🐛 aiida_localhost: suffix label with xdist-worker id

Fixes the ``UNIQUE constraint failed: db_dbcomputer.label`` flake on
``tests-presto`` (#7347): the ``aiida_localhost`` fixture's Computer now
lives under a label suffixed with the pytest-xdist worker id
(``localhost-main`` outside xdist, ``localhost-gw0`` etc. inside), so
it never collides with a literal ``'localhost'`` Computer that another
code path has created in the same profile.

Mechanism. The fixture's lookup filters on four fields (``label``,
``hostname``, ``scheduler_type``, ``transport_type``); the
``Computer.label`` UNIQUE constraint is single-column. When ``verdi
presto`` or ``verdi devel launch-shell-job`` had already inserted a
``localhost`` Computer in the same profile, the four-field
``Computer.collection.get`` would miss it (mismatched fields), and the
fallback ``store()`` would trip the single-column UNIQUE on label.
Worker-suffixing the fixture's label removes the collision at the source
— the fixture row simply lives under a different name from any
literal-label row that other code paths may create.

Note this also corrects a parallelism-correctness bug that predates the
flake: the fixture was written before xdist was a concern and assumed
exclusive use of "the" localhost row in the test profile. The literal
``'localhost'`` label remains the production contract for ``verdi
presto`` and ``verdi devel launch-shell-job`` and is left untouched.
Test usages that hardcoded ``orm.load_computer('localhost')`` to reach
the fixture-created computer are switched to depend on
``aiida_localhost`` directly.

Author: GeigerJ2

src/aiida/tools/pytest_fixtures/orm.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import os
 import pathlib
 import typing as t
 
@@ -11,6 +12,20 @@
     from aiida.orm import Computer
 
 
+def _xdist_worker_id() -> str:
+    """Return the pytest-xdist worker id, or ``'main'`` when not running under xdist.
+
+    Used to suffix shared fixture labels (e.g. ``aiida_localhost``) so that simultaneous test
+    sessions on the same machine, or independent test profiles within one session, cannot collide
+    on database UNIQUE constraints. Read from the environment rather than depending on the
+    ``worker_id`` fixture so this works even when pytest-xdist is not loaded.
+
+    Note pytest-xdist's own ``worker_id`` fixture uses ``'master'`` for the non-xdist case; we
+    diverge from that convention here because the value appears in user-visible labels.
+    """
+    return os.environ.get('PYTEST_XDIST_WORKER', 'main')
+
+
 @pytest.fixture(scope='session')
 def ssh_key(tmp_path_factory) -> t.Generator[pathlib.Path, None, None]:
     """Generate a temporary SSH key pair for the test session and return the filepath of the private key.
@@ -269,14 +284,20 @@ def if_exists_and_is_configured(label):
 def aiida_localhost(aiida_computer_local) -> 'Computer':
     """Return a :class:`aiida.orm.computers.Computer` instance representing localhost with ``core.local`` transport.
 
+    The computer's label is suffixed with the pytest-xdist worker id (e.g. ``localhost-gw0``,
+    or just ``localhost-main`` when not running under xdist). This keeps the fixture's row
+    distinct from any literal ``'localhost'`` Computer that other tests or commands (notably
+    ``verdi presto``) may create in the same profile, avoiding UNIQUE-constraint collisions
+    on ``Computer.label`` when fixture state and command state share a database.
+
     Usage::
 
         def test(aiida_localhost):
             assert aiida_localhost.transport_type == 'core.local'
 
     :return: The computer.
     """
-    return aiida_computer_local(label='localhost')
+    return aiida_computer_local(label=f'localhost-{_xdist_worker_id()}')
 
 
 @pytest.fixture

tests/calculations/test_stash.py

@@ -109,7 +109,7 @@ def test_code_vs_stash_mode_conflict(stash_mode, fixture_sandbox, aiida_localhos
     code = orm.InstalledCode(
         label='dummy_code',
         default_calc_job_plugin='core.stash',
-        computer=orm.load_computer(label='localhost'),
+        computer=aiida_localhost,
         filepath_executable='/doesnot/exist/script.sh',
     )
 
@@ -252,7 +252,7 @@ def test_submit_custom_code(fixture_sandbox, aiida_localhost, generate_calc_job,
     code = orm.InstalledCode(
         label='test_code',
         default_calc_job_plugin='core.stash',
-        computer=orm.load_computer(label='localhost'),
+        computer=aiida_localhost,
         filepath_executable=str(executable),
     )
     code.store()

tests/tools/pytest_fixtures/test_orm.py

@@ -16,8 +16,36 @@
 
 
 def test_aiida_localhost(aiida_localhost):
-    """Test the ``aiida_localhost`` fixture."""
-    assert aiida_localhost.label == 'localhost'
+    """Test the ``aiida_localhost`` fixture.
+
+    The label is suffixed with the pytest-xdist worker id to avoid collisions with literal
+    ``'localhost'`` Computers created by other tests or commands (e.g. ``verdi presto``).
+    """
+    assert aiida_localhost.label.startswith('localhost-')
+    assert aiida_localhost.hostname == 'localhost'
+    assert aiida_localhost.transport_type == 'core.local'
+    assert aiida_localhost.scheduler_type == 'core.direct'
+
+
+@pytest.mark.usefixtures('aiida_profile_clean')
+def test_aiida_localhost_does_not_collide_with_literal_label(aiida_localhost):
+    """The fixture's Computer must coexist with a literal ``'localhost'`` Computer.
+
+    Other code paths (``verdi presto``, ``verdi devel launch-shell-job``) create a Computer
+    with the literal ``'localhost'`` label in the same profile. Worker-suffixing the fixture's
+    label makes its row distinct from those, so no UNIQUE-constraint collision is possible
+    even when both rows live in the same profile.
+    """
+    Computer(
+        label='localhost',
+        hostname='localhost',
+        transport_type='core.ssh',
+        scheduler_type='core.direct',
+        workdir='/tmp',
+    ).store()
+
+    assert aiida_localhost.label != 'localhost'
+    assert aiida_localhost.transport_type == 'core.local'
 
 
 @pytest.mark.parametrize(