Re: [PATCH] for-each-repo: do nothing on empty config

On 1/5/2021 11:20 PM, Eric Sunshine wrote:

On Tue, Jan 5, 2021 at 9:20 PM Derrick Stolee [off-list ref] wrote:

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On 1/5/2021 12:55 PM, Eric Sunshine wrote:

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On Tue, Jan 5, 2021 at 9:44 AM Derrick Stolee via GitGitGadget
[off-list ref] wrote:
Probably not worth a re-roll, but the above has higher cognitive load than:

    if (!value)
        return 0;

which indicates clearly that the command succeeded, whereas `return
result` makes the reader scan all the code above the conditional to
figure out what values `result` could possibly hold.

True. Looking at this again, it might be better to just update the
loop to be

        for (i = 0; values && i < values->nr; i++) {

which would run the loop zero times when there are zero results.

I find the explicit `return 0` easier to reason about since I can see
immediately that it handles a particular boundary condition, after
which I no longer have to think about that condition as I continue
reading the code.

That said, I don't think it matters greatly one way or the other
whether you use `return result`, `return 0`, or adjust the loop
condition. It might matter if more functionality is added later, but
we don't have to worry about it now, and it's not worth re-rolling
just for this, or spending a lot of time discussing it.

Junio made a good point that 'values' doesn't change during the loop,
so I shouldn't use it in the sentinel. I'll use your "return 0;"

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+       git for-each-repo --config=bogus.config -- these args would fail

The `these args would fail` arguments add mystery to the test,
prompting the reader to wonder what exactly is going on: "Fail how?",
"Is it supposed to fail?". It might be less confusing to use something
more direct like `nonexistent` or `does not exist`.

I guess the point is that if we actually did try running a subcommand
on a repo (for instance, accidentally running it on the current repo)
then the command would fail when running "git these args would fail".

To me, "nonexistent" or "does not exist" doesn't adequately describe
this (hypothetical) situation.

Perhaps "fail-subcommand" might be better?

I had never even looked at git-for-each-repo before, so I took the
time to read the documentation and the source code before writing this
reply. Now that I understand what is supposed to happen, I might be
tempted to suggest `this-command-wont-be-run` as an argument, but
that's a mouthful. If you want to be really explicit that it should
fail if a bug gets re-introduced which causes the argument to be run
even though the config is empty, then perhaps use `false`:

    git for-each-repo --config=bogus.config -- false

I'm just going to repeat that this would run "git false". It achieves
the same goal where we interpret this as a failure if the subcommand
is run.

By the way, when reading the documentation, some questions came to mind.

Is the behavior implemented by this patch desirable? That is, if the
user mistypes the name of the configuration variable, would it be
better to let the user know about the problem by returning an error
code rather than succeeding silently? Or do you foresee people
intentionally running the command against an empty config variable?
That might be legitimate in automation situations. If legitimate to
have an empty config, then would it be helpful to somehow distinguish
between an empty config variable and a non-existent one (assuming we
can even do that)?

As mentioned in the message, this is the situation the background
maintenance would see if a user used 'git maintenance unregister' on
all of their repos or removed the 'maintenance.repo' config values
from their global config. I think it would be better to not start
failing the background commands.

Even outside of that context, we have no way to specify an "existing
but empty" multi-valued config value over a non-existing config
value. I'd rather prefer the interpretation "I succeeded in doing
nothing" instead of "I think you made a mistake, because there's
nothing to do."

Could we meet in the middle and print a warning to stderr?

	warning(_("supplied config key '%s' has no values"));

That would present a useful message to users running this command
manually (or constructing automation) without breaking scripts
that parse the output.

Is the API of this command ideal? It feels odd to force the user to
specify required input via a command-line option rather than just as a
positional argument. In other words, since the config variable name is
mandatory, an alternative invocation format would be:

    git for-each-repo <config-var> <cmd>

Or do you foresee future enhancements expanding the ways in which the
repo list can be specified (such as from a file or stdin or directly
via the command-line), in which case the `--config` option makes
sense.

I believe some voice interest in specifying a list of repositories
by providing a directory instead of a config value. That would
require scanning the immediate subdirectories to see if they are Git
repos.

A --stdin option would also be a good addition, if desired.

Since this command was intended for automation, not end-users, it
seemed that future-proofing the arguments in this way would be a good
idea. We want to remain flexible for future additions without
breaking compatibility.

Thanks,
-Stolee