kill the LHS command of a pipe once the RHS command terminates

[Vincent Lefevre <vincent@xxxxxxxxxx>]

With some commands I pipe the output to a pager, and when quitting
the pager, I want the command to terminate immediately (so that it
doesn't consume more resources) and want the shell prompt back.

Here are examples with "head -n 1" instead of a pager.

* With just a pipe:

zira% { echo foo; sleep 3; echo err >&2 } | head -n 1
foo
[wait for 3 seconds]
err
zira%

The issue is that when "head -n 1" terminates (just after "echo foo"),
the left-hand-side command is still running. Thus it still consumes
resources, and I also have to wait for the shell prompt, which is bad.

* With a process substitution:

zira% head -n 1 <(echo foo; sleep 3; echo err >&2)
foo
zira%

Here I get the shell prompt back immediately, which is good. But
the left-hand-side command is still running, so that it still
consumes resources; moreover, after the 3 seconds, I get

zira% err

In practice, one may want to redirect stderr to the pager too (not
always, though); this avoids this second issue, but not the first one,
which is that the left-hand-side command still consumes resources.

Does zsh provide a feature to kill the left-hand-side command in a
clean way (without a race condition, without a temporary file...)?

-- 
Vincent Lefèvre <vincent@xxxxxxxxxx> - Web: <https://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)