MDEV-10963 Fragmented BINLOG query

The problem was originally stated in
  http://bugs.mysql.com/bug.php?id=82212
The size of an base64-encoded Rows_log_event exceeds its
vanilla byte representation in 4/3 times.
When a binlogged event size is about 1GB mysqlbinlog generates
a BINLOG query that can't be send out due to its size.

It is fixed with fragmenting the BINLOG argument C-string into
(approximate) halves when the base64 encoded event is over 1GB size.
The mysqlbinlog in such case puts out

    SET @binlog_fragment_0='base64-encoded-fragment_0';
    SET @binlog_fragment_1='base64-encoded-fragment_1';
    BINLOG @binlog_fragment_0, @binlog_fragment_1;

to represent a big BINLOG.
For prompt memory release BINLOG handler is made to reset the BINLOG argument
user variables in the middle of processing, as if @binlog_fragment_{0,1} = NULL
is assigned.

Notice the 2 fragments are enough, though the client and server still may
need to tweak their @@max_allowed_packet to satisfy to the fragment
size (which they would have to do anyway with greater number of
fragments, should that be desired).

On the lower level the following changes are made:

Log_event::print_base64()
  remains to call encoder and store the encoded data into a cache but
  now *without* doing any formatting. The latter is left for time
  when the cache is copied to an output file (e.g mysqlbinlog output).
  No formatting behavior is also reflected by the change in the meaning
  of the last argument which specifies whether to cache the encoded data.

Rows_log_event::print_helper()
  is made to invoke a specialized fragmented cache-to-file copying function
  which is

copy_cache_to_file_wrapped()
  that takes care of fragmenting also optionally wraps encoded
  strings (fragments) into SQL stanzas.

my_b_copy_to_file()
  is refactored to into my_b_copy_all_to_file(). The former function
  is generalized
  to accepts more a limit argument to constraint the copying and does
  not reinitialize anymore the cache into reading mode.
  The limit does not do any effect on the fully read cache.

client/mysqlbinlog.cc

@@ -72,6 +72,7 @@ ulong mysqld_net_retry_count = 10L;
 ulong open_files_limit;
 ulong opt_binlog_rows_event_max_size;
 ulonglong test_flags = 0;
+ulong opt_binlog_rows_event_max_encoded_size= MAX_MAX_ALLOWED_PACKET;
 static uint opt_protocol= 0;
 static FILE *result_file;
 static char *result_file_name= 0;
@@ -813,7 +814,12 @@ write_event_header_and_base64(Log_event *ev, FILE *result_file,
 
   /* Write header and base64 output to cache */
   ev->print_header(head, print_event_info, FALSE);
-  ev->print_base64(body, print_event_info, FALSE);
+
+  DBUG_ASSERT(print_event_info->base64_output_mode == BASE64_OUTPUT_ALWAYS);
+
+  ev->print_base64(body, print_event_info,
+                   print_event_info->base64_output_mode !=
+                   BASE64_OUTPUT_DECODE_ROWS);
 
   /* Read data from cache and write to result file */
   if (copy_event_cache_to_file_and_reinit(head, result_file) ||
@@ -852,7 +858,9 @@ static bool print_base64(PRINT_EVENT_INFO *print_event_info, Log_event *ev)
     return 1;
   }
   ev->print(result_file, print_event_info);
-  return print_event_info->head_cache.error == -1;
+  return
+    print_event_info->head_cache.error == -1 ||
+    print_event_info->body_cache.error == -1;
 }
 
 
@@ -1472,6 +1480,15 @@ that may lead to an endless loop.",
    "This value must be a multiple of 256.",
    &opt_binlog_rows_event_max_size, &opt_binlog_rows_event_max_size, 0,
    GET_ULONG, REQUIRED_ARG, UINT_MAX,  256, ULONG_MAX,  0, 256,  0},
+#ifndef DBUG_OFF
+  {"debug-binlog-row-event-max-encoded-size", 0,
+   "The maximum size of base64-encoded rows-event in one BINLOG pseudo-query "
+   "instance. When the computed actual size exceeds the limit "
+   "the BINLOG's argument string is fragmented in two.",
+   &opt_binlog_rows_event_max_encoded_size,
+   &opt_binlog_rows_event_max_encoded_size, 0,
+   GET_ULONG, REQUIRED_ARG, UINT_MAX/4,  256, ULONG_MAX,  0, 256,  0},
+#endif
   {"verify-binlog-checksum", 'c', "Verify checksum binlog events.",
    (uchar**) &opt_verify_binlog_checksum, (uchar**) &opt_verify_binlog_checksum,
    0, GET_BOOL, NO_ARG, 0, 0, 0, 0, 0, 0},

include/my_sys.h

@@ -602,7 +602,9 @@ static inline size_t my_b_bytes_in_cache(const IO_CACHE *info)
   return *info->current_end - *info->current_pos;
 }
 
