }
 
@@ -1162,8 +1185,8 @@ xrow_decode_subscribe(struct xrow_header *row, struct tt_uuid *replicaset_uuid,
 	const char *end = data + row->body[0].iov_len;
 	const char *d = data;
 	if (mp_check(&d, end) != 0 || mp_typeof(*data) != MP_MAP) {
-		say_dump_row_hex(row->body[0].iov_base, end);
-		diag_set(ClientError, ER_INVALID_MSGPACK, "request body");
+		xrow_on_decode_err(row->body[0].iov_base, end, ER_INVALID_MSGPACK,
+				   "request body");
 		return -1;
 	}

Re: [tarantool-patches] [PATCH] xrow: print corrupted rows on decoding error.

[Vladimir Davydov]

On Tue, Mar 19, 2019 at 03:19:31PM +0300, Serge Petrenko wrote:
> Hi!
> Thankyou for review!
> The fixes are on the branch, and an incremental diff is below.
> 
> > 18 марта 2019 г., в 20:58, Vladimir Davydov <vdavydov.dev@gmail.com> написал(а):
> > 
> > On Fri, Mar 15, 2019 at 07:02:38PM +0300, Serge Petrenko wrote:
> >> Add row hex printing to log on verbose level. This would be useful
> >> during investigation of errors related to invalid msgpack packet
> >> arrival.
> >> 
> >> Related to #4040
> >> ---
> >> https://github.com/tarantool/tarantool/tree/sp/xrow-decode-verbose-err
> >> 
> >> 
> >> src/box/xrow.c | 27 +++++++++++++++++++++++++++
> >> 1 file changed, 27 insertions(+)
> >> 
> >> diff --git a/src/box/xrow.c b/src/box/xrow.c
> >> index bddae1d5b..8a5254902 100644
> >> --- a/src/box/xrow.c
> >> +++ b/src/box/xrow.c
> >> @@ -88,14 +88,29 @@ mp_decode_vclock(const char **data, struct vclock *vclock)
> >> 	return 0;
> >> }
> >> 
> >> +static inline void
> >> +say_dump_row_hex(const char *start, const char *end)
> >> +{
> >> +	char *buf = (char *)malloc(3 * (end - start) + 1);
> > 
> > Theoretically, malloc() may fail. You'd better check it and print
> > a warning in this case. Anyway, do we really need to use malloc()
> > here at all? Won't tt_static_buf be enough for typical cases.
> 
> Done, please see the incremental diff below.

There are too many changes for an incremental diff. It's time for v2.

> I still use malloc for sizes greater than TT_STATIC_BUF_LEN,
> but check for an allocation error.

No, please don't. I don't want branching here - it complicates the code
without yielding any benefits. I just suggested to use tt_static_buf
*instead of* malloc. If you think it's inadequate, then please use
malloc() as before.

> 
> > 
> >> +	char *pos = buf;
> >> +	for (const char *cur = start; cur < end; ++cur) {
> >> +		pos += sprintf(pos, "%02hhX ", *cur);
> >> +	}
> >> +	*pos = 0;
> >> +	say_verbose("Corrupted row is: %s", buf);
> > 
> > say_verbose() will turn into no-op, but you allocate a buffer and format
> > the message anyway. Please fix that by using say_log_level_is_enabled()
> > before doing anything.
> 
> Fixed.
> > 
> >> +	free(buf);
> >> +}
> >> +
> >> int
> >> xrow_header_decode(struct xrow_header *header, const char **pos,
> >> 		   const char *end, bool end_is_exact)
> >> {
> >> 	memset(header, 0, sizeof(struct xrow_header));
> >> 	const char *tmp = *pos;
> >> +	const char * const start = *pos;
> >> 	if (mp_check(&tmp, end) != 0) {
> >> error:
> >> +		say_dump_row_hex(start, end);
> >> 		diag_set(ClientError, ER_INVALID_MSGPACK, "packet header");
> >> 		return -1;
> >> 	}
> >> @@ -163,6 +178,7 @@ error:
> >> 	if (*pos < end && header->type != IPROTO_NOP) {
> >> 		const char *body = *pos;
> >> 		if (mp_check(pos, end)) {
> >> +			say_dump_row_hex(start, end);
> >> 			diag_set(ClientError, ER_INVALID_MSGPACK, "packet body");
> >> 			return -1;
> >> 		}
> >> @@ -171,6 +187,7 @@ error:
> >> 		header->body[0].iov_len = *pos - body;
> >> 	}
> >> 	if (end_is_exact && *pos < end) {
> >> +		say_dump_row_hex(start,end);
> >> 		diag_set(ClientError, ER_INVALID_MSGPACK, "packet body");
> > 
> > It would be better to add a helper function that would set diag and
> > print a corrupted row so that we don't miss it in case we add another
> > point of failure. Dunno, may be it's even worth adding a new exception
> > class with logging built in.
> 
> Done, please, see below.
> I don’t like the idea with a new exception class.
> If we introduce it, we’ll have to store the whole
> corrupted row, which can be quite big, until logging happens.

