perf script python: Add trace_context extension module to sys.modules

In Python3, the result of PyModule_Create (called from
scripts/python/Perf-Trace-Util/Context.c) is not automatically added to
sys.modules.  See: https://bugs.python.org/issue4592

Below is the observed behavior without the fix:

  # ldd /usr/bin/perf | grep -i python
	libpython3.6m.so.1.0 => /usr/lib64/libpython3.6m.so.1.0 (0x00007f8e1dfb2000)

  # perf record /bin/false
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.015 MB perf.data (17 samples) ]

  # perf script -g python | cat
  generated Python script: perf-script.py

  # perf script -s ./perf-script.py
  Traceback (most recent call last):
    File "./perf-script.py", line 18, in <module>
      from perf_trace_context import *
  ModuleNotFoundError: No module named 'perf_trace_context'
  Error running python script ./perf-script.py
  #

Committer notes:

To build with python3 use:

  $ make -C tools/perf PYTHON=python3

Use a non-const variable to pass the 'name' arg to
PyImport_AppendInittab(), as python2.6 has that as 'char *', which ends
up trowing this in some environments:

   CC       /tmp/build/perf/util/parse-branch-options.o
  util/scripting-engines/trace-event-python.c: In function 'python_start_script':
  util/scripting-engines/trace-event-python.c:1520:2: error: passing argument 1 of 'PyImport_AppendInittab' discards 'const' qualifier from pointer target type [-Werror]
    PyImport_AppendInittab("perf_trace_context", initfunc);
    ^
  In file included from /usr/include/python2.6/Python.h:130:0,
                   from util/scripting-engines/trace-event-python.c:22:
  /usr/include/python2.6/import.h:54:17: note: expected 'char *' but argument is of type 'const char *'
   PyAPI_FUNC(int) PyImport_AppendInittab(char *name, void (*initfunc)(void));
                   ^
  cc1: all warnings being treated as errors

Signed-off-by: Tony Jones <tonyj@suse.de>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jaroslav Škarvada <jskarvad@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Cc: Seeteena Thoufeek <s1seetee@linux.vnet.ibm.com>
Fixes: 66dfdff03d ("perf tools: Add Python 3 support")
Link: http://lkml.kernel.org/r/20190124005229.16146-2-tonyj@suse.de
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/scripting-engines/trace-event-python.c

@@ -1494,34 +1494,40 @@ static void _free_command_line(wchar_t **command_line, int num)
 static int python_start_script(const char *script, int argc, const char **argv)
 {
 	struct tables *tables = &tables_global;
+	PyMODINIT_FUNC (*initfunc)(void);
 #if PY_MAJOR_VERSION < 3
 	const char **command_line;
 #else
 	wchar_t **command_line;
 #endif
-	char buf[PATH_MAX];
+	/*
+	 * Use a non-const name variable to cope with python 2.6's
+	 * PyImport_AppendInittab prototype
+	 */
+	char buf[PATH_MAX], name[19] = "perf_trace_context";
 	int i, err = 0;
 	FILE *fp;
 
 #if PY_MAJOR_VERSION < 3
+	initfunc = initperf_trace_context;
 	command_line = malloc((argc + 1) * sizeof(const char *));
 	command_line[0] = script;
 	for (i = 1; i < argc + 1; i++)
 		command_line[i] = argv[i - 1];
 #else
+	initfunc = PyInit_perf_trace_context;
 	command_line = malloc((argc + 1) * sizeof(wchar_t *));
 	command_line[0] = Py_DecodeLocale(script, NULL);
 	for (i = 1; i < argc + 1; i++)
 		command_line[i] = Py_DecodeLocale(argv[i - 1], NULL);
 #endif
 
+	PyImport_AppendInittab(name, initfunc);
 	Py_Initialize();
 
 #if PY_MAJOR_VERSION < 3
-	initperf_trace_context();
 	PySys_SetArgv(argc + 1, (char **)command_line);
 #else
-	PyInit_perf_trace_context();
 	PySys_SetArgv(argc + 1, command_line);
 #endif