-int      my_b_copy_to_file(IO_CACHE *cache, FILE *file);
+int my_b_copy_to_file    (IO_CACHE *cache, FILE *file, size_t count);
+int my_b_copy_all_to_file(IO_CACHE *cache, FILE *file);
+
 my_off_t my_b_append_tell(IO_CACHE* info);
 my_off_t my_b_safe_tell(IO_CACHE* info); /* picks the correct tell() */
 int my_b_pread(IO_CACHE *info, uchar *Buffer, size_t Count, my_off_t pos);

mysql-test/suite/binlog/r/binlog_base64_flag.result

@@ -28,6 +28,25 @@ a
 1
 1
 3
+DELETE FROM t1 WHERE a=3;
+BINLOG '
+ODdYRw8BAAAAZgAAAGoAAAABAAQANS4xLjIzLXJjLWRlYnVnLWxvZwAAAAAAAAAAAAAAAAAAAAAA
+AAAAAAAAAAAAAAAAAAA4N1hHEzgNAAgAEgAEBAQEEgAAUwAEGggAAAAICAgC
+';
+SET @binlog_fragment_0='
+TFtYRxMBAAAAKQAAAH8BAAAAABAAAAAAAAAABHRlc3QAAnQxAAEDAAE=
+TFtYRxcBAAAAIgAAAKEBAAAQABAAAAAAAAEAAf/+AwAAAA==
+';
+SET @binlog_fragment_1='';
+BINLOG @binlog_fragment_0, @binlog_fragment_1;
+select * from t1;
+a
+1
+1
+3
+SELECT @binlog_fragment_0, @binlog_fragment_1 as 'NULL','NULL';
+@binlog_fragment_0	NULL	NULL
+NULL	NULL	NULL
 ==== Test --base64-output=never on a binlog with row events ====
 /*!50530 SET @@SESSION.PSEUDO_SLAVE_MODE=1*/;
 /*!40019 SET @@session.max_insert_delayed_threads=0*/;

mysql-test/suite/binlog/r/binlog_mysqlbinlog_row_frag.result

+CREATE TABLE t (a TEXT);
+RESET MASTER;
+INSERT INTO t SET a=repeat('a', 1024);
+SELECT a from t into @a;
+FLUSH LOGS;
+DELETE FROM t;
+FOUND /BINLOG @binlog_fragment_0, @binlog_fragment_1/ in mysqlbinlog.sql
+SELECT a LIKE @a as 'true' FROM t;
+true
+1
+BINLOG number-of-fragments must be exactly two
+BINLOG @binlog_fragment;
+ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near '' at line 1
+BINLOG @binlog_fragment, @binlog_fragment, @binlog_fragment;
+ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near ' @binlog_fragment' at line 1
+SET @binlog_fragment_0='012345';
+SET @binlog_fragment_1='012345';
+BINLOG @binlog_fragment_0, @binlog_fragment_1;
+ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use
+SET @binlog_fragment_0='012345';
+BINLOG @binlog_fragment_0, @binlog_fragment_not_exist;
+ERROR 42000: Incorrect argument type to variable 'binlog_fragment_not_exist'
+# Cleanup
+DROP TABLE t;

mysql-test/suite/binlog/t/binlog_base64_flag.test

@@ -67,6 +67,28 @@ TFtYRxcBAAAAIgAAAKEBAAAQABAAAAAAAAEAAf/+AwAAAA==
 # The above line should succeed and 3 should be in the table
 select * from t1;
 
+# The same as above with one-fragment BINLOG to prove
+# equivalency with the fragmented BINLOG @frag_0, @frag_1.
+DELETE FROM t1 WHERE a=3;
+# This is a binlog statement containing a Format_description_log_event
+# from the same version as the Table_map and Write_rows_log_event.
+BINLOG '
+ODdYRw8BAAAAZgAAAGoAAAABAAQANS4xLjIzLXJjLWRlYnVnLWxvZwAAAAAAAAAAAAAAAAAAAAAA
+AAAAAAAAAAAAAAAAAAA4N1hHEzgNAAgAEgAEBAQEEgAAUwAEGggAAAAICAgC
+';
+
+# This is a Table_map_log_event+Write_rows_log_event corresponding to:
+# INSERT INTO TABLE test.t1 VALUES (3)
+SET @binlog_fragment_0='
+TFtYRxMBAAAAKQAAAH8BAAAAABAAAAAAAAAABHRlc3QAAnQxAAEDAAE=
+TFtYRxcBAAAAIgAAAKEBAAAQABAAAAAAAAEAAf/+AwAAAA==
+';
+SET @binlog_fragment_1='';
+BINLOG @binlog_fragment_0, @binlog_fragment_1;
+# The above line should succeed and 3 should be in the table:
+select * from t1;
+# show "one-shot" feature of binlog_fragment variables
+SELECT @binlog_fragment_0, @binlog_fragment_1 as 'NULL','NULL';
 
