PATCH: bug fix: infinite loop in sysread

[Roman Perepelitsa <roman.perepelitsa@xxxxxxxxx>]

The attached patch fixes a bug in sysread from zsh/system. The bug
triggers in the following case:

1. zsh has been compiled with HAVE_SELECT and without HAVE_POLL
2. sysread is called with timeout (-t)
3. the input file descriptor is valid but there is no data to read
4. errno happens to be EINTR prior to the call to sysread

This results in an infinite loop in sysread:

  while ((ret = select(infd+1, (SELECT_ARG_2_T) &fds,
                       NULL, NULL,&select_tv)) < 1) {
      if (errno != EINTR || errflag || retflag || breaks || contflag)
          break;
  }

Here select() keeps returning 0, indicating timeout. This is not an
error, so errno doesn't get set. If it was EINTR prior to the call,
it stays EINTR, and the loop keeps spinning.

The fix is to replace `< 1` with `< 0` in the loop condition.

On GitHub:
https://github.com/zsh-users/zsh/compare/master...romkatv:fix-sysread-tmout

Roman.

diff --git a/Src/Modules/system.c b/Src/Modules/system.c
index 50de59cf9..fb3d80773 100644
--- a/Src/Modules/system.c
+++ b/Src/Modules/system.c
@@ -174,7 +174,7 @@ bin_sysread(char *nam, char **args, Options ops, UNUSED(int func))
 	}
 
 	while ((ret = select(infd+1, (SELECT_ARG_2_T) &fds,
-			     NULL, NULL,&select_tv)) < 1) {
+			     NULL, NULL,&select_tv)) < 0) {
 	    if (errno != EINTR || errflag || retflag || breaks || contflag)
 		break;
 	}