siginfo_init should maybe set SA_RESTART flag on sigaction (EINTR)

[cemeyer]

sigaction() in siginfo_init() sets sa_flags = 0. The SA_RESTART flag has the nice property of resuming some typical system calls without returning -1 / errno=EINTR to the application.

Alternatively, tarsnap should handle EINTR gracefully. I see some checks for EINTR in the source code.
But it does not appear to be consistently handled gracefully:

tarsnap: Can't read -: Interrupted system call

(Given this is EINTR and not ENOMEM, I think the printed line can only come from one location in the source code: https://github.com/Tarsnap/tarsnap/blob/master/tar/util.c#L358-L360 . I'm not sure what the right interaction between EINTR and FILE streams is. You could take the minimal approach of clearerror(3) if errno == EINTR, I guess.)

[cperciva]

What's your command line, what OS are you running on, and what are you doing to trigger a signal? It seems like fread should handle EINTR by itself.

[cemeyer]

1. tarsnap -xpkf XXX --null -T - with a extract list provided via stdin (modified redsnapper)
2. FreeBSD 13-CURRENT as of this week.
3. ^T (SIGINFO)
4. I agree — it seems odd that fread(3) would not be handling the EINTR.

[cemeyer]

Re: fread, it can be blamed on lib/libc/stdio/refill.c:

  136         fp->_r = _sread(fp, (char *)fp->_p, fp->_bf._size);
  137         fp->_flags &= ~__SMOD;  /* buffer contents are again pristine */
  138         if (fp->_r <= 0) {
  139                 if (fp->_r == 0)
  140                         fp->_flags |= __SEOF;
  141                 else {
  142                         fp->_r = 0;
  143                         fp->_flags |= __SERR;
  144                 }
  145                 return (EOF);
  146         }

(_sread does not appear to do anything magic like re-start EINTR operations.)

Edit: In fact, it seems like very little of FreeBSD libc (and none of libc/stdio) handles EINTR in any explicit way.

[cemeyer]

FWIW, POSIX 2017 specifically calls out EINTR as a valid error for fgetc(3), and by extension, fread(3). And e.g. musl libc does not do any special handling of EINTR either. It seems that stdio FILE-using programs need to be EINTR-aware in the same ways that programs using bare syscalls do.