Use NULL instead of 0

GCC 15 introduced a new warning "-Wzero-as-null-pointer-constant" which
checks for places where '0' has been used in place of the NULL macro.

This showed a few places in Unit where we were using '0' instead of the
more correct NULL. E.g.

  $ make -j4 EXTRA_CFLAGS=-Wzero-as-null-pointer-constant
  ...
  src/nxt_buf.c: In function ‘nxt_buf_mmap_alloc’:
  src/nxt_buf.h:192:21: error: zero as null pointer constant [-Werror=zero-as-null-pointer-constant]
    192 |         (bm)->start = 0;                                                      \
        |                     ^
  src/nxt_buf.c:135:9: note: in expansion of macro ‘nxt_buf_mem_set_size’
    135 |         nxt_buf_mem_set_size(&b->mem, size);
        |         ^~~~~~~~~~~~~~~~~~~~

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

Author: ac000

src/nxt_buf.h

@@ -189,7 +189,7 @@ struct nxt_buf_s {
 
 #define nxt_buf_mem_set_size(bm, size)                                        \
     do {                                                                      \
-        (bm)->start = 0;                                                      \
+        (bm)->start = NULL;                                                   \
         (bm)->end = (void *) size;                                            \
     } while (0)
 

src/nxt_unit.c

@@ -716,7 +716,7 @@ nxt_unit_ctx_init(nxt_unit_impl_t *lib, nxt_unit_ctx_impl_t *ctx_impl,
     ctx_impl->req.req.unit = &lib->unit;
 
     ctx_impl->read_port = NULL;
-    ctx_impl->requests.slot = 0;
+    ctx_impl->requests.slot = NULL;
 
     return NXT_UNIT_OK;
 }

src/python/nxt_python_asgi_http.c

@@ -45,10 +45,10 @@ static PyObject *nxt_py_asgi_http_done(PyObject *self, PyObject *future);
 
 
 static PyMethodDef nxt_py_asgi_http_methods[] = {
-    { "receive",   nxt_py_asgi_http_receive, METH_NOARGS, 0 },
-    { "send",      nxt_py_asgi_http_send,    METH_O,      0 },
-    { "_done",     nxt_py_asgi_http_done,    METH_O,      0 },
-    { NULL, NULL, 0, 0 }
+    { "receive",   nxt_py_asgi_http_receive, METH_NOARGS, NULL },
+    { "send",      nxt_py_asgi_http_send,    METH_O,      NULL },
+    { "_done",     nxt_py_asgi_http_done,    METH_O,      NULL },
+    {}
 };
 
 static PyAsyncMethods nxt_py_asgi_async_methods = {

src/python/nxt_python_asgi_lifespan.c

@@ -48,10 +48,10 @@ static void nxt_py_asgi_lifespan_dealloc(PyObject *self);
 
 
 static PyMethodDef nxt_py_asgi_lifespan_methods[] = {
-    { "receive",   nxt_py_asgi_lifespan_receive, METH_NOARGS, 0 },
-    { "send",      nxt_py_asgi_lifespan_send,    METH_O,      0 },
-    { "_done",     nxt_py_asgi_lifespan_done,    METH_O,      0 },
-    { NULL, NULL, 0, 0 }
+    { "receive",   nxt_py_asgi_lifespan_receive, METH_NOARGS, NULL },
+    { "send",      nxt_py_asgi_lifespan_send,    METH_O,      NULL },
+    { "_done",     nxt_py_asgi_lifespan_done,    METH_O,      NULL },
+    {}
 };
 
 static PyMemberDef nxt_py_asgi_lifespan_members[] = {

src/python/nxt_python_asgi_websocket.c

@@ -70,10 +70,10 @@ static PyObject *nxt_py_asgi_websocket_done(PyObject *self, PyObject *future);
 
 
 static PyMethodDef nxt_py_asgi_websocket_methods[] = {
-    { "receive",   nxt_py_asgi_websocket_receive, METH_NOARGS, 0 },
-    { "send",      nxt_py_asgi_websocket_send,    METH_O,      0 },
-    { "_done",     nxt_py_asgi_websocket_done,    METH_O,      0 },
-    { NULL, NULL, 0, 0 }
+    { "receive",   nxt_py_asgi_websocket_receive, METH_NOARGS, NULL },
+    { "send",      nxt_py_asgi_websocket_send,    METH_O,      NULL },
+    { "_done",     nxt_py_asgi_websocket_done,    METH_O,      NULL },
+    {}
 };
 
 static PyAsyncMethods nxt_py_asgi_async_methods = {

src/python/nxt_python_wsgi.c

@@ -108,10 +108,10 @@ static PyMethodDef nxt_py_write_method[] = {
 
 
 static PyMethodDef nxt_py_input_methods[] = {
-    { "read",      (PyCFunction) nxt_py_input_read,      METH_VARARGS, 0 },
-    { "readline",  (PyCFunction) nxt_py_input_readline,  METH_VARARGS, 0 },
-    { "readlines", (PyCFunction) nxt_py_input_readlines, METH_VARARGS, 0 },
-    { NULL, NULL, 0, 0 }
+    { "read",      (PyCFunction) nxt_py_input_read,      METH_VARARGS, NULL },
+    { "readline",  (PyCFunction) nxt_py_input_readline,  METH_VARARGS, NULL },
+    { "readlines", (PyCFunction) nxt_py_input_readlines, METH_VARARGS, NULL },
+    {}
 };
 
 

src/ruby/nxt_ruby.c

@@ -1081,7 +1081,7 @@ nxt_ruby_rack_result_body(nxt_unit_request_info_t *req, VALUE result)
         }
 
     } else if (rb_respond_to(body, rb_intern("each"))) {
-        rb_block_call(body, rb_intern("each"), 0, 0,
+        rb_block_call(body, rb_intern("each"), 0, NULL,
                       nxt_ruby_rack_result_body_each, (VALUE) (uintptr_t) req);
 
     } else {