BUG#14726272- BACKPORT FIX FOR BUG 11746142 TO 5.5 AND 5.1

Details of BUG#11746142: CALLING MYSQLD WHILE ANOTHER 
INSTANCE IS RUNNING, REMOVES PID FILE
Fix: Before removing the pid file, ensure it was created
by the same process, leave it intact otherwise.

sql/mysqld.cc:
  delete_pid_file() introduced, which checks that the pid file
          belongs to the process before removing it

sql/mysqld.cc

@@ -867,6 +867,7 @@ static void clean_up(bool print_message);
 static int test_if_case_insensitive(const char *dir_name);
 
 #ifndef EMBEDDED_LIBRARY
+static bool pid_file_created= false;
 static void usage(void);
 static void start_signal_handler(void);
 static void close_server_sock();
@@ -875,6 +876,7 @@ static void wait_for_signal_thread_to_end(void);
 static void create_pid_file();
 static void end_ssl();
 #endif
+static void delete_pid_file(myf flags);
 
 
 #ifndef EMBEDDED_LIBRARY
@@ -1395,10 +1397,7 @@ void clean_up(bool print_message)
   debug_sync_end();
 #endif /* defined(ENABLED_DEBUG_SYNC) */
 
-#if !defined(EMBEDDED_LIBRARY)
-  if (!opt_bootstrap)
-    (void) my_delete(pidfile_name,MYF(0));	// This may not always exist
-#endif
+  delete_pid_file(MYF(0));
   if (print_message && errmesg && server_start_time)
     sql_print_information(ER(ER_SHUTDOWN_COMPLETE),my_progname);
   thread_scheduler.end();
@@ -4387,9 +4386,7 @@ we force server id to 2, but this MySQL server will not act as a slave.");
     (void) pthread_kill(signal_thread, MYSQL_KILL_SIGNAL);
 #endif /* __NETWARE__ */
 
-    if (!opt_bootstrap)
-      (void) my_delete(pidfile_name,MYF(MY_WME));	// Not needed anymore
-
+    delete_pid_file(MYF(MY_WME));
     if (unix_sock != INVALID_SOCKET)
       unlink(mysqld_unix_port);
     exit(1);
@@ -9098,12 +9095,13 @@ static void create_pid_file()
   if ((file = my_create(pidfile_name,0664,
 			O_WRONLY | O_TRUNC, MYF(MY_WME))) >= 0)
   {
-    char buff[21], *end;
+    char buff[MAX_BIGINT_WIDTH + 1], *end;
     end= int10_to_str((long) getpid(), buff, 10);
     *end++= '\n';
     if (!my_write(file, (uchar*) buff, (uint) (end-buff), MYF(MY_WME | MY_NABP)))
     {
       (void) my_close(file, MYF(0));
+      pid_file_created= true;
       return;
     }
     (void) my_close(file, MYF(0));
@@ -9113,6 +9111,38 @@ static void create_pid_file()
 }
 #endif /* EMBEDDED_LIBRARY */
 
+
+/**
+  Remove the process' pid file.
+  
+  @param  flags  file operation flags
+*/
+
+static void delete_pid_file(myf flags)
+{
+#ifndef EMBEDDED_LIBRARY
+  File file;
+  if (opt_bootstrap ||
+      !pid_file_created ||
+      !(file= my_open(pidfile_name, O_RDONLY, flags)))
+    return;
+
+  /* Make sure that the pid file was created by the same process. */
+  uchar buff[MAX_BIGINT_WIDTH + 1];
+  size_t error= my_read(file, buff, sizeof(buff), flags);
+  my_close(file, flags);
+  buff[sizeof(buff) - 1]= '\0';
+  if (error != MY_FILE_ERROR &&
+      atol((char *) buff) == (long) getpid())
+  {
+    my_delete(pidfile_name, flags);
+    pid_file_created= false;
+  }
+#endif /* EMBEDDED_LIBRARY */
+  return;
+}
+
+
 /** Clear most status variables. */
 void refresh_status(THD *thd)
 {