Python: avoiding buffering of exception backtraces.

A quote from the Python 3 documentation:

 | When interactive, stdout and stderr streams are line-buffered.
 | Otherwise, they are block-buffered like regular text files.

As a result, if an exception occurred and PyErr_Print() was called, its output
could be buffered but not printed to the log for a while (ultimately, until
the interpreter finalization).  If the application process crashed shortly,
the backtrace was completely lost.

Buffering can be disabled by redefining the sys.stderr stream object.
However, interference with standard environment objects was deemed undesirable.
Instead, sys.stderr.flush() is called every time after printing exceptions.
A potential advantage here is that lines from backtraces won't be mixed
with other lines in the log.

src/nxt_python_wsgi.c

@@ -86,6 +86,7 @@ static PyObject *nxt_py_input_read(nxt_py_input_t *self, PyObject *args);
 static PyObject *nxt_py_input_readline(nxt_py_input_t *self, PyObject *args);
 static PyObject *nxt_py_input_readlines(nxt_py_input_t *self, PyObject *args);
 
+static void nxt_python_print_exception(void);
 static int nxt_python_write(nxt_python_run_ctx_t *ctx, PyObject *bytes);
 
 struct nxt_python_run_ctx_s {
@@ -140,6 +141,7 @@ static PyTypeObject nxt_py_input_type = {
 };
 
 
+static PyObject           *nxt_py_stderr_flush;
 static PyObject           *nxt_py_application;
 static PyObject           *nxt_py_start_resp_obj;
 static PyObject           *nxt_py_write_obj;
@@ -238,6 +240,21 @@ nxt_python_init(nxt_task_t *task, nxt_common_app_conf_t *conf)
 
     module = NULL;
 
+    obj = PySys_GetObject((char *) "stderr");
+    if (nxt_slow_path(obj == NULL)) {
+        nxt_alert(task, "Python failed to get \"sys.stderr\" object");
+        goto fail;
+    }
+
+    nxt_py_stderr_flush = PyObject_GetAttrString(obj, "flush");
+    if (nxt_slow_path(nxt_py_stderr_flush == NULL)) {
+        nxt_alert(task, "Python failed to get \"flush\" attribute of "
+                        "\"sys.stderr\" object");
+        goto fail;
+    }
+
+    Py_DECREF(obj);
+
     if (c->path.length > 0) {
         obj = PyString_FromStringAndSize((char *) c->path.start,
                                          c->path.length);
@@ -308,7 +325,7 @@ nxt_python_init(nxt_task_t *task, nxt_common_app_conf_t *conf)
     module = PyImport_ImportModule(nxt_py_module);
     if (nxt_slow_path(module == NULL)) {
         nxt_alert(task, "Python failed to import module \"%s\"", nxt_py_module);
-        PyErr_Print();
+        nxt_python_print_exception();
         goto fail;
     }
 
@@ -404,7 +421,7 @@ nxt_python_request_handler(nxt_unit_request_info_t *req)
 
     if (nxt_slow_path(response == NULL)) {
         nxt_unit_req_error(req, "Python failed to call the application");
-        PyErr_Print();
+        nxt_python_print_exception();
 
         rc = NXT_UNIT_ERROR;
         goto done;
@@ -427,7 +444,7 @@ nxt_python_request_handler(nxt_unit_request_info_t *req)
                     if (nxt_slow_path(PyErr_Occurred() != NULL)) {
                         nxt_unit_req_error(req, "Python failed to iterate over "
                                            "the application response object");
-                        PyErr_Print();
+                        nxt_python_print_exception();
 
                         rc = NXT_UNIT_ERROR;
                     }
@@ -456,7 +473,7 @@ nxt_python_request_handler(nxt_unit_request_info_t *req)
         } else {
             nxt_unit_req_error(req,
                             "the application returned not an iterable object");
-            PyErr_Print();
+            nxt_python_print_exception();
 
             rc = NXT_UNIT_ERROR;
         }
@@ -468,7 +485,7 @@ nxt_python_request_handler(nxt_unit_request_info_t *req)
             if (nxt_slow_path(result == NULL)) {
                 nxt_unit_req_error(req, "Python failed to call the close() "
                                         "method of the application response");
-                PyErr_Print();
+                nxt_python_print_exception();
 
             } else {
                 Py_DECREF(result);
@@ -495,6 +512,7 @@ done:
 static void
 nxt_python_atexit(void)
 {
+    Py_XDECREF(nxt_py_stderr_flush);
     Py_XDECREF(nxt_py_application);
     Py_XDECREF(nxt_py_start_resp_obj);
     Py_XDECREF(nxt_py_write_obj);
@@ -733,7 +751,7 @@ nxt_python_add_sptr(nxt_python_run_ctx_t *ctx, const char *name,
         nxt_unit_req_error(ctx->req,
                            "Python failed to create value string \"%.*s\"",
                            (int) size, src);
-        PyErr_Print();
+        nxt_python_print_exception();
 
         return NXT_UNIT_ERROR;
     }
@@ -768,7 +786,7 @@ nxt_python_add_str(nxt_python_run_ctx_t *ctx, const char *name,
         nxt_unit_req_error(ctx->req,
                            "Python failed to create value string \"%.*s\"",
                            (int) size, str);
-        PyErr_Print();
+        nxt_python_print_exception();
 
         return NXT_UNIT_ERROR;
     }
@@ -1103,6 +1121,28 @@ nxt_py_input_readlines(nxt_py_input_t *self, PyObject *args)
 }
 
 
+static void
+nxt_python_print_exception(void)
+{
+    PyErr_Print();
+
+#if PY_MAJOR_VERSION == 3
+    /* The backtrace may be buffered in sys.stderr file object. */
+    {
+        PyObject  *result;
+
+        result = PyObject_CallFunction(nxt_py_stderr_flush, NULL);
+        if (nxt_slow_path(result == NULL)) {
+            PyErr_Clear();
+            return;
+        }
+
+        Py_DECREF(result);
+    }
+#endif
+}
+
+
 static int
 nxt_python_write(nxt_python_run_ctx_t *ctx, PyObject *bytes)
 {