memprof: set default path to profiling output file

sysprof has an optional parameter `path`, that sets a path to
the profiling output file. By default, the path is `sysprof.bin`.
`misc.memprof.start()` requires setting a path to the profiling
output file. The patch fixes this inconsistency by introducing a
default path to the memprof profiling output file - `memprof.bin`.

src/lib_misc.c

@@ -386,6 +386,8 @@ LJLIB_CF(misc_sysprof_report)
 
 #define LJLIB_MODULE_misc_memprof
 
+#define MEMPROF_DEFAULT_OUTPUT "memprof.bin"
+
 /* local started, err, errno = misc.memprof.start(fname) */
 LJLIB_CF(misc_memprof_start)
 {
@@ -394,7 +396,8 @@ LJLIB_CF(misc_memprof_start)
   return sysprof_error(L, PROFILE_ERRUSE, err_details);
 #else
   struct lj_memprof_options opt = {0};
-  const char *fname = strdata(lj_lib_checkstr(L, 1));
+  GCstr *s = lj_lib_optstr(L, 1);
+  const char *fname = s ? strdata(s) : MEMPROF_DEFAULT_OUTPUT;
   struct profile_ctx *ctx;
   int memprof_status;
 

test/tarantool-tests/profilers/misclib-memprof-lapi-default-file.test.lua

@@ -0,0 +1,37 @@
+local tap = require('tap')
+local test = tap.test('misc-memprof-lapi-default-file'):skipcond({
+  ['Memprof is implemented for x86_64 only'] = jit.arch ~= 'x86' and
+                                               jit.arch ~= 'x64',
+  ['Memprof is disabled'] = os.getenv('LUAJIT_DISABLE_MEMPROF'),
+})
+
+test:plan(1)
+
+local tools = require('utils.tools')
+
+test:test('default-output-file', function(subtest)
+
+  subtest:plan(1)
+
+  local default_output_file = 'memprof.bin'
+  os.remove(default_output_file)
+
+  local _, _ = misc.memprof.start()
+
+  local res, err = misc.memprof.stop()
+
+  -- Want to cleanup carefully if something went wrong.
+  if not res then
+    os.remove(default_output_file)
+    error(err)
+  end
+
+  local profile_buf = tools.read_file(default_output_file)
+  subtest:ok(profile_buf ~= nil and #profile_buf ~= 0,
+             'default output file is not empty')
+
+  -- We don't need it any more.
+  os.remove(default_output_file)
+end)
+
+test:done(true)