Re: t0301-credential-cache test failure on cygwin

On Wed, Jul 13, 2022 at 03:42:38PM +0100, Adam Dinwoodie wrote:

Hmm. So I had assumed that the problem was unlink()ing the socket path while the client was still trying to read(). If that's the case, then I _think_ the minimal reproduction below should also trigger the problem. That might give you something more useful to show to Cygwin folks.

But...

...this is about select outcomes. Which makes me think two things (though these are really pretty blind guesses, as I'm not at all familiar with Cygwin's socket code):

  - it may be that this situation was always ECONNABORTED (it's not on
    Linux, but perhaps due to details of the underlying socket code on
    Windows, it's different there). And this patch is just surfacing
    that state to the caller better.

  - it could be unrelated to the unlink() entirely. The socket code in
    Git dups the client descriptor and opens two FILE* handles, one for
    reading and one for writing. Could it be that it's important to
    fclose() one before the other (and the implicit closing done by
    exit() does the wrong order)? It seems like a stretch, but this
    commit message is talking about a shutdown(SHUT_RD), which is kind
    of what you get by fclose-ing one side (though again, not on Linux,
    because the "FILE*" are aware that they are read/write, but the
    underlying descriptors aren't).

Or maybe those are just totally off track. I know you said you don't have time to dig further, and that's fine. But if you or anybody has a chance to try the program below, it might be interesting to see the result (it would confirm whether it's the unlink() that's the problem).

-- >8 -- #include <stdio.h> #include <stdlib.h>

#include <unistd.h> #include <sys/socket.h> #include <sys/un.h>

#define SOCKET_PATH "mysocket"

static void diesys(const char *syscall)
{
	perror(syscall);
	exit(1);
}

static int create_server(void)
{
	int fd;
	struct sockaddr_un sa = {
		.sun_family = AF_UNIX,
		.sun_path = SOCKET_PATH,
	};

	fd = socket(AF_UNIX, SOCK_STREAM, 0);
	if (fd < 0)
		diesys("socket");
	if (bind(fd, (struct sockaddr *)&sa, sizeof(sa)) < 0)
		diesys("bind");
	if (listen(fd, 5) < 0)
		diesys("listen");

	return fd;
}

static void do_server(int listen_fd)
{
	int client_fd = accept(listen_fd, NULL, NULL);
	if (client_fd < 0)
		diesys("accept");
	if (unlink(SOCKET_PATH) < 0)
		diesys("unlink");
	if (close(client_fd) < 0)
		diesys("close(client)");
	close(listen_fd);
}

static void do_client(void)
{
	int fd;
	char buf[64];
	struct sockaddr_un sa = {
		.sun_family = AF_UNIX,
		.sun_path = SOCKET_PATH,
	};

	fd = socket(AF_UNIX, SOCK_STREAM, 0);
	if (fd < 0)
		diesys("socket");
	if (connect(fd, (struct sockaddr *)&sa, sizeof(sa)) < 0)
		diesys("connect");
	if (read(fd, buf, sizeof(buf)) < 0)
		diesys("read");
	close(fd);
}

int main(void)
{
	int fd = create_server();

	if (fork()) {
		do_server(fd);
	} else {
		close(fd);
		do_client();
	}
	return 0;
}