 # Test that mysqlbinlog stops with an error message when the
 # --base64-output=never flag is used on a binlog with base64 events.

mysql-test/suite/binlog/t/binlog_mysqlbinlog_row_frag.test

+--source include/have_debug.inc
+--source include/have_binlog_format_row.inc
+
+--let $MYSQLD_DATADIR= `select @@datadir`
+
+CREATE TABLE t (a TEXT);
+# events of interest are guaranteed to stay in 000001 log
+RESET MASTER;
+--eval INSERT INTO t SET a=repeat('a', 1024)
+SELECT a from t into @a;
+FLUSH LOGS;
+DELETE FROM t;
+
+--exec $MYSQL_BINLOG --debug-binlog-row-event-max-encoded-size=256 $MYSQLD_DATADIR/master-bin.000001 > $MYSQLTEST_VARDIR/tmp/mysqlbinlog.sql
+
+--let SEARCH_PATTERN= BINLOG @binlog_fragment_0, @binlog_fragment_1
+--let SEARCH_FILE= $MYSQLTEST_VARDIR/tmp/mysqlbinlog.sql
+--source include/search_pattern_in_file.inc
+
+--exec $MYSQL test < $MYSQLTEST_VARDIR/tmp/mysqlbinlog.sql
+
+SELECT a LIKE @a as 'true' FROM t;
+
+# improper syntax error
+--echo BINLOG number-of-fragments must be exactly two
+--error ER_PARSE_ERROR
+BINLOG @binlog_fragment;
+--error ER_PARSE_ERROR
+BINLOG @binlog_fragment, @binlog_fragment, @binlog_fragment;
+
+# corrupted fragments error check (to the expected error code notice,
+# the same error code occurs in a similar unfragmented case)
+SET @binlog_fragment_0='012345';
+SET @binlog_fragment_1='012345';
+--error ER_SYNTAX_ERROR
+BINLOG @binlog_fragment_0, @binlog_fragment_1;
+
+# Not existing fragment is not allowed
+SET @binlog_fragment_0='012345';
+--error ER_WRONG_TYPE_FOR_VAR
+BINLOG @binlog_fragment_0, @binlog_fragment_not_exist;
+
+--echo # Cleanup
+--remove_file $MYSQLTEST_VARDIR/tmp/mysqlbinlog.sql
+DROP TABLE t;

mysys/mf_iocache2.c

@@ -23,51 +23,56 @@
 #include <stdarg.h>
 #include <m_ctype.h>
 
-/*
-  Copy contents of an IO_CACHE to a file.
-
-  SYNOPSIS
-    my_b_copy_to_file()
-    cache  IO_CACHE to copy from
-    file   File to copy to
-
-  DESCRIPTION
-    Copy the contents of the cache to the file. The cache will be
-    re-inited to a read cache and will read from the beginning of the
-    cache.
-
-    If a failure to write fully occurs, the cache is only copied
-    partially.
+/**
+  Copy the cache to the file. Copying can be constrained to @c count
+  number of bytes when the parameter is less than SIZE_T_MAX.  The
+  cache will be optionally re-inited to a read cache and will read
+  from the beginning of the cache.  If a failure to write fully
+  occurs, the cache is only copied partially.
 
   TODO
-    Make this function solid by handling partial reads from the cache
-    in a correct manner: it should be atomic.
-
-  RETURN VALUE
-    0  All OK
-    1  An error occurred
+  Make this function solid by handling partial reads from the cache
+  in a correct manner: it should be atomic.
+
+  @param cache      IO_CACHE to copy from
+  @param file       File to copy to
+  @param count      the copied size or the max of the type
+                    when the whole cache is to be copied.
+  @return
+         0          All OK
+         1          An error occurred
 */
 int
-my_b_copy_to_file(IO_CACHE *cache, FILE *file)
+my_b_copy_to_file(IO_CACHE *cache, FILE *file,
+                  size_t count)
 {
-  size_t bytes_in_cache;
+  size_t curr_write, bytes_in_cache;
   DBUG_ENTER("my_b_copy_to_file");
 
