mysqld: --dumpable option for KSPP w/ hidepid=2

This provides a mysqld option to force prctl(PR_SET_DUMPABLE, 1).
The existing code-path to 

wsrep_sst_rsync does not work under KSPP with hidepid=2 cannot see the mysqld
process, because it has PR_SET_DUMPABLE==0.

Users with KSPP & hidepid=2 should set --dumpable for now. 

In future it would better if wsrep_sst_rsync did not assume that mysqld was not
present just because it could not see the PID.

Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>

diff -Nuar --exclude '*~' mysql.orig/sql/mysqld.cc mysql/sql/mysqld.cc
--- mysql.orig/sql/mysqld.cc	2018-02-25 19:27:16.000000000 -0800
+++ mysql/sql/mysqld.cc	2018-04-26 16:20:28.085268150 -0700
@@ -2485,7 +2485,7 @@
 static inline void allow_coredumps()
 {
 #ifdef PR_SET_DUMPABLE
-  if (test_flags & TEST_CORE_ON_SIGNAL)
+  if (test_flags & TEST_DUMPABLE)
   {
     /* inform kernel that process is dumpable */
     (void) prctl(PR_SET_DUMPABLE, 1);
@@ -7364,6 +7364,8 @@
    0, 0, 0},
   {"core-file", OPT_WANT_CORE, "Write core on errors.", 0, 0, 0, GET_NO_ARG,
    NO_ARG, 0, 0, 0, 0, 0, 0},
+  {"dumpable", OPT_DUMPABLE, "Force prctl(2) PR_SET_DUMPABLE", 0, 0, 0, GET_NO_ARG,
+   NO_ARG, 0, 0, 0, 0, 0, 0},
 #ifdef DBUG_OFF
   {"debug", '#', "Built in DBUG debugger. Disabled in this build.",
    &current_dbug_option, &current_dbug_option, 0, GET_STR, OPT_ARG,
@@ -9336,6 +9338,9 @@
     opt_skip_name_resolve= 1;
     opt_specialflag|=SPECIAL_NO_RESOLVE;
     break;
+  case (int) OPT_DUMPABLE:
+    test_flags |= TEST_DUMPABLE;
+    break;
   case (int) OPT_WANT_CORE:
-    test_flags |= TEST_CORE_ON_SIGNAL;
+    test_flags |= TEST_CORE_ON_SIGNAL | TEST_DUMPABLE;
     break;
diff -Nuar --exclude '*~' mysql.orig/sql/mysqld.h mysql/sql/mysqld.h
--- mysql.orig/sql/mysqld.h	2018-02-25 19:27:16.000000000 -0800
+++ mysql/sql/mysqld.h	2018-04-26 16:21:47.516871026 -0700
@@ -59,6 +59,9 @@
 #define TEST_SIGINT		1024U	/**< Allow sigint on threads */
 #define TEST_SYNCHRONIZATION    2048U   /**< get server to do sleep in
                                            some places */
+#define TEST_DUMPABLE 4096U /**< Allow PR_SET_DUMPABLE without other 
+                               side-effects of TEST_CORE_ON_SIGNAL,
+                               for use with KSPP & SST */
 
 /* Keep things compatible */
 #define OPT_DEFAULT SHOW_OPT_DEFAULT
@@ -691,6 +694,7 @@
   OPT_SSL_KEY,
   OPT_THREAD_CONCURRENCY,
   OPT_WANT_CORE,
+  OPT_DUMPABLE,
 #ifdef WITH_WSREP
   OPT_WSREP_CAUSAL_READS,
   OPT_WSREP_SYNC_WAIT,