Okay.

> 
> diff --git a/src/box/xrow.c b/src/box/xrow.c
> index 3854874cc..000647111 100644
> --- a/src/box/xrow.c
> +++ b/src/box/xrow.c
> @@ -88,17 +88,44 @@ mp_decode_vclock(const char **data, struct vclock *vclock)
>  	return 0;
>  }
>  
> +/**
> + * Set diagnostics with an error and log the corrupted row
> + * which caused the error.
> + * @param what - exception to set.
> + * @param desc_str - error description string.
> + */
>  static inline void
> -say_dump_row_hex(const char *start, const char *end)
> +xrow_on_decode_err(const char *start, const char *end, uint32_t what,
> +		   const char *desc_str)

This should be a macro (perhaps a macro + a helper function), otherwise
the line number where an error occurred will be lost.

>  {
> -	char *buf = (char *)malloc(3 * (end - start) + 1);
> +	diag_set(ClientError, what, desc_str);
> +
> +	if (!say_log_level_is_enabled(S_VERBOSE))
> +		return;
> +
> +	size_t len = 3 * (end - start + 1);
> +	char *buf;
> +
> +	if (len <= TT_STATIC_BUF_LEN) {
> +		buf = tt_static_buf();
> +	} else {
> +		buf = (char *)malloc(len);
> +		if (!buf) {
> +			say_verbose("Got a corrupted row during decoding. "\
> +				    "Not enough memory to dump row contents.");
> +			return;
> +		}
> +	}
> +
>  	char *pos = buf;
>  	for (const char *cur = start; cur < end; ++cur) {
>  		pos += sprintf(pos, "%02hhX ", *cur);
>  	}

Please use snprintf.

I'm not quite familiar with the %02hhX specifier. What does the output
look like? Could you give an example in the commit message? BTW, it'd be
really nice to have a test covering this code - should be pretty easy to
add. Also, is the output format consistent with some unix tool (hexdump
maybe)?

>  	*pos = 0;
>  	say_verbose("Corrupted row is: %s", buf);
> -	free(buf);
> +
> +	if (len > TT_STATIC_BUF_LEN)
> +		free(buf);
>  }
>  
>  int
> @@ -110,8 +137,7 @@ xrow_header_decode(struct xrow_header *header, const char **pos,
>  	const char * const start = *pos;
>  	if (mp_check(&tmp, end) != 0) {
>  error:
> -		say_dump_row_hex(start, end);
> -		diag_set(ClientError, ER_INVALID_MSGPACK, "packet header");
> +		xrow_on_decode_err(start, end, ER_INVALID_MSGPACK, "packet header");
>  		return -1;
>  	}
>  
> @@ -178,8 +204,7 @@ error:
>  	if (*pos < end && header->type != IPROTO_NOP) {
>  		const char *body = *pos;
>  		if (mp_check(pos, end)) {
> -			say_dump_row_hex(start, end);
> -			diag_set(ClientError, ER_INVALID_MSGPACK, "packet body");
> +			xrow_on_decode_err(start, end, ER_INVALID_MSGPACK, "packet body");

You skipped quite a few places where a decoding error might occur, e.g.
xrow_decode_sql => ER_MISSING_REQUEST_FIELD doesn't have extra logging
while xrow_decode_dml => ER_MISSING_REQUEST_FIELD does.

I'd like to see some pattern here. Perhaps, logging every error doesn't
make much sense, especially if you primarily target at corrupted input.
Let's discuss.