Handling change file message in libunit.

This is required for proper log file rotation action.

src/nxt_unit.c

@@ -767,6 +767,16 @@ nxt_unit_process_msg(nxt_unit_ctx_t *ctx, nxt_unit_port_id_t *port_id,
     case _NXT_PORT_MSG_CHANGE_FILE:
         nxt_unit_debug(ctx, "#%"PRIu32": change_file: fd %d",
                        port_msg->stream, recv_msg.fd);
+
+        if (dup2(recv_msg.fd, lib->log_fd) == -1) {
+            nxt_unit_alert(ctx, "#%"PRIu32": dup2(%d, %d) failed: %s (%d)",
+                           port_msg->stream, recv_msg.fd, lib->log_fd,
+                           strerror(errno), errno);
+
+            goto fail;
+        }
+
+        rc = NXT_UNIT_OK;
         break;
 
     case _NXT_PORT_MSG_MMAP:

test/python/log_body/wsgi.py

@@ -2,7 +2,7 @@ def application(environ, start_response):
     content_length = int(environ.get('CONTENT_LENGTH', 0))
     body = bytes(environ['wsgi.input'].read(content_length))
 
-    environ['wsgi.errors'].write(body)
+    environ['wsgi.errors'].write(body.decode())
     environ['wsgi.errors'].flush()
 
     start_response('200', [('Content-Length', '0')])

test/test_usr1.py

@@ -51,7 +51,6 @@ class TestUSR1(TestApplicationPython):
             self.search_in_log(r'/usr1', log_new), 'rename new 2'
         )
 
-    @unittest.skip('not yet')
     def test_usr1_unit_log(self):
         self.load('log_body')