-  /* Reinit the cache to read from the beginning of the cache */
-  if (reinit_io_cache(cache, READ_CACHE, 0L, FALSE, FALSE))
-    DBUG_RETURN(1);
   bytes_in_cache= my_b_bytes_in_cache(cache);
   do
   {
-    if (my_fwrite(file, cache->read_pos, bytes_in_cache,
+    curr_write= MY_MIN(bytes_in_cache, count);
+    if (my_fwrite(file, cache->read_pos, curr_write,
                   MYF(MY_WME | MY_NABP)) == (size_t) -1)
       DBUG_RETURN(1);
-  } while ((bytes_in_cache= my_b_fill(cache)));
+
+    cache->read_pos += curr_write;
+    count -= curr_write;
+  } while (count && (bytes_in_cache= my_b_fill(cache)));
   if(cache->error == -1)
     DBUG_RETURN(1);
   DBUG_RETURN(0);
 }
 
+int my_b_copy_all_to_file(IO_CACHE *cache, FILE *file)
+{
+  DBUG_ENTER("my_b_copy_all_to_file");
+  /* Reinit the cache to read from the beginning of the cache */
+  if (reinit_io_cache(cache, READ_CACHE, 0L, FALSE, FALSE))
+    DBUG_RETURN(1);
+  DBUG_RETURN(my_b_copy_to_file(cache, file, SIZE_T_MAX));
+}
 
 my_off_t my_b_append_tell(IO_CACHE* info)
 {

sql/item_func.cc

@@ -4828,7 +4828,7 @@ bool Item_func_set_user_var::register_field_in_bitmap(uchar *arg)
     true    failure
 */
 
-static bool
+bool
 update_hash(user_var_entry *entry, bool set_null, void *ptr, uint length,
             Item_result type, CHARSET_INFO *cs,
             bool unsigned_arg)

sql/item_func.h

@@ -2283,4 +2283,8 @@ bool eval_const_cond(COND *cond);
 
 extern bool volatile  mqh_used;
 
+bool update_hash(user_var_entry *entry, bool set_null, void *ptr, uint length,
+                 Item_result type, CHARSET_INFO *cs,
+                 bool unsigned_arg);
+
 #endif /* ITEM_FUNC_INCLUDED */

sql/log_event.cc

@@ -2783,9 +2783,23 @@ void free_table_map_log_event(Table_map_log_event *event)
   delete event;
 }
 
+/**
+  Encode the event, optionally per 'do_print_encoded' arg store the
+  result into the argument cache; optionally per event_info's
+  'verbose' print into the cache a verbose representation of the event.
+  Note, no extra wrapping is done to the being io-cached data, like
+  to producing a BINLOG query. It's left for a routine that extracts from
+  the cache.
+
+  @param file               pointer to IO_CACHE
+  @param print_event_info   pointer to print_event_info specializing
+                            what out of and how to print the event
+  @param do_print_encoded   whether to store base64-encoded event
+                            into @file.
+*/
 void Log_event::print_base64(IO_CACHE* file,
                              PRINT_EVENT_INFO* print_event_info,
-                             bool more)
+                             bool do_print_encoded)
 {
   const uchar *ptr= (const uchar *)temp_buf;
   uint32 size= uint4korr(ptr + EVENT_LEN_OFFSET);
@@ -2804,17 +2818,9 @@ void Log_event::print_base64(IO_CACHE* file,
     DBUG_ASSERT(0);
   }
 
-  if (print_event_info->base64_output_mode != BASE64_OUTPUT_DECODE_ROWS)
-  {
-    if (my_b_tell(file) == 0)
-      my_b_write_string(file, "\nBINLOG '\n");
-
+  if (do_print_encoded)
     my_b_printf(file, "%s\n", tmp_str);
 
-    if (!more)
-      my_b_printf(file, "'%s\n", print_event_info->delimiter);
-  }
-  
   if (print_event_info->verbose)
   {
     Rows_log_event *ev= NULL;
@@ -4851,9 +4857,17 @@ void Start_log_event_v3::print(FILE* file, PRINT_EVENT_INFO* print_event_info)
       print_event_info->base64_output_mode != BASE64_OUTPUT_NEVER &&
       !print_event_info->short_form)
   {
-    if (print_event_info->base64_output_mode != BASE64_OUTPUT_DECODE_ROWS)
+    /* BINLOG is matched with the delimiter below on the same level */
+    bool do_print_encoded=
+      print_event_info->base64_output_mode != BASE64_OUTPUT_DECODE_ROWS;
+    if (do_print_encoded)
       my_b_printf(&cache, "BINLOG '\n");
-    print_base64(&cache, print_event_info, FALSE);
+
+    print_base64(&cache, print_event_info, do_print_encoded);
+
+    if (do_print_encoded)
+      my_b_printf(&cache, "'%s\n", print_event_info->delimiter);
+
     print_event_info->printed_fd_event= TRUE;
   }
   DBUG_VOID_RETURN;
@@ -10491,12 +10505,128 @@ void Rows_log_event::pack_info(Protocol *protocol)
 #endif
 
