bug#37093: wc runs 100% cpu when in pipeline or tee >(wc)

[Edward Huff]

In the demo below, dd uses 0.665s to write 1GiB of zeros.
sha256sum uses 4.285s to calculate the sha256 of 1GiB of zeros.
wc uses 32.160s to count 1GiB of zeros.

Linux localhost 5.2.8-200.fc30.x86_64 #1 SMP Sat Aug 10 13:21:39 UTC 2019
x86_64 x86_64 x86_64 GNU/Linux
coreutils-8.31-2.fc30.x86_64
dd (coreutils) 8.31
wc (GNU coreutils) 8.31
sha256sum (GNU coreutils) 8.31

baseline results:
$ dd if=/dev/zero count=$((1024*1024)) bs=1024 | tee >(sha256sum>&2) | wc
1048576+0 records in
1048576+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 32.5007 s, 33.0 MB/s
49bc20df15e412a64472421e13fe86ff1c5165e18b2afccf160d4dc19fe68a14  -
      0       0 1073741824
$

Demo script:
$ cat wc-at-100pct
#!/bin/bash
set -xv
rm pipe* tee*
{
   {
      time dd if=/dev/zero count=$((1024*1024)) bs=1024
   } 2>>pipedd
} | {
   tee >(
      {
         time sha256sum
      } >teesha256 2>&1
   )
} | {
   {
      time wc
   } > pipewc 2>&1
}

{
   {
      time dd if=/dev/zero count=$((1024*1024)) bs=1024
   } 2>>pipedd
} | {
   tee >(
      {
         time wc
      } >teewc 2>&1
   )
} | {
   {
      time sha256sum
   } > pipesha256sum 2>&1
}

head pipe* tee*
$

Results:
./wc-at-100pct
rm pipe* tee*
+ rm pipedd pipesha256sum pipewc teesha256 teewc
{
   {
      time dd if=/dev/zero count=$((1024*1024)) bs=1024
   } 2>>pipedd
} | {
   tee >(
      {
         time sha256sum
      } >teesha256 2>&1
   )
} | {
   {
      time wc
   } > pipewc 2>&1
}
+ tee /dev/fd/63

{
   {
      time dd if=/dev/zero count=$((1024*1024)) bs=1024
   } 2>>pipedd
} | {
   tee >(
      {
         time wc
      } >teewc 2>&1
   )
} | {
   {
      time sha256sum
   } > pipesha256sum 2>&1
}
+ tee /dev/fd/63

head pipe* tee*
+ head pipedd pipesha256sum pipewc teesha256 teewc
==> pipedd <==
+ dd if=/dev/zero count=1048576 bs=1024
1048576+0 records in
1048576+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 32.5495 s, 33.0 MB/s

real 0m32.550s
user 0m0.665s
sys 0m1.503s
+ dd if=/dev/zero count=1048576 bs=1024
1048576+0 records in

==> pipesha256sum <==
+ sha256sum
49bc20df15e412a64472421e13fe86ff1c5165e18b2afccf160d4dc19fe68a14  -

real 0m31.457s
user 0m4.285s
sys 0m0.562s

==> pipewc <==
+ wc
      0       0 1073741824

real 0m32.555s
user 0m32.160s
sys 0m0.247s

==> teesha256 <==
++ sha256sum
49bc20df15e412a64472421e13fe86ff1c5165e18b2afccf160d4dc19fe68a14  -

real 0m32.553s
user 0m4.333s
sys 0m0.704s

==> teewc <==
++ wc
      0       0 1073741824

real 0m31.456s
user 0m31.121s
sys 0m0.221s