[PATCH] BUG#30379 Better randomise time before retry in timeout check (DBTC)

timoOutLoopStartLab() checks if any transactions have been delayed
for so long that we are forced to perform some action (e.g. abort,
resend etc).

It is *MEANT* to (according to the comment):
> To avoid aborting both transactions in a deadlock detected by time-out
> we insert a random extra time-out of upto 630 ms by using the lowest
> six bits of the api connect reference.
> We spread it out from 0 to 630 ms if base time-out is larger than 3 sec,
> we spread it out from 0 to 70 ms if base time-out is smaller than 300 msec,
> and otherwise we spread it out 310 ms.

The comment (as all do) lies.

the API connect reference is not very random, producing incredibly
predictable "random" numbers. This could lead to both txns being
aborted instead of just one.

Before:
timeout value: 123 3
timeout value: 122 2
timeout value: 122 2
timeout value: 122 2
timeout value: 123 3

After:
timeout value: 127 7
timeout value: 126 6
timeout value: 129 9
timeout value: 139 19
timeout value: 137 17
timeout value: 151 31
timeout value: 130 10
timeout value: 132 12

Index: ndb-work/ndb/src/kernel/blocks/dbtc/DbtcMain.cpp
===================================================================

ndb/include/util/ndb_rand.h

+/* Copyright (C) 2003 MySQL AB
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; version 2 of the License.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA */
+
+#ifndef NDB_RAND_H
+#define NDB_RAND_H
+
+#define NDB_RAND_MAX 32767
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+int ndb_rand(void);
+
+void ndb_srand(unsigned seed);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif

ndb/src/common/util/Makefile.am

@@ -24,7 +24,8 @@ libgeneral_la_SOURCES = \
             uucode.c random.c version.c \
             strdup.c \
             ConfigValues.cpp ndb_init.c basestring_vsnprintf.c \
-            Bitmask.cpp
+            Bitmask.cpp \
+	    ndb_rand.c
 
 EXTRA_PROGRAMS = testBitmask
 testBitmask_SOURCES =  testBitmask.cpp

ndb/src/common/util/ndb_rand.c

+/* Copyright (C) 2003 MySQL AB
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; version 2 of the License.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA */
+
+#include <ndb_rand.h>
+
+static unsigned long next= 1;
+
+/**
+ * ndb_rand
+ *
+ * constant time, cheap, pseudo-random number generator.
+ *
+ * NDB_RAND_MAX assumed to be 32767
+ *
+ * This is the POSIX example for "generating the same sequence on
+ * different machines". Although that is not one of our requirements.
+ */
+int ndb_rand(void)
+{
+  next= next * 1103515245 + 12345;
+  return((unsigned)(next/65536) % 32768);
+}
+
+void ndb_srand(unsigned seed)
+{
+  next= seed;
+}
+

ndb/src/kernel/blocks/dbtc/DbtcMain.cpp

@@ -20,6 +20,7 @@
 #include <RefConvert.hpp>
 #include <ndb_limits.h>
 #include <my_sys.h>
+#include <ndb_rand.h>
 
 #include <signaldata/EventReport.hpp>
 #include <signaldata/TcKeyReq.hpp>
@@ -6278,7 +6279,8 @@ void Dbtc::timeOutLoopStartLab(Signal* signal, Uint32 api_con_ptr)
     jam();
     if (api_timer != 0) {
       Uint32 error= ZTIME_OUT_ERROR;
-      time_out_value= time_out_param + (api_con_ptr & mask_value);
+      time_out_value= time_out_param + (ndb_rand() & mask_value);
+      ndbout_c("timeout value: %u %u",time_out_value, time_out_value-time_out_param);
       if (unlikely(old_mask_value)) // abort during single user mode
       {
         apiConnectptr.i = api_con_ptr;