Re: [PATCH] pager: die when paging to non-existing command

> When trying to execute a non-existent program from GIT_PAGER, we display
> an error.  However, we also send the complete text to the terminal
> and return a successful exit code.  This can be confusing for the user
> and the displayed error could easily become obscured by a lengthy
> text.
> 
> For example, here the error message would be very far above after
> sending 50 MB of text:
> 
>     $ GIT_PAGER=non-existent t/test-terminal.perl git log | wc -c
>     error: cannot run non-existent: No such file or directory
>     50314363
> 
> Let's make the error clear by aborting the process and return an error
> so that the user can easily correct their mistake.
> 
> This will be the result of the change:
> 
>     $ GIT_PAGER=non-existent t/test-terminal.perl git log | wc -c
>     error: cannot run non-existent: No such file or directory
>     fatal: unable to start the pager: 'non-existent'
>     0

> Finally, it's worth noting that we are not changing the behavior if the
> command specified in GIT_PAGER is a shell command.  In such cases, it
> is:
> 
>     $ GIT_PAGER=:\;non-existent t/test-terminal.perl git log
>     :;non-existent: 1: non-existent: not found
>     died of signal 13 at t/test-terminal.perl line 33.

> The behavior change we're introducing in this commit affects two tests
> in t7006, which is a good sign regarding test coverage and requires us
> to address it.
> 
> The first test is 'git skips paging non-existing command'.  This test
> comes from f7991f01f2 (t7006: clean up SIGPIPE handling in trace2 tests,
> 2021-11-21,) where a modification was made to a test that was originally
> introduced in c24b7f6736 (pager: test for exit code with and without
> SIGPIPE, 2021-02-02).  That original test was, IMHO, in the same
> direction we're going in this commit.

> The second test being affected is: 'non-existent pager doesnt cause
> crash', introduced in f917f57f40 (pager: fix crash when pager program
> doesn't exist, 2021-11-24).  As its name states, it has the intention of
> checking that we don't introduce a regression that produces a crash when
> GIT_PAGER points to a nonexistent program.
> 
> This test could be considered redundant nowadays, due to us already
> having several tests checking implicitly what a non-existent command in
> GIT_PAGER produces.  However, let's maintain a good belt-and-suspenders
> strategy; adapt it to the new world.