BUG#38826 Race in MYSQL_LOG::purge_logs is impossible to debug in production

BUG#39325 Server crash inside MYSQL_LOG::purge_first_log halts replicaiton

The patch reverses the order of the purging and updating events for log and
relay-log.info/index files respectively.

This solves the problem of having holes caused by crashes happening between updating
info/index files and purging logs.

This patch also contains an aditional test case for testing the crashing before purge logs.
      
NOTE1: This is a combined patch for BUG#38826 and BUG#39325. This patch is based on
bugteam tree and takes into account reviewers suggestions.

NOTE2: Merge from 5.0-bugteam

mysql-test/include/wait_until_connected_again.inc

@@ -6,7 +6,7 @@
 let $counter= 500;
 while ($mysql_errno)
 {
-  --error 0,2002,2006
+  --error 0,2002,2006,2003
   show status;
 
   dec $counter;

mysql-test/r/binlog_index.result

@@ -21,6 +21,7 @@ flush logs;
 *** must be a warning master-bin.000001 was not found ***
 Warnings:
 Warning	1612	Being purged log MYSQLTEST_VARDIR/log/master-bin.000001 was not found
+Warning	1612	Being purged log MYSQLTEST_VARDIR/log/master-bin.000001 was not found
 *** must show one record, of the active binlog, left in the index file after PURGE ***
 show binary logs;
 Log_name	File_size

mysql-test/suite/rpl/r/rpl_crash_before_purge_logs.result

+stop slave;
+drop table if exists t1,t2,t3,t4,t5,t6,t7,t8,t9;
+reset master;
+reset slave;
+drop table if exists t1,t2,t3,t4,t5,t6,t7,t8,t9;
+start slave;
+stop slave;
+DROP TABLE IF EXISTS t1;
+CREATE TABLE t1 (id INT);
+start slave;
+SET GLOBAL debug= "+d,crash_before_purge_logs";
+FLUSH LOGS;
+ERROR HY000: Lost connection to MySQL server during query
+start slave;

mysql-test/suite/rpl/t/rpl_crash_before_purge_logs.test

+--source include/master-slave.inc
+--source include/have_debug.inc
+--disable_reconnect
+
+# We have to sync with master, to ensure slave had time to start properly
+# # before we stop it. If not, we get errors about UNIX_TIMESTAMP() in the
+# log.
+sync_slave_with_master;
+stop slave;
+--source include/wait_for_slave_to_stop.inc
+
+# ON MASTER 
+
+connection master;
+
+--disable_warnings
+DROP TABLE IF EXISTS t1;
+--enable_warnings
+
+CREATE TABLE t1 (id INT);
+
+let $1=100;
+disable_query_log;
+begin;
+while ($1)
+{
+  eval INSERT INTO t1 VALUES( $1 );
+  dec $1;
+}
+DROP TABLE t1;
+save_master_pos;
+
+enable_query_log;
+
+## ON SLAVE
+
+connection slave;
+start slave;
+--source include/wait_for_slave_to_start.inc
+sync_with_master 0;
+connection master;
+save_master_pos;
+connection slave;
+
+--exec echo "restart" > $MYSQLTEST_VARDIR/tmp/slave0.expect
+SET GLOBAL debug= "+d,crash_before_purge_logs";
+
+--error 2013
+# try to rotate logs
+FLUSH LOGS;
+
+--enable_reconnect
+--source include/wait_until_connected_again.inc
+start slave;
+--source include/wait_for_slave_to_start.inc
+
+sync_with_master 0;

sql/log.h

@@ -233,6 +233,13 @@ class MYSQL_BIN_LOG: public TC_LOG, private MYSQL_LOG
   pthread_cond_t update_cond;
   ulonglong bytes_written;
   IO_CACHE index_file;
+  /*
+    purge_temp is a temp file used in purge_logs so that the index file
+    can be updated before deleting files from disk, yielding better crash
+    recovery. It is created on demand the first time purge_logs is called
+    and then reused for subsequent calls. It is cleaned up in cleanup().
+  */
+  IO_CACHE purge_temp;
   char index_file_name[FN_REFLEN];
   /*
      The max size before rotation (usable only if log_type == LOG_BIN: binary