What is the value of errno after epoll_ctl() fails?
On Thu, 12 Oct 2017 at 18:40, Ed Hemphill <[email protected]> wrote:
> It was an FD created with pipe() and, which was used to redirect a new
> process' stdout, the uv_poll_init() is called in the parent, and the in the
> child a dup2() is called to attach it to stdout or stderr, before the exec.
>
> So:
>
> fd = pipe()
>
> uv_poll_init(fd)
> uv_poll_start()
>
> and then in the child:
>
> dup2(fd,CHILD_STDOUT)
>
>
> Here is how I fixed it. And I think this may be an issue in libuv... I
> made an alternate version of uv_poll_init() with an alt uv_poll_check_fd() :
>
>
> int ww_alt_uv__io_check_fd(uv_loop_t* loop, int fd) {
> // struct uv__epoll_event e;
> int rc = 0;
> // see:
> https://stackoverflow.com/questions/12340695/how-to-check-if-a-given-file-descriptor-stored-in-a-variable-is-still-valid
> rc = fcntl(fd, F_GETFD);
> if (rc == -1) {
> rc = errno;
> }
> // e.events = POLLIN;
> // e.data = -1;
> //
> // rc = 0;
> // if (uv__epoll_ctl(loop->backend_fd, UV__EPOLL_CTL_ADD, fd, &e))
> // if (errno != EEXIST)
> // rc = -errno;
> //
> // // no abort() on failure, just report back fail
> // if (rc == 0)
> // if (uv__epoll_ctl(loop->backend_fd, UV__EPOLL_CTL_DEL, fd, &e))
> // rc = -errno;
> //
> return rc;
> }
>
>
> https://github.com/WigWagCo/greasego/blob/master/deps/src/greaseLib/deps/libuv-v1.10.1/src/unix/linux-core.c#L169
>
> I'm not sure how or why the check_fd code came about... I do get why the
> code abort()s, b/c if you can add but not delete, the FD set for epoll is
> totally messed up. But the problem with that check is you run the risk of
> messing the epoll set up, and having to do an abort(). And an abort() is a
> no go for us.
>
> What's interesting is while the del FD part of the code failed in the
> original, using this check above, the FD passes, and the redirect works as
> expected.
>
> -Ed
>
> On Sunday, October 8, 2017 at 6:03:15 PM UTC-5, Saúl Ibarra Corretgé wrote:
>
>>
>>
>> > On Oct 7, 2017, at 11:13, Ed Hemphill <[email protected]> wrote:
>> >
>> > I have a problem with an abort() getting called by the following code
>> in linux-core.c:
>> >
>> >
>> > int uv__io_check_fd(uv_loop_t* loop, int fd) {
>> > struct uv__epoll_event e;
>> > int rc;
>> >
>> > e.events = POLLIN;
>> > e.data = -1;
>> >
>> > rc = 0;
>> > if (uv__epoll_ctl(loop->backend_fd, UV__EPOLL_CTL_ADD, fd, &e))
>> > if (errno != EEXIST)
>> > rc = -errno;
>> >
>> > if (rc == 0)
>> > if (uv__epoll_ctl(loop->backend_fd, UV__EPOLL_CTL_DEL, fd, &e))
>> > abort();
>> >
>> > return rc;
>> > }
>> >
>> > (gdb) frame 2
>> > #2 0x000000000080d842 in uv__io_check_fd (loop=0x7ffff7dd8c40
>> <libLoop>, fd=33) at ../src/unix/linux-core.c:164
>> > 164 ../src/unix/linux-core.c: No such file or directory.
>> > (gdb) p errno
>> > $1 = 2
>> >
>> >
>> >
>> > My understanding is this code is to check whether the FD can actually
>> be added to the epoll set. What I don't get, is why the abort() would ever
>> be reached. I.e. why does epol_ctl() allow you to add the FD if it can't
>> remove it (meaning it really could not add it)?
>> >
>> > I was wondering if someone could shed some light on how this code came
>> about, and why an FD would be in such a state that it would reach the
>> abort(). The amount knowledge on the epoll inner workings out there on the
>> net is minimal.
>> >
>> > In my case, the errno is 2 on second epoll_ctl fail, which is ENOENT
>> >
>> > Thanks so much,
>> > Ed
>> >
>> >
>> >
>> > more backtrace
>> >
>> > (gdb) bt
>> > #0 0x00007ffff6c08c37 in __GI_raise (sig=sig@entry=6) at
>> ../nptl/sysdeps/unix/sysv/linux/raise.c:56
>> > #1 0x00007ffff6c0c028 in __GI_abort () at abort.c:89
>> > #2 0x000000000080d842 in uv__io_check_fd (loop=0x7ffff7dd8c40
>> <libLoop>, fd=33) at ../src/unix/linux-core.c:164
>> > #3 0x000000000080273c in uv_poll_init (loop=0x7ffff7dd8c40 <libLoop>,
>> handle=0x1780858, fd=33) at ../src/unix/poll.c:58
>> > #4 0x00007ffff7bc8715 in
>> fdRedirectorTicket::fdRedirectorTicket(unsigned int, int, void
>> (*)(GreaseLibError*, int, int)) ()
>> > from /home/ed/work/gostuff/src/
>> github.com/WigWagCo/maestro/../greasego/deps/lib/libgrease.so.1
>> >
>>
>> What kind of fd is that?
>>
>> --
>> Saúl
>>
>> --
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