BUG#51866 - crash with repair by sort and fulltext keys

Repairing MyISAM table with fulltext indexes and low
myisam_sort_buffer_size may crash the server.

Estimation of number of index entries was done incorrectly,
causing further assertion failure or server crash.

Docs note: min value for myisam_sort_buffer_size has been
changed from 4 to 4096.

mysql-test/r/fulltext.result:
  A test case for BUG#51866.
mysql-test/r/myisam.result:
  Min value for myisam_sort_buffer_size is 4096.
mysql-test/r/variables.result:
  Min value for myisam_sort_buffer_size is 4096.
mysql-test/suite/sys_vars/r/myisam_sort_buffer_size_basic_32.result:
  Min value for myisam_sort_buffer_size is 4096.
mysql-test/t/fulltext.test:
  A test case for BUG#51866.
sql/mysqld.cc:
  Min value for myisam_sort_buffer_size is 4096.
storage/myisam/mi_check.c:
  When estimating number of index entries for external
  fulltext parser, take into account that key_length may
  be bigger than myisam_sort_buffer_size. Reuse logic
  from _create_index_by_sort(): force MIN_SORT_BUFFER to
  be min value for myisam_sort_buffer_size.
  
  Another problem is that ftkey_nr has no other meaning
  than serial number of fulltext index starting with 1.
  We can't say if this key using built-in or external
  parser basing on it's value. In other words we always
  entered if-branch for external parser. At this point,
  the only way to check if we use default parser is to
  compare keyinfo::parser with &ft_default_parser.
storage/myisam/sort.c:
  Get rid of MIN_SORT_MEMORY, use MIN_SORT_BUFFER instead
  (defined in myisamdef.h, has the same value and purpose).

1 files changed, 4 insertions, 0 deletions

diff --git a/mysql-test/r/myisam.result b/mysql-test/r/myisam.result
index ba417d54998..508523fda21 100644
--- a/mysql-test/r/myisam.result
+++ b/mysql-test/r/myisam.result
@@ -2313,6 +2313,8 @@ INSERT INTO t1 SELECT a+1280,b FROM t1;
 INSERT INTO t1 SELECT a+2560,b FROM t1;
 INSERT INTO t1 SELECT a+5120,b FROM t1;
 SET myisam_sort_buffer_size=4;
+Warnings:
+Warning	1292	Truncated incorrect myisam_sort_buffer_size value: '4'
 REPAIR TABLE t1;
 Table	Op	Msg_type	Msg_text
 test.t1	repair	error	myisam_sort_buffer_size is too small
@@ -2363,6 +2365,8 @@ CREATE TABLE t1(a CHAR(255), KEY(a));
 SELECT * FROM t1, t1 AS a1;
 a	a
 SET myisam_sort_buffer_size=4;
+Warnings:
+Warning	1292	Truncated incorrect myisam_sort_buffer_size value: '4'
 INSERT INTO t1 VALUES
 ('0'),('0'),('0'),('0'),('0'),('0'),('0'),('0'),('0'),('0'),
 ('0'),('0'),('0'),('0'),('0'),('0'),('0'),('0'),('0'),('0'),