Trivial interface test fails when the charms don't have a requirements.txt file

[theoctober19th]

When writing interface tests for a new interface azure_storage, even a very trivial interface test failed; and I traced the reason to be due to unavailability of requirements.txt file in the charm being downloaded.

Steps to reproduce:
I followed these instructions to write a trivial integration test for the interface: https://github.com/canonical/charm-relation-interfaces/blob/main/README_INTERFACE_TESTS.md

1. Clone https://github.com/theoctober19th/charm-relation-interfaces/tree/azure-storage-credentials and checkout to branch azure-storage-credentials.

2. Verify that the tests are specified correctly:

interface_tester discover --include azure_storage

3. Run the test.

python run_matrix.py --include azure_storage

Observation
The command fails with the following error logs:

INFO:root:Running tests for interface: azure_storage
INFO:root:Running tests for version: v0
INFO:root:Running tests for role: provider
INFO:root:No tests specified for azure_storage/provider; skipping...
INFO:root:Running tests for role: requirer
INFO:root:Running 1 azure_storage interface tests on: ['spark-integration-hub-k8s', 'spark-history-server-k8s']...
INFO:root:Running tests for azure_storage
INFO:root:Running tests for charm: spark-integration-hub-k8s
INFO:root:Preparing testing environment for: spark-integration-hub-k8s
INFO:root:Cloning: spark-integration-hub-k8s from (https://github.com/canonical/spark-integration-hub-k8s-operator@main)
INFO:root:Preparing venv for /tmp/charm-relation-interfaces-tests/spark-integration-hub-k8s
INFO:root:Installing dependencies in venv for /tmp/charm-relation-interfaces-tests/spark-integration-hub-k8s
WARNING:root:test setup failed for spark-integration-hub-k8s azure_storage requirer
Traceback (most recent call last):
  File "/home/ubuntu/charm-relation-interfaces/run_matrix.py", line 179, in _setup_venv
    subprocess.check_call(
  File "/usr/lib/python3.10/subprocess.py", line 369, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '.interface-venv/bin/python -m pip install -r requirements.txt' returned non-zero exit status 1.

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/ubuntu/charm-relation-interfaces/run_matrix.py", line 218, in _test_charm
    charm_path, test_path = _prepare_repo(
  File "/home/ubuntu/charm-relation-interfaces/run_matrix.py", line 90, in _prepare_repo
    _setup_venv(charm_path)
  File "/home/ubuntu/charm-relation-interfaces/run_matrix.py", line 188, in _setup_venv
    raise SetupError("venv setup failed") from e
SetupError: venv setup failed
INFO:root:Result: FAILED
INFO:root:Running tests for charm: spark-history-server-k8s
INFO:root:Preparing testing environment for: spark-history-server-k8s
INFO:root:Cloning: spark-history-server-k8s from (https://github.com/canonical/spark-history-server-k8s-operator@main)
INFO:root:Preparing venv for /tmp/charm-relation-interfaces-tests/spark-history-server-k8s
INFO:root:Installing dependencies in venv for /tmp/charm-relation-interfaces-tests/spark-history-server-k8s
WARNING:root:test setup failed for spark-history-server-k8s azure_storage requirer
Traceback (most recent call last):
  File "/home/ubuntu/charm-relation-interfaces/run_matrix.py", line 179, in _setup_venv
    subprocess.check_call(
  File "/usr/lib/python3.10/subprocess.py", line 369, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '.interface-venv/bin/python -m pip install -r requirements.txt' returned non-zero exit status 1.

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/ubuntu/charm-relation-interfaces/run_matrix.py", line 218, in _test_charm
    charm_path, test_path = _prepare_repo(
  File "/home/ubuntu/charm-relation-interfaces/run_matrix.py", line 90, in _prepare_repo
    _setup_venv(charm_path)
  File "/home/ubuntu/charm-relation-interfaces/run_matrix.py", line 188, in _setup_venv
    raise SetupError("venv setup failed") from e
SetupError: venv setup failed
INFO:root:Result: FAILED
+++ Results +++
{
  "azure_storage": {
    "v0": {
      "provider": {},
      "requirer": {
        "spark-integration-hub-k8s": false,
        "spark-history-server-k8s": false
      }
    }
  }
}

[PietroPasotti]

This is because the interface test runner requires an interface.txt file in order to know how to set up a venv to test the charm.
Now more charms are using uv and lockfiles, and therefore omitting a requirements.txt file, we should consider adding support for this (e.g. a way to configure the test runner to run something other than pip install -r requirements.txt to set up the env; for example uv ...