dereferencing result of ngx_http_lua_get_req() without checking for NULL

[deem0n]

Hello, static checker svacer found several places where we are not checking req object for NULL, while it provided counter-examples where we are doing so.

Problematic files:

lua-nginx-module/src/ngx_http_lua_socket_tcp.c

Lines 5077 to 5083 in 5bd471a

	r = ngx_http_lua_get_req(L);
	if (r != r->main) {
	return luaL_error(L, "attempt to read the request body in a "
	"subrequest");
	}

lua-nginx-module/src/ngx_http_lua_semaphore.c

Lines 486 to 488 in 5bd471a

	r = ngx_http_lua_get_req(wait_co_ctx->co);
	c = r->connection;

lua-nginx-module/src/ngx_http_lua_semaphore.c

Lines 530 to 532 in 5bd471a

	r = ngx_http_lua_get_req(wait_co_ctx->co);
	c = r->connection;

lua-nginx-module/src/ngx_http_lua_output.c

Lines 490 to 496 in 5bd471a

	r = ngx_http_lua_get_req(L);
	if (n == 1 && r == r->main) {
	luaL_checktype(L, 1, LUA_TBOOLEAN);
	wait = lua_toboolean(L, 1);
	}

We have other places where we do proper checks

lua-nginx-module/src/ngx_http_lua_socket_tcp.c

Lines 4521 to 4525 in 5bd471a

	r = ngx_http_lua_get_req(L);
	if (r == NULL) {
	return luaL_error(L, "no request found");
	}

and 4 more cases in ngx_http_lua_socket_tcp.c, plus one case in ngx_http_lua_util.c:

lua-nginx-module/src/ngx_http_lua_util.c

Lines 3125 to 3129 in 5bd471a

	r = ngx_http_lua_get_req(L);
	if (r == NULL) {
	return 0;
	}

definition of ngx_http_lua_get_req() follows:

static ngx_inline ngx_http_request_t *
ngx_http_lua_get_req(lua_State *L)
{
#ifdef OPENRESTY_LUAJIT
    return lua_getexdata(L);
#else
    ngx_http_request_t    *r;

    lua_getglobal(L, ngx_http_lua_req_key);
    r = lua_touserdata(L, -1);
    lua_pop(L, 1);

    return r;
#endif
}

And it seems, that we don't need to check result of the ngx_http_lua_get_req() as it just returns global object, which is http request object and it must be always available in context of the Lua code execution, because Lua code is always executed inside http request processing pipeline.

Is it right? Can you please confirm that it is false positive static analyzer case ?

If not, I can provide boring MR with trivial fixes.

Thanks!