`sleep 10 | read` hangs interactive shell after ^Z

[McDutchie]

Found on comp.unix.shell:

From: Christian Weisgerber <naddy@mips.inka.de>
Newsgroups: comp.unix.shell
Subject: ksh93: pipelines vs. job control
Date: Mon, 13 May 2024 14:35:32 -0000 (UTC)
Message-ID: <slrnv4499k.30g3.naddy@lorvorc.mips.inka.de>
User-Agent: slrn/1.0.3 (FreeBSD)

... or "how to accidentally hang your ksh93 session".

Bash has this shell option:

    lastpipe
            If set, and job control is not active, the shell runs
            the last command of a pipeline not executed in the
            background in the current shell environment.

The bit about job control sounds like a weird stipulation, but makes
sense once you think about it.  I wonder how ksh93 handles that.
Famously, the AT&T ksh executes the last command of a pipeline in
the current shell:

  $ x=foo
  $ { /bin/sleep 10; echo bar; } | read x
  $ echo $x
  bar

However, a background pipeline runs in a subshell, as documented
in the man page:

  $ x=foo
  $ { /bin/sleep 10; echo bar; } | read x &
  [1]     78726
  $ 
  [1] +  Done                    { /bin/sleep 10; echo bar; } | read x &
  $ echo $x  
  foo

But we are in job control environment.  I can start a pipeline in
the foreground, suspend it, and put it into the background.  ksh93
can't know about that in advance.

  $ { /bin/sleep 10; echo bar; } | read x  
  ^Z

... and now the pipeline is suspended...

    PID  PGID STAT   COMMAND
  71496 71496 S      - ksh93 ksh93
  62980 62980 T+     `-- ksh93 ksh93
   3778 62980 T+p      `-- /bin/sleep 10

... and you're stuck.  There's no shell prompt, so you can't bg or fg
anything, intr (^C) or quit (^\) don't help, and there's no tty
control character to continue a suspended process (group).

You need to take radical measures like hanging up, or sending a
SIGCONT or SIGKILL from a different terminal.

I can't quite tell if this behavior constitutes a bug, but it seems
to follow from the design decision to not run the last command of
a pipeline in a subshell.  And it can trap the unwary.

A more minimal example:

  $ /bin/sleep 10 | read
  ^Z

  PID  PGID STAT   COMMAND
  30796 30796 S      - ksh93 ksh93
  31981 31981 T+p    `-- /bin/sleep 10

-- 
Christian "naddy" Weisgerber                          naddy@mips.inka.de

[lzaoral]

Thank you for the pointers in https://github.com/ksh93/ksh/pull/763#issuecomment-2249668652, @McDutchie! I'd say it is a different bug than #750, tough. The following line seems to cause the problem:

https://github.com/ksh93/ksh/blob/9ea4e32028411e2ad2dedb89974085937d9d7980/src/cmd/ksh93/bltins/sleep.c#L143

t is a double but inf cannot be properly represented in int so the assignment of n will evaluate -1 on my machine when debugged in GDB:

...
(gdb) p t
$6 = inf
(gdb) p n
$7 = -1
...

This conversion is actually not valid in C as shown by this experiment

#include <math.h>
#include <stdio.h>

int main(void) {
    double d = INFINITY;
    printf("double = %f, int = %d\n", d, (int) d);
}

$ clang -fsanitize=undefined inf.c && ./a.out
inf.c:6:42: runtime error: inf is outside the range of representable values of type 'int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior inf.c:6:42
double = inf, int = 2147483647

and the C99 standard itself:

6.3.1.4 Real floating and integer

When a finite value of real floating type is converted to an integer type other than _Bool, the fractional part is discarded (i.e., the value is truncated toward zero). If the value of the integral part cannot be represented by the integer type, the behavior is undefined.50)

[McDutchie]

Thanks for that. Looks like sleep inf was simply unimplemented and accidentally appeared to work. Also, since the Tv_t members are of type uint32_t, n should match that.

Here is a patch that should fix undefined behaviour and implement sleep inf on C99 and up. Please test:

diff --git a/src/cmd/ksh93/bltins/sleep.c b/src/cmd/ksh93/bltins/sleep.c
index e849ef44b..a899e8279 100644
--- a/src/cmd/ksh93/bltins/sleep.c
+++ b/src/cmd/ksh93/bltins/sleep.c
@@ -29,6 +29,7 @@
 #include   <error.h>
 #include   <errno.h>
 #include   <tmx.h>
+#include   <ast_float.h>
 #include   "builtins.h"
 #include   "FEATURE/time"
 #include   "FEATURE/poll"
@@ -140,9 +141,27 @@ skip:
  */
 void sh_delay(double t, int sflag)
 {
-   int n = (int)t;
+   uint32_t n;
    Tv_t ts, tx;

+#if _lib_fpclassify
+   switch (fpclassify(t))
+   {
+   case FP_NAN:
+       errormsg(SH_DICT,ERROR_exit(1),e_number,"NaN");
+       UNREACHABLE();
+   case FP_INFINITE:
+       ts.tv_sec = 0xFFFFFFFF;  /* uint32_t max */
+       ts.tv_nsec = 0;
+       while (1)
+       {
+           tvsleep(&ts, NULL);
+           if ((sh.trapnote & (SH_SIGSET | SH_SIGTRAP)) || sflag)
+               return;
+       }
+   }
+#endif
+   n = (uint32_t)t;
    ts.tv_sec = n;
    ts.tv_nsec = 1000000000 * (t - (double)n);
    while(tvsleep(&ts, &tx) < 0)

[lzaoral]

Thank you, @McDutchie! I confirm that your patch fixes the issue described in https://github.com/ksh93/ksh/pull/763#issuecomment-2249668652. clock_nanosleep is now called only once and with correct values!

$ strace arch/linux.arm64-64/bin/ksh -c 'sleep inf'
...
rt_sigaction(SIGWINCH, {sa_handler=0x44ee5c, sa_mask=[], sa_flags=SA_INTERRUPT}, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0
rt_sigaction(SIGALRM, {sa_handler=0x44ee5c, sa_mask=[], sa_flags=SA_INTERRUPT}, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0
clock_nanosleep(CLOCK_REALTIME, 0, {tv_sec=4294967295, tv_nsec=0},