Re: runsv and EAGAIN

[Charlie Brady <charlieb-supervision@budge.apana.org.au>]

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On Sat, 12 Jul 2008, Dra¾en Kaèar wrote:

> Charlie Brady wrote:
>>> Here's a sample strace:
>>>
>>> rt_sigprocmask(SIG_UNBLOCK, [TERM], NULL, 8) = 0
>>> rt_sigprocmask(SIG_UNBLOCK, [CHLD], NULL, 8) = 0
>>> poll([{fd=3, events=POLLIN}, {fd=9, events=POLLIN, revents=POLLIN},
>>> {fd=11, events=POLLIN}], 3, 1000020) = 1
>
> You only have POLLIN in revents for fd 9. Also, the return value is 1,
> meaning that only one file descriptor has readable data.
>
>>> rt_sigprocmask(SIG_BLOCK, [TERM], NULL, 8) = 0
>>> rt_sigprocmask(SIG_BLOCK, [CHLD], NULL, 8) = 0
>>> read(3, 0xbfa256c3, 1)                  = -1 EAGAIN (Resource temporarily
>>> unavailable)
>>> waitpid(-1, 0xbfa256bc, WNOHANG)        = 0
>>> read(9, "", 1)                          = 0
>>> read(11, 0xbfa256c3, 1)                 = -1 EAGAIN (Resource temporarily
>>> unavailable)
>>> gettimeofday({1215792640, 591768}, NULL) = 0
>> ...
>
>> So poll i saying that data is available, and read is saying that it isn't.
>> Is anyone else confused?
>
> poll() is saying that the data is available on fd 9 and read() on fd 9
> does not return an error. The other two file descriptors should have not
> been read.

Thanks. I didn't read the strace at all carefully, did I?

I see in iopause() that poll() is called without checking the return 
status.

fd 3 is runsv's selfpipe. fd 9 is the control fifo and fd 11 is the log 
control fifo. Poll tells us that fd 9 had data to read, but then read 
returned 0.

I'd better check (on Monday) to see whether I somehow have multiple runsv 
processes in the same directory...

Here's the relevant part of runsv.c:

...
     sig_unblock(sig_term);
     sig_unblock(sig_child);
     iopause(x, 2 +haslog, &deadline, &now);
     sig_block(sig_term);
     sig_block(sig_child);

     while (read(selfpipe[0], &ch, 1) == 1)
       ;
     for (;;) {
       int child;
       int wstat;

       child =wait_nohang(&wstat);
       if (!child) break;
       if ((child == -1) && (errno != error_intr)) break;
       if (child == svd[0].pid) {
         svd[0].pid =0;
         pidchanged =1;
         svd[0].wstat =wstat;
         svd[0].ctrl &=~C_TERM;
         if (svd[0].state != S_FINISH)
           if ((fd =open_read("finish")) != -1) {
             close(fd);
             svd[0].state =S_FINISH;
             update_status(&svd[0]);
             continue;
           }
         svd[0].state =S_DOWN;
         taia_uint(&deadline, 1);
         taia_add(&deadline, &svd[0].start, &deadline);
         taia_now(&svd[0].start);
         update_status(&svd[0]);
         if (taia_less(&svd[0].start, &deadline)) sleep(1);
       }
       if (haslog) {
         if (child == svd[1].pid) {
           svd[1].pid =0;
           pidchanged =1;
           svd[1].state =S_DOWN;
           svd[1].ctrl &=~C_TERM;
           taia_uint(&deadline, 1);
           taia_add(&deadline, &svd[1].start, &deadline);
           taia_now(&svd[1].start);
           update_status(&svd[1]);
           if (taia_less(&svd[1].start, &deadline)) sleep(1);
         }
       }
     }
     if (read(svd[0].fdcontrol, &ch, 1) == 1) ctrl(&svd[0], ch);
     if (haslog)
       if (read(svd[1].fdcontrol, &ch, 1) == 1) ctrl(&svd[1], ch);

...