bug#77535: timeout treats very short durations as `0`

[Pádraig Brady]

On 04/04/2025 16:26, Nicolas Boichat wrote:

Hi,

Version: timeout (GNU coreutils) 9.6, Archlinux, x86-64.

While playing with different duration parameters to `timeout`, I
noticed that extremely short durations, like `1e-3000`, are rounded
down to `0`. The problem is that `0` has a special meaning (disabling
the timeout), so I don't think this is desired.

All of these commands exit immediately:
```
timeout 0.0001 cat
timeout 1e-100 cat
timeout 1e-300 cat
timeout 1e-323 cat
```

But these never exits:
```
timeout 1e-324 cat
timeout 1e-3000 cat
```

As if we had typed:
```
timeout 0 cat
```

I think there is some logic in `printf` to handle float parsing
underflow, so maybe this can be reused in `timeout` as well.

The following should avoid this issue.

Marking this as done.

thanks!
Pádraig

diff --git a/src/timeout.c b/src/timeout.c
index 578d71070..6756cd888 100644
--- a/src/timeout.c
+++ b/src/timeout.c
@@ -371,6 +371,10 @@ parse_duration (char const *str)
       usage (EXIT_CANCELED);
     }

+  /* Clamp underflow to 1ns, as 0 disables the timeout.  */
+  if (duration == 0 && errno == ERANGE)
+    duration = 1e-9;
+
   return duration;
 }