“There are no easy answers when it comes to reporting and handling errors.”
“P. J. Plauger”
I release a new version of the greylist daemon [1] and what happens? Mark [2] finds a bug.
Or rather, the new version didn't fix his current problem.
I take a look into the issue and it's not terribly surprising that I botched handing a particular error.
Sigh.
Error handling was taught by osmosis at college (Computer Science and Engineering at Florida Atlantic University) [3]. It was expected that we just magically pick up on handling errors but really, as long as the program didn't outright crash and produced something vaguely like the output, all was fine.
In fact, when I expressly asked one my instructors what to do when handling a particular thorny error, I was told, point blank: “if you don't know how to handle the error, then don't check for it.” The instructor, sadly, had a point—you could go mad from trying to handle every possible error condition.
It's not hard to test for errors. In C, just about every function you can think of can return an error (with a few exceptions, like getpid() under Unix—if a process can't get its own process ID, then you have other things to worry about). But checking the return of every function call gets tedious, fast. What was once a small concise function:
>
```
int create_socket(struct sockaddr *paddr,socklen_t saddr)
{
ListenNode listen;
struct epoll_event ev;
int reuse = 1;
assert(paddr != NULL);
assert(saddr > 0);
listen = malloc(sizeof(struct listen_node));
memset(listen,0,sizeof(struct listen_node));
memcpy(&listen->local,paddr,saddr);
listen->fn = event_read;
listen->sock = socket(paddr->sa_family,SOCK_DGRAM,0);
setsockopt(listen->sock,SOL_SOCKET,SO_REUSEADDR,&reuse,sizeof(reuse));
fcntl(listen->sock,F_GETFL,0);
fcntl(listen->sock,F_SETFL,rc | O_NONBLOCK);
bind(listen->sock,paddr,saddr);
memset(&ev,0,sizeof(ev));
ev.events = EPOLLIN;
ev.data.ptr = listen;
epoll_ctl(g_queue,EPOLL_CTL_ADD,listen->sock,&ev);
return listen->sock;
}
```
becomes twice as big as each function call is wrapped up in an if statement:
>
```
int create_socket(struct sockaddr *paddr,socklen_t saddr)
{
ListenNode listen;
struct epoll_event ev;
int reuse = 1;
assert(paddr != NULL);
assert(saddr > 0);
listen = malloc(sizeof(struct listen_node));
if (listen == NULL)
return -1;
memset(listen,0,sizeof(struct listen_node));
memcpy(&listen->local,paddr,saddr);
listen->fn = event_read;
listen->sock = socket(paddr->sa_family,SOCK_DGRAM,0);
if (listen->sock == -1)
{
perror("socket()");
return -1;
}
if (setsockopt(listen->sock,SOL_SOCKET,SO_REUSEADDR,&reuse,sizeof(reuse)) == -1)
{
perror("setsockopt()");
return -1;
}
if fcntl(listen->sock,F_GETFL,0) == -1)
{
perror("fcntl(GETFL)");
return -1;
}
if (fcntl(listen->sock,F_SETFL,rc | O_NONBLOCK) == -1)
{
perror("fcntl(SETFL)");
return -1;
}
if (bind(listen->sock,paddr,saddr) == -1)
{
perror("bind()");
return -1;
}
memset(&ev,0,sizeof(ev));
ev.events = EPOLLIN;
ev.data.ptr = listen;
if (epoll_ctl(g_queue,EPOLL_CTL_ADD,listen->sock,&ev) == -1)
{
perror("epoll_ctl(ADD)");
return -1;
}
return listen->sock;
}
```
And even this isn't all that great. The function prints what happened (via the perror() call) but that presupposes that stderr (the standard error reporting file) is open! If it's not open, well then … if you're lucky, perror() (or whatever code it eventually calls) checks to see if stderr is open and if not, just … fail … gracefully? I guess? Hopefully? Maybe?
But even if we could return what failed to the caller, then that means the implementation details get pushed out of the create_socket() function (which is typically considered a “bad idea”). Even setting that aside, what can the caller do? Well, not much really.
The socket() call could fail because there's not enough kernel memory, or there's too many files already open, we don't have permission to create the socket, or the protocol isn't supported. Don't have the right privileges? If the process isn't root, there's not much that can be done (and if the process was root, there wouldn't be an issue). Not enough kernel memory? I wouldn't even know what to do in that case except kill off a few processes or reboot the box. But in each case, there isn't much the program can do except give up (and maybe attempt to log the error somewhere).
And all the other calls are pretty much the same—no memory, no privileges, can't do it, etc. We can break the errors down into a few categories:
The problem being: a program can run for ages before you see an error (Mark ran my greylist daemon [4] for a few years before the error manifested itself, and only then because the server operating system was upgraded—the bug he hit was of the last category—something it could have handled, but I didn't handle it properly) so it's not uncommon for error paths in programs to have, well, errors (ironically enough).
In fact, there's really only three ways to handle errors:
Each method has its pros and cons and nobody is really happy with any of the methods, but really, that's all there is when you get down to it. The first is tedious, but doesn't require any special langauge features; the second requires support in the language, and even so, is really spaghetti code [6] in hiding and the third … well, again, no special language support is needed, isn't tedious and it tends to make the code fast but, well … let's just say that the error recovery can make a programmer go postal [7].
I just wish there was a better way …
I could claim I left finding the errors in the code above as an exercise for the reader, but really, I blew it [8].
[5] http://www.leconcombre.com/concpost/us/postcard4/alfred_e_neuman.html
[6] http://en.wikipedia.org/wiki/Spaghetti_code