Issues with `mix xref` on umbrella apps

[japhib]

Elixir and Erlang/OTP versions

Erlang/OTP 25 [erts-13.1.1] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [jit:ns]

Elixir 1.14.3 (compiled with Erlang/OTP 25)

Operating system

MacOS Ventura 13.4

Current behavior

The behavior of various mix xref tools is weird, and in some cases just plain doesn't work, on umbrella apps. Here's a sample umbrella app that I did testing on: https://github.com/japhib/simple_umbrella_app

1. mix xref trace doesn't work with relative paths

$ mix xref trace apps/app_b/lib/app_b.ex
** (Code.LoadError) could not load /Users/japhib/projects/simple_umbrella_app/apps/app_a/apps/app_b/lib/app_b.ex. Reason: enoent
    (elixir 1.14.3) lib/code.ex:1826: Code.find_file!/2
    (elixir 1.14.3) lib/code.ex:1521: anonymous fn/2 in Code.compile_file/2
    (mix 1.14.3) lib/mix/tasks/xref.ex:471: Mix.Tasks.Xref.handle_trace/2
    (mix 1.14.3) lib/mix/task.ex:421: anonymous fn/3 in Mix.Task.run_task/4
    (mix 1.14.3) lib/mix/project.ex:397: Mix.Project.in_project/4
    (elixir 1.14.3) lib/file.ex:1607: File.cd!/2
    (mix 1.14.3) lib/mix/task.ex:577: anonymous fn/4 in Mix.Task.run_in_children_projects/2

Note that above ^ the error comes from attempting to load the path I passed in, apps/app_b/lib/app_b.ex, under the prefix of apps/app_a. My guess is that it tries to find the provided path in every app rather than just a single one.

When I provide an absolute path, it works fine:

$ mix xref trace /Users/japhib/projects/simple_umbrella_app/apps/app_b/lib/app_b.ex
==> app_a
/Users/japhib/projects/simple_umbrella_app/apps/app_b/lib/app_b.ex:4: struct StructInAppA (export)
/Users/japhib/projects/simple_umbrella_app/apps/app_b/lib/app_b.ex:4: alias StructInAppA (runtime)
/Users/japhib/projects/simple_umbrella_app/apps/app_b/lib/app_b.ex:9: call AppA.hello/0 (runtime)

2. mix xref graph --include-siblings does not include dependencies on other apps

According to the docs, using the --include-siblings flag should:

includes dependencies that have :in_umbrella set to true in the current project in the reports. This can be used to find callers or to analyze graphs between projects.

In my test project simple_umbrella_app, there is a dependency from lib/app_b.ex on both lib/app_a.ex and lib/struct_in_app_a.ex. Neither of these dependencies are shown.

$ mix xref graph --include-siblings
==> app_a
lib/app_a.ex
└── lib/struct_in_app_a.ex (export)
lib/struct_in_app_a.ex
==> app_b
lib/app_b.ex

3. The --sink and --source options on mix xref graph do not work

The --sink option is supposed to show a graph of modules that reference the specified module, while the --source option is supposed to show a graph of modules that the specified module references. This rarely works (see below) and seems to have something to do with the way the path is specified.

# app_a.ex

$ mix xref graph --sink lib/app_a.ex
** (Mix) Sinks could not be found: lib/app_a.ex
$ mix xref graph --sink apps/app_a/lib/app_a.ex
** (Mix) Sinks could not be found: apps/app_a/lib/app_a.ex
$ mix xref graph --sink /Users/japhib/projects/simple_umbrella_app/apps/app_a/lib/app_a.ex
** (Mix) Sinks could not be found: /Users/japhib/projects/simple_umbrella_app/apps/app_a/lib/app_a.ex
$ mix xref graph --source lib/app_a.ex
==> app_a
lib/app_a.ex
└── lib/struct_in_app_a.ex (export)
** (Mix) Sources could not be found: lib/app_a.ex
$ mix xref graph --source apps/app_a/lib/app_a.ex
** (Mix) Sources could not be found: apps/app_a/lib/app_a.ex
$ mix xref graph --source /Users/japhib/projects/simple_umbrella_app/apps/app_a/lib/app_a.ex
** (Mix) Sources could not be found: /Users/japhib/projects/simple_umbrella_app/apps/app_a/lib/app_a.ex

