Fix for BUG#41541 - Valgrind warnings on packed MyISAM table

            
After the table is compressed by the myisampack utility,
opening the table by the server produces valgrind warnings.
      
This happens because when we try to read a record into the buffer
we alway assume that the remaining buffer to read is always equal 
to word size(4 or 8 or 2 bytes) we read. Sometimes we have 
remaining buffer size less than word size and trying to read the 
entire word size will end up in valgrind errors.
            
Fixed by reading byte by byte when we detect the remaining buffer 
size is less than the word size.

myisam/mi_packrec.c:
  Fixed fill_buffer() to read byte by byte when the remaining 
  buffer size is less than word size.
mysql-test/r/myisampack.result:
  Result file for BUG#41541
mysql-test/t/myisampack.test:
  Testcase for BUG#41541

myisam/mi_packrec.c

@@ -1430,6 +1430,32 @@ static void fill_buffer(MI_BIT_BUFF *bit_buff)
     bit_buff->current_byte=0;
     return;
   }
+  else
+  {
+    uint len= 0;
+    uint i= 0;
+    /*
+      Check if the remaining buffer/record to read is less than the word size.
+      If so read byte by byte
+
+      Note: if this branch becomes a bottleneck it can be removed, assuming
+      that the second memory segment allocates 7 extra bytes (see
+      _mi_read_pack_info()).
+    */
+    len= bit_buff->end - bit_buff->pos;
+    if (len < (BITS_SAVED / 8))
+    {
+      bit_buff->current_byte= 0;
+      for (i=0 ; i < len ; i++)
+      {
+        bit_buff->current_byte+= (((uint) ((uchar) bit_buff->pos[len - i - 1]))
+                                                                    << (8 * i));
+      }
+      bit_buff->pos= bit_buff->end;
+      return;
+    }
+  }
+
 #if BITS_SAVED == 64
   bit_buff->current_byte=  ((((uint) ((uchar) bit_buff->pos[7]))) +
 			     (((uint) ((uchar) bit_buff->pos[6])) << 8) +

mysql-test/r/myisampack.result

@@ -27,3 +27,14 @@ CHECK TABLE t1 EXTENDED;
 Table	Op	Msg_type	Msg_text
 test.t1	check	status	OK
 DROP TABLE t1;
+#
+# BUG#41541 - Valgrind warnings on packed MyISAM table
+#
+CREATE TABLE  t1(f1 VARCHAR(200), f2 TEXT);
+INSERT INTO  t1 VALUES ('foo', 'foo1'), ('bar', 'bar1');
+FLUSH TABLE t1;
+# Compress the table using MYISAMPACK tool
+SELECT COUNT(*) FROM t1;
+COUNT(*)
+1024
+DROP TABLE t1;

mysql-test/t/myisampack.test

@@ -31,3 +31,22 @@ FLUSH TABLES;
 --exec $MYISAMCHK -s --unpack $MYSQLTEST_VARDIR/master-data/test/t1
 CHECK TABLE t1 EXTENDED;
 DROP TABLE t1;
+
+--echo #
+--echo # BUG#41541 - Valgrind warnings on packed MyISAM table
+--echo #
+CREATE TABLE  t1(f1 VARCHAR(200), f2 TEXT);
+INSERT INTO  t1 VALUES ('foo', 'foo1'), ('bar', 'bar1');
+let $i=9;
+--disable_query_log
+while ($i)
+{
+ INSERT INTO t1 SELECT * FROM t1; 
+ dec $i; 
+}
+--enable_query_log
+FLUSH TABLE t1; 
+--echo # Compress the table using MYISAMPACK tool
+--exec $MYISAMPACK $MYSQLTEST_VARDIR/master-data/test/t1
+SELECT COUNT(*) FROM t1;
+DROP TABLE t1;