 #ifdef MYSQL_CLIENT
+/**
+  Print an event "body" cache to @c file possibly in two fragments.
+  Each fragement is optionally per @c do_wrap to produce an SQL statement.
+
+  @param file      a file to print to
+  @param body      the "body" IO_CACHE of event
+  @param do_wrap   whether to wrap base64-encoded strings with
+                   SQL cover.
+  @param delimiter delimiter string
+
+  The function signals on any error through setting @c body->error to -1.
+*/
+void copy_cache_to_file_wrapped(FILE *file,
+                                IO_CACHE *body,
+                                bool do_wrap,
+                                const char *delimiter)
+{
+  const char str_binlog[]= "\nBINLOG '\n";
+  const char fmt_delim[]=   "'%s\n";
+  const char fmt_n_delim[]= "\n'%s";
+  const my_off_t cache_size= my_b_tell(body);
+
+  if (reinit_io_cache(body, READ_CACHE, 0L, FALSE, FALSE))
+  {
+    body->error= -1;
+    goto end;
+  }
+
+  if (!do_wrap)
+  {
+    my_b_copy_to_file(body, file, SIZE_T_MAX);
+  }
+  else if (4 + sizeof(str_binlog) + cache_size + sizeof(fmt_delim) >
+           opt_binlog_rows_event_max_encoded_size)
+  {
+    /*
+      2 fragments can always represent near 1GB row-based
+      base64-encoded event as two strings each of size less than
+      max(max_allowed_packet). Greater number of fragments does not
+      save from potential need to tweak (increase) @@max_allowed_packet
+      before to process the fragments. So 2 is safe and enough.
+
+      Split the big query when its packet size's estimation exceeds a
+      limit. The estimate includes the maximum packet header
+      contribution of non-compressed packet.
+    */
+    const char fmt_frag[]= "\nSET @binlog_fragment_%d ='\n";
+
+    my_fprintf(file, fmt_frag, 0);
+    if (my_b_copy_to_file(body, file, cache_size/2 + 1))
+    {
+      body->error= -1;
+      goto end;
+    }
+    my_fprintf(file, fmt_n_delim, delimiter);
+
+    my_fprintf(file, fmt_frag, 1);
+    if (my_b_copy_to_file(body, file, SIZE_T_MAX))
+    {
+      body->error= -1;
+      goto end;
+    }
+    my_fprintf(file, fmt_delim, delimiter);
+
+    my_fprintf(file, "BINLOG @binlog_fragment_0, @binlog_fragment_1%s\n",
+               delimiter);
+  }
+  else
+  {
+    my_fprintf(file, str_binlog);
+    if (my_b_copy_to_file(body, file, SIZE_T_MAX))
+    {
+      body->error= -1;
+      goto end;
+    }
+    my_fprintf(file, fmt_delim, delimiter);
+  }
+  reinit_io_cache(body, WRITE_CACHE, 0, FALSE, TRUE);
+
+end:
+  return;
+}
+
+/**
+  The function invokes base64 encoder to run on the current
+  event string and store the result into two caches.
+  When the event ends the current statement the caches are is copied into
+  the argument file.
+  Copying is also concerned how to wrap the event, specifically to produce
+  a valid SQL syntax.
+  When the encoded data size is within max(MAX_ALLOWED_PACKET)
+  a regular BINLOG query is composed. Otherwise it is build as fragmented
+
+    SET @binlog_fragment_0='...';
+    SET @binlog_fragment_1='...';
+    BINLOG @binlog_fragment_0, @binlog_fragment_1;
+
+  where fragments are represented by a pair of indexed user
+  "one shot" variables.
+
+  @note
+  If any changes made don't forget to duplicate them to
+  Old_rows_log_event as long as it's supported.
+
+  @param file               pointer to IO_CACHE
+  @param print_event_info   pointer to print_event_info specializing
+                            what out of and how to print the event
+  @param name               the name of a table that the event operates on
+
+  The function signals on any error of cache access through setting
+  that cache's @c error to -1.
+*/
 void Rows_log_event::print_helper(FILE *file,
                                   PRINT_EVENT_INFO *print_event_info,
                                   char const *const name)
 {
   IO_CACHE *const head= &print_event_info->head_cache;
   IO_CACHE *const body= &print_event_info->body_cache;
+  bool do_print_encoded=
+    print_event_info->base64_output_mode != BASE64_OUTPUT_DECODE_ROWS &&
+    !print_event_info->short_form;
+
   if (!print_event_info->short_form)
   {
     bool const last_stmt_event= get_flags(STMT_END_F);
@@ -10504,13 +10634,18 @@ void Rows_log_event::print_helper(FILE *file,
     my_b_printf(head, "\t%s: table id %lu%s\n",
                 name, m_table_id,
                 last_stmt_event ? " flags: STMT_END_F" : "");
-    print_base64(body, print_event_info, !last_stmt_event);
+    print_base64(body, print_event_info, do_print_encoded);
   }
 
