[PATCH v4] 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 execute pager 'non-existent'
    0

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.

At any rate, this test obviously needs to be adjusted to check the new behavior we are introducing. Do it.

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.

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.

Signed-off-by: Rubén Justo <rjusto@gmail.com>
---

Error pointed out by Hannes in:

https://lore.kernel.org/git/f031c152-1b97-4598-92f3-a72aefd701a4@kdbg.org

Thanks!

Range-diff against v2:
1:  60e852bffb ! 1:  93d6074a17 pager: die when paging to non-existing command
    @@ Commit message
     
             $ 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'
    +        fatal: unable to execute pager 'non-existent'
             0
     
         The behavior change we're introducing in this commit affects two tests

 pager.c          |  2 +-
 t/t7006-pager.sh | 17 +++++------------
 2 files changed, 6 insertions(+), 13 deletions(-)