non blocking connect() never succeeds

[mcgoo]

WSL never returns 0 for success if connect() is called multiple times on a non-blocking socket, it transitions straight from EINPROGRESS to EISCONN.

Build 10.0.16299

repro:

#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <errno.h>
#include <netinet/in.h>

int main() {

    int fd = socket( AF_INET, SOCK_STREAM | SOCK_NONBLOCK, 0 );
    if (fd < 0)
        perror("socket: ");

    struct sockaddr_in ip4addr;
    ip4addr.sin_family = AF_INET;
    ip4addr.sin_port = htons(80);
    inet_pton(AF_INET, "74.125.69.99", &ip4addr.sin_addr);


    for(;;) {

        int res = connect(fd, (struct sockaddr*)&ip4addr, sizeof(ip4addr));
        printf("called connect, res=%d ", res);
        if(res < 0) {
            printf("    errno=%d ", errno);
            switch(errno) {
                case EINPROGRESS:
                    printf("        in progress\n");
                    continue;
                case EALREADY:
                    printf("        already in progress\n");
                    continue;
                case EISCONN:
                    printf("        is connected\n");
                    goto done;
                default:
                    printf("        something else, bailing out\n");
                    goto done;
            }
        } else {
            printf("\n");
        }

    }
done:

    return 0;
}

Linux

called connect, res=-1     errno=115         in progress
called connect, res=-1     errno=114         already in progress
called connect, res=-1     errno=114         already in progress
...
called connect, res=-1     errno=114         already in progress
called connect, res=-1     errno=114         already in progress
called connect, res=0
called connect, res=-1     errno=106         is connected

WSL

called connect, res=-1     errno=115         in progress
called connect, res=-1     errno=114         already in progress
called connect, res=-1     errno=114         already in progress
...
called connect, res=-1     errno=114         already in progress
called connect, res=-1     errno=114         already in progress
called connect, res=-1     errno=106         is connected

Possibly related to #2846

[mcgoo]

Also present in 10.0.17093.1000.

[therealkenc]

Possibly related to #2846

I'd be surprised if it wasn't. Was irssi what you were tracking, or did you just happen across that issue? Which is to say, if you have another breaking case, the better. Great post on the merits either way.

[mcgoo]

No, it's a binary vendor library. I found the irssi issue by searching for WSL connect EINPROGRESS I think.

[therealkenc]

Just did the exercise and the OP (thank you for the concise repro) still does not behave correctly (return res=0) on WSL1 20180.

Better in WSL2.