   if (get_flags(STMT_END_F))
   {
-    copy_event_cache_to_file_and_reinit(head, file);
-    copy_event_cache_to_file_and_reinit(body, file);
+    if (copy_event_cache_to_file_and_reinit(head, file))
+    {
+      head->error= -1;
+      return;
+    }
+    copy_cache_to_file_wrapped(file, body, do_print_encoded,
+                               print_event_info->delimiter);
   }
 }
 #endif
@@ -11379,7 +11514,9 @@ void Table_map_log_event::print(FILE *file, PRINT_EVENT_INFO *print_event_info)
                 m_dbnam, m_tblnam, m_table_id,
                 ((m_flags & TM_BIT_HAS_TRIGGERS_F) ?
                  " (has triggers)" : ""));
-    print_base64(&print_event_info->body_cache, print_event_info, TRUE);
+    print_base64(&print_event_info->body_cache, print_event_info,
+                 print_event_info->base64_output_mode !=
+                 BASE64_OUTPUT_DECODE_ROWS);
     copy_event_cache_to_file_and_reinit(&print_event_info->head_cache, file);
   }
 }

sql/log_event.h

@@ -1157,7 +1157,7 @@ class Log_event
   void print_header(IO_CACHE* file, PRINT_EVENT_INFO* print_event_info,
                     bool is_more);
   void print_base64(IO_CACHE* file, PRINT_EVENT_INFO* print_event_info,
-                    bool is_more);
+                    bool do_print_encoded);
 #endif
   /*
     read_log_event() functions read an event from a binlog or relay
@@ -4891,15 +4891,22 @@ class Ignorable_log_event : public Log_event {
   virtual int get_data_size() { return IGNORABLE_HEADER_LEN; }
 };
 
+#ifdef MYSQL_CLIENT
+void copy_cache_to_file_wrapped(FILE *file,
+                                PRINT_EVENT_INFO *print_event_info,
+                                IO_CACHE *body,
+                                bool do_wrap);
+#endif
 
 static inline bool copy_event_cache_to_file_and_reinit(IO_CACHE *cache,
                                                        FILE *file)
 {
-  return         
-    my_b_copy_to_file(cache, file) ||
+  return
+    my_b_copy_all_to_file(cache, file) ||
     reinit_io_cache(cache, WRITE_CACHE, 0, FALSE, TRUE);
 }
 
+
 #ifdef MYSQL_SERVER
 /*****************************************************************************
 

sql/log_event_old.cc

@@ -1850,12 +1850,17 @@ void Old_rows_log_event::pack_info(Protocol *protocol)
 
 
 #ifdef MYSQL_CLIENT
+/* Method duplicates Rows_log_event's one */
 void Old_rows_log_event::print_helper(FILE *file,
                                       PRINT_EVENT_INFO *print_event_info,
                                       char const *const name)
 {
   IO_CACHE *const head= &print_event_info->head_cache;
   IO_CACHE *const body= &print_event_info->body_cache;
+  bool do_print_encoded=
+    print_event_info->base64_output_mode != BASE64_OUTPUT_DECODE_ROWS &&
+    !print_event_info->short_form;
+
   if (!print_event_info->short_form)
   {
     bool const last_stmt_event= get_flags(STMT_END_F);
@@ -1863,13 +1868,18 @@ void Old_rows_log_event::print_helper(FILE *file,
     my_b_printf(head, "\t%s: table id %lu%s\n",
                 name, m_table_id,
                 last_stmt_event ? " flags: STMT_END_F" : "");
-    print_base64(body, print_event_info, !last_stmt_event);
+    print_base64(body, print_event_info, do_print_encoded);
   }
 
   if (get_flags(STMT_END_F))
   {
-    copy_event_cache_to_file_and_reinit(head, file);
-    copy_event_cache_to_file_and_reinit(body, file);
+    if (copy_event_cache_to_file_and_reinit(head, file))
+    {
+      head->error= -1;
+      return;
+    }
+    copy_cache_to_file_wrapped(file, body, do_print_encoded,
+                               print_event_info->delimiter);
   }
 }
 #endif