# app_b.ex

$ mix xref graph --sink lib/app_b.ex
** (Mix) Sinks could not be found: lib/app_b.ex
$ mix xref graph --sink apps/app_b/lib/app_b.ex
** (Mix) Sinks could not be found: apps/app_b/lib/app_b.ex
$ mix xref graph --sink /Users/japhib/projects/simple_umbrella_app/apps/app_b/lib/app_b.ex
** (Mix) Sinks could not be found: /Users/japhib/projects/simple_umbrella_app/apps/app_b/lib/app_b.ex
$ mix xref graph --source lib/app_b.ex
** (Mix) Sources could not be found: lib/app_b.ex
$ mix xref graph --source apps/app_b/lib/app_b.ex
** (Mix) Sources could not be found: apps/app_b/lib/app_b.ex
$ mix xref graph --source /Users/japhib/projects/simple_umbrella_app/apps/app_b/lib/app_b.ex
** (Mix) Sources could not be found: /Users/japhib/projects/simple_umbrella_app/apps/app_b/lib/app_b.ex

My unproven guess is that, like above, Mix is trying to use this path on every app rather than just the app it belongs to. At least, if it goes in alphabetical order (tries app_a, then app_b) then that would explain why above, with mix xref graph --source lib/app_a.ex it actually does appear to work at first (when it runs on app_a), but then still prints out an error message (when it tries to run on app_b).

Expected behavior

I would expect mix xref graph and mix xref trace to be just as useful on umbrella apps as they are on regular/non-umbrella apps.

[josevalim]

The task is marked as recursive, which means it runs in all umbrella children, which is probably a legacy from when xref was a compiler callback. We should definitely remove the @recursive true annotation.

[japhib]

Hi Jose! While I appreciate the quick response, the merged changes are actually not that helpful to me. Obviously it's your call as language creator how to handle this, but I'd like to at least explain why this isn't my ideal resolution.

--

The first thing is that it's a partial regression. mix xref graph, without any args, actually did work on the whole umbrella app -- it just didn't print "sibling" dependencies from other subapps within the umbrella. Now that command is unusable from the parent app, as you can only run it from the children apps' directories, and it will raise an error when run from the parent app.

(I guess running it from each subapp and concatenating the results would have the same result as running from the parent dir did before ... so, maybe this doesn't really matter.)

--

The second thing is that I was really hoping to get my hands on the --sink argument for use with mix xref graph in my umbrella app. Some background:

I work on a quite large umbrella app, with a lot of subapps (45 at current count), and presumably a lot of compile-time dependencies. I'd like to track down these compile-time dependencies in order to improve overall compile times for the project.

When I posted about reducing my compile times on Elixir Forum, the general response was "use mix xref to find compile-time dependencies and convert them to run-time dependencies." With mix xref trace fixed, I can now use it to analyze a single file and see what its dependencies are. (Thanks for fixing that!) But in order to go the other way -- that is, given a file name, what other files are dependent on it? -- I believe the best tool to use is mix xref graph --sink, which will provide sort of a reverse-tree with all the files that are directly or indirectly dependent on the file specified.

With the changes that you merged to close this issue, mix xref graph will now only be able to run on a single app, and support for --include-siblings in that command is explicitly dropped. It appears that I'm now out of luck, and while mix xref graph --sink could be used inside a subapp to find dependencies, it will not find any dependencies across subapp boundaries.

So, what should I do? Is there another way to do what I'm trying to do?

My imagined ideal solution is that mix xref graph --sink would be fixed/updated to A) work on full umbrella apps, and B) display cross-app dependencies, a la --include-siblings. I know you're busy, so it's totally understandable if you don't have time to work on that. But if someone else were to contribute that change, would you accept it? Or are there some fundamental limits to the way these tools work, that would make such a PR hacky or impossible?

If I were to try implementing that change, would you have any pointers or thoughts on how I'd go about doing so?

--

This is a long comment, so thank you for your time reading/responding. I also appreciate that mix xref trace now appears to be fixed for umbrella apps.

[josevalim]

Hi @japhib ! I think we can support those features, it is just that they didn’t work, so the first step is to acknowledge that.

A pull request that introduced support for actual umbrella behavior would be welcome. I think doing it for graphs would be easy, I can take a look if that’s the case and confirm tomorrow. Otherwise we would need someone to dig deeper into it. :)

[japhib]

Sounds great!

[japhib]

Would it be possible to backport these changes to 1.14/15?

[japhib]

Just opened 2 PRs for backporting this to versions 1.14: #12654

and 1.15: #12655