Commit d78ada4a ("perf tests attr: Do not store failed events") does
not create an event file in the /tmp directory when the
perf_open_event() system call failed.

This can lead to a situation where not /tmp/event-xx-yy-zz result file
exists at all (for example on a s390x virtual machine environment) where
no CPUMF hardware is available.

The following command then fails with a python call back chain instead
of printing failure:

  [root@s8360046 perf]# /usr/bin/python2 ./tests/attr.py -d ./tests/attr/ \
      -p ./perf -v -ttest-stat-basic
  running './tests/attr//test-stat-basic'
  Traceback (most recent call last):
    File "./tests/attr.py", line 379, in <module>
      main()
    File "./tests/attr.py", line 370, in main
      run_tests(options)
    File "./tests/attr.py", line 311, in run_tests
      Test(f, options).run()
    File "./tests/attr.py", line 300, in run
      self.compare(self.expect, self.result)
    File "./tests/attr.py", line 248, in compare
      exp_event.diff(res_event)
  UnboundLocalError: local variable 'res_event' referenced before assignment
  [root@s8360046 perf]#

This patch catches this pitfall and prints an error message instead:

  [root@s8360047 perf]# /usr/bin/python2 ./tests/attr.py -d ./tests/attr/ \
       -p ./perf  -vvv -ttest-stat-basic
  running './tests/attr//test-stat-basic'
    loading expected events
      Event event:base-stat
        fd = 1
        group_fd = -1
        flags = 0|8
        [....]
        sample_regs_user = 0
        sample_stack_user = 0
    'PERF_TEST_ATTR=/tmp/tmpJbMQMP ./perf stat -o /tmp/tmpJbMQMP/perf.data -e cycles kill >/dev/null 2>&1' ret '1', expected '1'
    loading result events
    compare
      matching [event:base-stat]
      match: [event:base-stat] matches []
      res_event is empty
  FAILED './tests/attr//test-stat-basic' - match failure
  [root@s8360047 perf]#
Signed-off-by: Thomas-Mich Richter <tmricht@linux.vnet.ibm.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Thomas-Mich Richter <tmricht@linux.vnet.ibm.com>
LPU-Reference: 20170913081209.39570-1-tmricht@linux.vnet.ibm.com
Link: http://lkml.kernel.org/n/tip-04d63nn7svfgxdhi60gq2mlm@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>