sql/sql_binlog.cc

@@ -28,6 +28,65 @@
                                                 // START_EVENT_V3,
                                                 // Log_event_type,
                                                 // Log_event
+
+/**
+  Copy fragments into the standard placeholder thd->lex->comment.str.
+
+  Compute the size of the (still) encoded total,
+  allocate and then copy fragments one after another.
+  The size can exceed max(max_allowed_packet) which is not a
+  problem as no String instance is created off this char array.
+
+  @param thd  THD handle
+  @return
+     0        at success,
+    -1        otherwise.
+*/
+int binlog_defragment(THD *thd)
+{
+  user_var_entry *entry[2];
+  LEX_STRING name[2]= { thd->lex->comment, thd->lex->ident };
+
+  /* compute the total size */
+  thd->lex->comment.str= NULL;
+  thd->lex->comment.length= 0;
+  for (uint k= 0; k < 2; k++)
+  {
+    entry[k]=
+      (user_var_entry*) my_hash_search(&thd->user_vars, (uchar*) name[k].str,
+                                       name[k].length);
+    if (!entry[k] || entry[k]->type != STRING_RESULT)
+    {
+      my_error(ER_WRONG_TYPE_FOR_VAR, MYF(0), name[k].str);
+      return -1;
+    }
+    thd->lex->comment.length += entry[k]->length;
+  }
+
+  thd->lex->comment.str=                            // to be freed by the caller
+    (char *) my_malloc(thd->lex->comment.length, MYF(MY_WME));
+  if (!thd->lex->comment.str)
+  {
+    my_error(ER_OUTOFMEMORY, MYF(ME_FATALERROR), 1);
+    return -1;
+  }
+
+  /* fragments are merged into allocated buf while the user var:s get reset */
+  size_t gathered_length= 0;
+  for (uint k=0; k < 2; k++)
+  {
+    memcpy(thd->lex->comment.str + gathered_length, entry[k]->value,
+           entry[k]->length);
+    gathered_length += entry[k]->length;
+    update_hash(entry[k], true, NULL, 0, STRING_RESULT, &my_charset_bin, 0);
+  }
+
+  DBUG_ASSERT(gathered_length == thd->lex->comment.length);
+
+  return 0;
+}
+
+
 /**
   Execute a BINLOG statement.
 
@@ -53,14 +112,6 @@ void mysql_client_binlog_statement(THD* thd)
   if (check_global_access(thd, SUPER_ACL))
     DBUG_VOID_RETURN;
 
-  size_t coded_len= thd->lex->comment.length;
-  if (!coded_len)
-  {
-    my_error(ER_SYNTAX_ERROR, MYF(0));
-    DBUG_VOID_RETURN;
-  }
-  size_t decoded_len= base64_needed_decoded_length(coded_len);
-
   /*
     option_bits will be changed when applying the event. But we don't expect
     it be changed permanently after BINLOG statement, so backup it first.
@@ -81,6 +132,8 @@ void mysql_client_binlog_statement(THD* thd)
   int err;
   Relay_log_info *rli;
   rpl_group_info *rgi;
+  char *buf= NULL;
+  size_t coded_len= 0, decoded_len= 0;
 
   rli= thd->rli_fake;
   if (!rli)
@@ -102,15 +155,13 @@ void mysql_client_binlog_statement(THD* thd)
   rgi->thd= thd;
 
   const char *error= 0;
-  char *buf= (char *) my_malloc(decoded_len, MYF(MY_WME));
   Log_event *ev = 0;
+  my_bool is_fragmented= FALSE;
 
   /*
     Out of memory check
   */
-  if (!(rli &&
-        rli->relay_log.description_event_for_exec &&
-        buf))
+  if (!(rli && rli->relay_log.description_event_for_exec))
   {
     my_error(ER_OUTOFMEMORY, MYF(ME_FATALERROR), 1);  /* needed 1 bytes */
     goto end;
@@ -119,6 +170,23 @@ void mysql_client_binlog_statement(THD* thd)
   rli->sql_driver_thd= thd;
   rli->no_storage= TRUE;
 
+  if (unlikely(is_fragmented= thd->lex->comment.str && thd->lex->ident.str))
+    if (binlog_defragment(thd))
+      goto end;
+
+  if (!(coded_len= thd->lex->comment.length))
+  {
+    my_error(ER_SYNTAX_ERROR, MYF(0));
+    goto end;
+  }
+
+  decoded_len= base64_needed_decoded_length(coded_len);
+  if (!(buf= (char *) my_malloc(decoded_len, MYF(MY_WME))))
+  {
+    my_error(ER_OUTOFMEMORY, MYF(ME_FATALERROR), 1);
+    goto end;
+  }
+
   for (char const *strptr= thd->lex->comment.str ;
        strptr < thd->lex->comment.str + thd->lex->comment.length ; )
   {
@@ -272,6 +340,8 @@ void mysql_client_binlog_statement(THD* thd)
   my_ok(thd);
 
