Re: read with very small timeout sometime hand on stdin

[felix]

On Wed, Oct 28, 2020 at 09:23:09AM -0400, Chet Ramey wrote:
> 
> I can't reproduce this using the following stripped-down reproducer:

> trap 'echo $f ; exit' SIGINT
> 
> for f in {1..10000}; do
>       read -t .000001 v
>       if [ $? -ne 142 ]; then
>               echo $f: $?
>       fi
> done
This is strange: $? seem to be alway 142! On my host:
  Debian 10.4 - Intel(R) Core(TM) i5-4570 CPU @ 3.20GHz - 32Gb ram

  $ TIMEFORMAT="U:%U S:%S R:%R"
  $ time for f in {1..1000};do read -t .000001 v; [ $? -ne 142 ] &&
      echo f:$f, v:$v, RC:$?. ; done < <(
          for i in {1..1000};do sleep 3;echo;done)
  U:0.206 S:0.032 R:0.240

  $ time for f in {1..1000};do read -t .000001 v; [ $? -ne 142 ] &&
      echo f:$f, v:$v, RC:$?. ; done < <(
          for i in {1..1000};do sleep 3;echo;done)
  U:0.167 S:0.068 R:0.236

  $ time for f in {1..1000};do read -t .000001 v; [ $? -ne 142 ] &&
      echo f:$f, v:$v, RC:$?. ; done < <(
          for i in {1..1000};do sleep 3;echo;done)
  U:0.207 S:0.049 R:24.152

  $ time for f in {1..1000};do read -t .000001 v; [ $? -ne 142 ] &&
      echo f:$f, v:$v, RC:$?. ; done < <(
          for i in {1..1000};do sleep 3;echo;done)
  U:0.177 S:0.067 R:6.233

Where 6 second seem represent two `sleep 3;echo` ...

Same on my raspberry-pi (BCM2835 ARMv6-compatible rev 7 (v6l) - 512M),
  Machine Type: armv6l-unknown-linux-gnueabihf - 5.1.0.rc1

...but with 100 loop and 10 second sleep:


  $ TIMEFORMAT="U:%U S:%S R:%R"
  $ time for f in {1..100};do read -t .000001 v; [ $? -ne 142 ] &&
        echo f:$f, v:$v, RC:$?. ; done < <(
            for i in {1..100};do sleep 10;echo;done ) 
  U:0.447 S:0.021 R:0.995
  
  $ time for f in {1..100};do read -t .000001 v; [ $? -ne 142 ] &&
        echo f:$f, v:$v, RC:$?. ; done < <(
            for i in {1..100};do sleep 10;echo;done ) 
  U:8.589 S:0.108 R:10.918

  $ time for f in {1..100};do read -t .000001 v; [ $? -ne 142 ] &&
        echo f:$f, v:$v, RC:$?. ; done < <(
            for i in {1..100};do sleep 10;echo;done ) 
  U:0.445 S:0.022 R:0.999

  $ time for f in {1..100};do read -t .000001 v; [ $? -ne 142 ] &&
        echo f:$f, v:$v, RC:$?. ; done < <(
            for i in {1..100};do sleep 10;echo;done ) 
  U:0.540 S:0.065 R:10.958

Again, no output! (Result code seem to be alway 142)

But clearly without `sleep 10; echo` loop as input, this test may hang...
until you hit <return>...  maybe several times...

Nota: None of the host used to make this tests are really ``quiet'':
 My host loadavg: 0.48 0.78 0.91 2/1074 15425
 Raspbery load  : 0.93 1.01 1.12 2/87 28120

This seem harder to reproduce on quiet system
 Other host load: 0.00 0.04 0.00 1/772 468
 U:0.015 S:0.008 R:0.023
 U:0.021 S:0.000 R:0.023
 U:0.021 S:0.000 R:0.022
 U:0.015 S:0.010 R:3.023
 U:0.015 S:0.004 R:0.020
 U:0.018 S:0.004 R:0.023
 U:0.010 S:0.008 R:0.019
 U:0.010 S:0.012 R:0.023
 U:0.023 S:0.000 R:0.024
 U:0.015 S:0.004 R:0.020
 U:0.019 S:0.000 R:0.020
 U:0.015 S:0.008 R:0.023
 U:0.038 S:0.008 R:3.023
 U:0.023 S:0.000 R:0.023
 U:0.030 S:0.000 R:3.025
 U:0.009 S:0.008 R:0.018
 U:0.020 S:0.007 R:3.031


-- 
 Félix Hauri  -  <felix@f-hauri.ch>  -  http://www.f-hauri.ch