 end:
+  if (unlikely(is_fragmented))
+    my_free(thd->lex->comment.str);
   thd->variables.option_bits= thd_options;
   rgi->slave_close_thread_tables(thd);
   my_free(buf);

sql/sql_lex.h

@@ -2459,6 +2459,10 @@ struct LEX: public Query_tables_list
   String *wild; /* Wildcard in SHOW {something} LIKE 'wild'*/ 
   sql_exchange *exchange;
   select_result *result;
+  /**
+    @c the two may also hold BINLOG arguments: either comment holds a
+    base64-char string or both represent the BINLOG fragment user variables.
+  */
   LEX_STRING comment, ident;
   LEX_USER *grant_user;
   XID *xid;

sql/sql_yacc.yy

@@ -7950,8 +7950,17 @@ binlog_base64_event:
           {
             Lex->sql_command = SQLCOM_BINLOG_BASE64_EVENT;
             Lex->comment= $2;
+            Lex->ident.str=    NULL;
+            Lex->ident.length= 0;
           }
-        ;
+          |
+          BINLOG_SYM '@' ident_or_text ',' '@' ident_or_text
+          {
+            Lex->sql_command = SQLCOM_BINLOG_BASE64_EVENT;
+            Lex->comment= $3;
+            Lex->ident=   $6;
+          }
+          ;
 
 check_view_or_table:
           table_or_tables table_list opt_mi_check_type

unittest/sql/mf_iocache-t.cc

@@ -370,10 +370,77 @@ void mdev17133()
 }
 
 
+void mdev10963()
+{
+  int res;
+  uint n_checks= 8;
+  uchar buf[1024 * 512];
+  uint n_frag= sizeof(buf)/(2 * CACHE_SIZE);
+  FILE *file;
+  myf my_flags= MYF(MY_WME);
+  const char *file_name="cache.log";
+
+  memset(buf, FILL, sizeof(buf));
+  diag("MDEV-10963 Fragmented BINLOG query");
+
+  init_io_cache_encryption();
+  srand((uint) time(NULL));
+
+  /* copying source */
+  res= open_cached_file(&info, 0, 0, CACHE_SIZE, 0);
+  ok(res == 0, "open_cached_file" INFO_TAIL);
+  res= my_b_write(&info, buf, sizeof(buf));
+
+  ulong total_size= my_b_tell(&info);
+  ok(res == 0 && total_size == sizeof(buf), "cache is written");
+
+  /* destination */
+  file= my_fopen(file_name, O_RDWR | O_TRUNC | O_CREAT, my_flags);
+  ok(my_fileno(file) > 0, "opened file fd = %d", my_fileno(file));
+
+  /*
+    For n_checks times verify a sequence of copying with random fragment
+    size ranging from zero to about the double of the cache read buffer size.
+  */
+  for (; n_checks; n_checks--, rewind(file))
+  {
+    // copied size is an estimate can be incremeneted to greater than total_size
+    ulong copied_size= 0;
+
+    res= reinit_io_cache(&info, READ_CACHE, 0L, FALSE, FALSE);
+    ok(res == 0, "cache turned to read");
+
+    for (ulong i= 0, curr_size= 0; i < n_frag; i++, copied_size += curr_size)
+    {
+      curr_size= rand() % (2 * (total_size - copied_size) / (n_frag - i));
+
+      DBUG_ASSERT(curr_size <= total_size - copied_size || i == n_frag - 1);
+
+      res= my_b_copy_to_file(&info, file, curr_size);
+      ok(res == 0, "%lu of the cache copied to file", curr_size);
+    }
+    /*
+      Regardless of total_size <> copied_size the function succeeds:
+      when total_size < copied_size the huge overflowed value of the last
+      argument is ignored because nothing already left uncopied in the cache.
+    */
+    res= my_b_copy_to_file(&info, file, total_size - copied_size);
+    ok(res == 0, "%lu of the cache copied to file", total_size - copied_size);
+    ok(my_ftell(file, my_flags) == sizeof(buf),
+       "file written in %d fragments", n_frag+1);
+
+    res= reinit_io_cache(&info, WRITE_CACHE, total_size, 0, 0);
+    ok(res == 0 && my_b_tell(&info) == sizeof(buf), "cache turned to write");
+  }
+  close_cached_file(&info);
+  my_fclose(file, my_flags);
+  my_delete(file_name, MYF(MY_WME));
+}
+
 int main(int argc __attribute__((unused)),char *argv[])
 {
   MY_INIT(argv[0]);
-  plan(114);
+  plan(277);
 
   /* temp files with and without encryption */
   encrypt_tmp_files= 1;
@@ -391,6 +458,7 @@ int main(int argc __attribute__((unused)),char *argv[])
 
   mdev14014();
   mdev17133();
+  mdev10963();
 
   my_end(0);
   return exit_status();